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Mixtures, Solutions and Separation PYQs – Chemistry, SSC & Railway

104 Mixtures, Solutions and Separation (Chemistry) questions asked in recent SSC and Railway exams, newest first, with exam, date & shift, answers and solutions.

📚 104 questionsSSC 6 · Railway 98
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RRB Group D 2025-26 – 25 August 2026 Shift 1 · Solutions, solubility and concentration

Q1. Which of the following is a characteristic of a true solution?

  1. (a) The solute particles settle rapidly at the bottom.
  2. (b) The solute particles are easily separated by filtration.
  3. (c) The solute particles can be seen with the naked eye.
  4. (d) The solute particles are evenly distributed in the solution.
Answer & solution

Answer: (d) The solute particles are evenly distributed in the solution.

Correct Answer:
Correct Answer: Option 4 – The solute particles are evenly distributed in the solution.
Explanation:
Atrue solutionis a homogeneous mixture where the solute particles are so small (less than 1 nm) that they areevenly and uniformly distributedthroughout the solvent at the molecular level. They do not settle over time, cannot be seen with the naked eye, and cannot be separated by ordinary filtration.
Important Facts:
• A true solution is optically clear/transparent and does not scatter light (no Tyndall effect).
• Suspensions, by contrast, have visible, settling particles that can be filtered out.
• Colloids are intermediate — particles too small to settle but large enough to scatter light (Tyndall effect).
RRB Group D 2025-26 – 25 August 2026 Shift 2 · Suspensions and colloids

Q2. Which of the following is NOT an example of a colloid?

  1. (a) Blood
  2. (b) Milk
  3. (c) Sugar solution
  4. (d) Ice cream
Answer & solution

Answer: (c) Sugar solution

Correct Answer:
Correct Answer: Option 3 – Sugar solution
Explanation:
Blood, milk, and ice cream are allcolloids(blood is a colloidal suspension of cells/proteins in plasma; milk is an emulsion of fat globules in water; ice cream is a foam/emulsion colloid). Asugar solution, however, is a true (homogeneous) solution where sugar molecules are completely dissolved at the molecular level — it is not a colloid.
RRB Group D 2025-26 – 25 August 2026 Shift 3 · Solutions, solubility and concentration

Q3. Which of the following statements about the solubility of solids and gases in liquids is generally CORRECT?

  1. (a) Solubility of both solids and gases in liquids decreases with increase in temperature.
  2. (b) Solubility of solids in liquids increases, while that of gases in liquids decreases with an increase in temperature.
  3. (c) Solubility of both solids and gases in liquids increases with increase in temperature.
  4. (d) Solubility of solids in liquids decreases, while that of gases in liquids increases with an increase in temperature.
Answer & solution

Answer: (b) Solubility of solids in liquids increases, while that of gases in liquids decreases with an increase in temperature.

Correct Answer:
Correct Answer: Option 2 – Solubility of solids in liquids increases, while that of gases in liquids decreases with an increase in temperature.
Explanation:
Dissolving most solids in a liquid is an endothermic process, so raising temperature generally increases solid solubility. Gas solubility, however, depends on the gas molecules staying trapped in the liquid; higher temperature gives gas molecules more kinetic energy to escape, so gas solubility generally decreases with rising temperature.
Important Facts:
• Warm water holds less dissolved oxygen than cold water, which is why fish are stressed in warm water.
• Sugar and salt dissolve faster and more in hot water than cold water.
• This is a general trend; a few exceptions exist (e.g., some sulphate salts), but it holds for most solids and gases.
RRB Group D 2025-26 – 21 August 2026 Shift 1 · Suspensions and colloids

Q4. Which of the following is an example of coagulation in everyday life?

  1. (a) Separating pigments using chromatography
  2. (b) Extracting salt from sea water
  3. (c) Filtering sand from a mixture of water and sand
  4. (d) Adding vinegar in milk to form paneer
Answer & solution

Answer: (d) Adding vinegar in milk to form paneer

Explanation:
Coagulation refers to the clumping together of dispersed colloidal particles into larger aggregates. Adding vinegar (an acid) to milk destabilises the milk protein (casein) colloid, causing it to coagulate and separate as paneer — a classic everyday example of coagulation.
Correct Answer: Option 4 – Adding vinegar in milk to form paneer
RRB Group D 2025-26 – 21 August 2026 Shift 2 · Suspensions and colloids

Q5. Which of the following is true about suspensions?

  1. (a) The solute particles settle down on standing
  2. (b) They do not scatter light
  3. (c) The solute dissolves completely
  4. (d) They are transparent
Answer & solution

Answer: (a) The solute particles settle down on standing

Correct Answer:
Correct Answer: Option 1 – The solute particles settle down on standing
Explanation:
A suspension is a heterogeneous mixture in which relatively large, undissolved solid particles are dispersed in a liquid. These particles are visible and settle at the bottom on standing. Suspensions also scatter light (Tyndall effect) and are not transparent.
RRB Group D 2025-26 – 21 August 2026 Shift 2 · Separation techniques (distillation, chromatography, centrifugation, crystallisation)

Q6. In an industrial process, a mixture of products is obtained when a hydrocarbon undergoes halogenation. Which method can be used to separate the monosubstituted product?

  1. (a) Distillation
  2. (b) Combustion
  3. (c) Precipitation
  4. (d) Filtration
Answer & solution

Answer: (a) Distillation

Correct Answer:
Correct Answer: Option 1 – Distillation
Explanation:
Halogenation of a hydrocarbon typically produces a mixture of mono-, di-, and poly-substituted products along with unreacted hydrocarbon, each having different boiling points. Fractional distillation is used to separate these liquid components based on their boiling point differences, isolating the monosubstituted product.
RRB Group D 2025-26 – 21 August 2026 Shift 3 · Suspensions and colloids

Q7. Which of the following is a property of a suspension?

  1. (a) Particles do not settle down
  2. (b) Can be separated by filtration
  3. (c) Particles are smaller than 1 nm
  4. (d) Homogeneous mixture
Answer & solution

Answer: (b) Can be separated by filtration

Correct Answer:
Correct Answer: Option 2 – Can be separated by filtration
Explanation:
A suspension is a heterogeneous mixture with large, visible particles that settle down on standing andcan be separated by filtration— unlike a true solution.
Important Facts:
• Suspension particles are larger than 1000 nm (not smaller than 1 nm — that's a true solution).
• Suspensions are heterogeneous, not homogeneous, and their particles do settle down over time.
RRB Group D 2025-26 – 20 August 2026 Shift 1 · Suspensions and colloids

Q8. Which of the following is a characteristic of a colloidal solution?

  1. (a) The solute particles can be seen with the naked eye.
  2. (b) The solute particles settle down on standing.
  3. (c) The solute particles do not scatter a beam of light.
  4. (d) The solute particles scatter a beam of light but do not settle down.
Answer & solution

Answer: (d) The solute particles scatter a beam of light but do not settle down.

Correct Answer:
Correct Answer: Option 4 – The solute particles scatter a beam of light but do not settle down.
Explanation:
Acolloidal solutionhas particle sizes intermediate between true solutions and suspensions. These particles are large enough to scatter light (theTyndall effect) but too small and light to settle under gravity, so the colloid remains stable/uniform without separating on standing.
Important Facts:
• True solutions do not show the Tyndall effect (particles too small to scatter light).
• Suspensions show visible particles that settle on standing.
• Examples of colloids: milk, fog, gel, blood plasma.
RRB Group D 2025-26 – 20 August 2026 Shift 2 · Suspensions and colloids

Q9. Which of the following mixtures shows the Tyndall effect but the particles settle down on standing?

  1. (a) Solution
  2. (b) Suspension
  3. (c) Compound
  4. (d) Colloid
Answer & solution

Answer: (b) Suspension

Correct Answer:
Correct Answer: Option 2 – Suspension
Explanation:
A suspension has relatively large, heterogeneously dispersed particles that can scatter/reflect light (making the light path visible) but, being large and heavy, are unstable and settle down on standing. This is in contrast to a true colloid, whose particles also scatter light (Tyndall effect) but remain permanently suspended without settling.
Important Facts:
• True solutions do not scatter light and their particles never settle out.
• Colloidal solutions show the Tyndall effect but are stable (do not settle on standing) — the key differentiator from suspensions.
RRB Group D 2025-26 – 20 August 2026 Shift 3 · Suspensions and colloids

Q10. A colloidal solution is a type of:

  1. (a) Pure substance
  2. (b) Heterogeneous mixture
  3. (c) True solution
  4. (d) Homogeneous mixture
Answer & solution

Answer: (b) Heterogeneous mixture

Correct Answer:
Option 2 – Heterogeneous mixture
Explanation:
A colloidal solution (colloid) consists of dispersed particles (1 nm–1000 nm) distributed through a dispersion medium; it is aheterogeneous mixture, distinct from a true (homogeneous) solution, and shows the Tyndall effect.
RRB Group D 2025-26 – 19 August 2026 Shift 1 · Suspensions and colloids

Q11. Which of the following is a characteristic property of a suspension?

  1. (a) Its components cannot be separated
  2. (b) It is a true solution
  3. (c) It only burns with a blue flame
  4. (d) Its particles can be seen with the naked eye
Answer & solution

Answer: (d) Its particles can be seen with the naked eye

Correct Answer:
Correct Answer: Option 4 – Its particles can be seen with the naked eye
Explanation:
A suspension is aheterogeneousmixture in which relatively large solute particles remainvisible to the naked eyeand are not truly dissolved. These particles scatter light, settle down on standing, and can be separated by filtration.
Important Facts:
• Suspension particles are larger than 1000 nm (e.g., muddy water, chalk in water).
• Suspensions show the Tyndall effect and can be separated by simple filtration.
• A true solution is homogeneous with particles too small to be seen (unlike a suspension).
RRB Group D 2025-26 – 19 August 2026 Shift 2 · Solutions, solubility and concentration

Q12. A student adds salt to water until no more salt dissolves and some salt remains undissolved at the bottom. What does this indicate?

  1. (a) The solution formed is a colloidal solution
  2. (b) Salt has very low solubility in water
  3. (c) The solution formed is saturated salt solution
  4. (d) Salt is completely insoluble in water
Answer & solution

Answer: (c) The solution formed is saturated salt solution

Correct Answer:
Correct Answer: Option 3 – The solution formed is saturated salt solution
Explanation:
When a solvent has dissolved the maximum amount of solute possible at a given temperature, and additional solute remains undissolved, the solution is called asaturated solution. Since salt does dissolve substantially in water (just not indefinitely), this rules out 'very low solubility' or 'completely insoluble'; and a true salt solution is not colloidal.
Important Facts:
• A saturated solution is in dynamic equilibrium with undissolved solute at a given temperature.
• Heating the solution can often dissolve more solute (making it unsaturated), showing solubility depends on temperature.
RRB Group D 2025-26 – 19 August 2026 Shift 3 · Suspensions and colloids

Q13. Which of the following is a common example of a suspension found in daily life?

  1. (a) Starch solution
  2. (b) Milk
  3. (c) Muddy water
  4. (d) Salt water
Answer & solution

Answer: (c) Muddy water

Correct Answer:
Option 3 – Muddy water
Explanation:
Asuspensionis a heterogeneous mixture in which solute particles do not dissolve but remain suspended and are large enough to be visible and settle over time. Muddy water is a classic example, as mud/soil particles remain suspended and settle when left undisturbed.
Important Facts:
• Salt water and starch solution (colloid) are not true suspensions.
• Milk is a colloid (colloidal emulsion), not a suspension.
• Suspension particles scatter light (Tyndall effect) and can be separated by filtration.
Correct Answer: Option 3 – Muddy water
RRB Group D 2025-26 – 18 August 2026 Shift 1 · Suspensions and colloids

Q14. Which of the following is a correct property of a colloidal solution?

  1. (a) The solute can be separated by filtration.
  2. (b) The particles scatter a beam of light.
  3. (c) The solute settles down after some time.
  4. (d) The solute particles are invisible under a microscope.
Answer & solution

Answer: (b) The particles scatter a beam of light.

Properties of colloidal solutions
Colloidal particles are intermediate in size between true solutions and suspensions. They are large enough to scatter a beam of light passing through them — this is known as theTyndall effect, a defining property of colloids.
Colloidal particles cannot be separated by ordinary filtration, do not settle on standing, and while individually invisible to the naked eye, they can be seen using an ultramicroscope (not a regular microscope) via light scattering.
Correct Answer: The particles scatter a beam of light.
RRB Group D 2025-26 – 18 August 2026 Shift 2 · Separation techniques (distillation, chromatography, centrifugation, crystallisation)

Q15. Which of the following statements about centrifugation is INCORRECT?

  1. (a) Centrifugation can be applied in laboratory to separate the components of blood.
  2. (b) Centrifugation is used for separating components of different densities in a mixture.
  3. (c) Centrifugation involves spinning a mixture at high speed to separate its components.
  4. (d) Only solid mixtures with similar particle sizes can be separated by centrifugation.
Answer & solution

Answer: (d) Only solid mixtures with similar particle sizes can be separated by centrifugation.

Centrifugation — Identifying the Incorrect Statement
Centrifugation separates components of a mixture based on differences indensity(not particle size), by spinning the mixture at high speed. It is used for both liquid-liquid mixtures (e.g., cream from milk) and solid-liquid mixtures (e.g., blood components).
The statement 'Only solid mixtures with similar particle sizes can be separated by centrifugation' isincorrect— centrifugation works on density differences and is not restricted to solids of similar particle size.
Correct Answer: Only solid mixtures with similar particle sizes can be separated by centrifugation.
RRB Group D 2025-26 – 18 August 2026 Shift 3 · Pure substances and mixtures

Q16. Which of the following is a pure substance?

  1. (a) Only compound
  2. (b) Only mixture
  3. (c) Only element
  4. (d) Both compound and element
Answer & solution

Answer: (d) Both compound and element

Concept: Pure Substances vs Mixtures
A pure substance has a fixed, definite composition and uniform properties throughout. Pure substances are classified into two categories:
• Elements — made of only one type of atom (e.g., oxygen, iron)
• Compounds — made of two or more elements chemically combined in a fixed ratio (e.g., water, salt)
Mixtures (made of two or more substances not chemically combined, in variable proportions) are NOT pure substances. Hence both compounds and elements qualify as pure substances.
Correct Answer: Both compound and element
RRB Group D 2025-26 – 17 August 2026 Shift 1 · Separation techniques (distillation, chromatography, centrifugation, crystallisation)

Q17. A scientist wants to obtain pure solid iodine from its mixture with sand. Which change of temperature process should they use?

  1. (a) Freezing
  2. (b) Condensation
  3. (c) Sublimation
  4. (d) Melting
Answer & solution

Answer: (c) Sublimation

Separating Iodine from Sand
Iodine sublimes— it changes directly from solid to vapour on heating, without passing through a liquid state, and re-solidifies (deposits) directly back to solid on cooling. Sand does not sublime.
So heating the mixture allows iodine vapours to rise and be collected/condensed separately as pure solid iodine on a cool surface, leaving sand behind — this technique is calledsublimation.
Correct Answer: Sublimation
RRB Group D 2025-26 – 17 August 2026 Shift 2 · Suspensions and colloids

Q18. What are the two main components of a colloidal solution?

  1. (a) Reactant phase and product medium
  2. (b) Solute phase and solvent medium
  3. (c) Dispersed phase and dispersion medium
  4. (d) Metal phase and non-metal medium
Answer & solution

Answer: (c) Dispersed phase and dispersion medium

Components of a Colloid
A colloidal solution has two components: thedispersed phase(the solute-like component present in small particles, e.g., 1 nm-1000 nm in size) and thedispersion medium(the continuous phase in which the dispersed phase is distributed).
• Example: In milk, fat globules are the dispersed phase and water is the dispersion medium.
• This terminology (dispersed phase / dispersion medium) is specific to colloids, distinguishing them from true solutions (solute/solvent).
Correct Answer: Dispersed phase and dispersion medium
RRB Group D 2025-26 – 17 August 2026 Shift 3 · Separation techniques (distillation, chromatography, centrifugation, crystallisation)

Q19. Which factor must be considered before using a separating funnel for two liquids?

  1. (a) Both liquids have high boiling point
  2. (b) The liquids must have different colors
  3. (c) The liquids must be immiscible
  4. (d) The liquids must have strong odors
Answer & solution

Answer: (c) The liquids must be immiscible

Separating Funnel Requirement
A separating funnel is used to separate two immiscible liquids based on the difference in their densities (e.g., oil and water).
• The essential condition is that the two liquids must beimmiscible(do not mix/form separate layers).
• Boiling point, colour, and odour are not relevant requirements for using a separating funnel.
Correct Answer: The liquids must be immiscible
RRB Group D 2025-26 – 14 August 2026 Shift 3 · Solutions, solubility and concentration

Q20. Which of the following statements is true for homogeneous mixtures?

  1. (a) The solute particles settle down when it is left undisturbed.
  2. (b) The particles of a solution are smaller and cannot be seen with the naked eye.
  3. (c) They show the Tyndall effect.
  4. (d) The components of the mixture can be separated by filtration.
Answer & solution

Answer: (b) The particles of a solution are smaller and cannot be seen with the naked eye.

Answer: Option 2 – The particles of a solution are too small to be seen with the naked eye.
• In a homogeneous mixture (true solution) particles are tiny, don't settle, and don't show the Tyndall effect.
RRB Group D 2025-26 – 13 August 2026 Shift 1 · Homogeneous and heterogeneous mixtures

Q21. Which of the following mixtures is NOT homogeneous?

  1. (a) Vinegar
  2. (b) Oil and water
  3. (c) Salt water
  4. (d) Air
Answer & solution

Answer: (b) Oil and water

Answer: Option 2 – Oil and water.
•Oil and waterdon't mix uniformly ⇒ heterogeneous. The others are homogeneous.
RRB Group D 2025-26 – 13 August 2026 Shift 2 · Homogeneous and heterogeneous mixtures

Q22. Which of the following is a heterogenous mixture?

  1. (a) Sand and Iron fillings
  2. (b) Air
  3. (c) Vinegar
  4. (d) Salt solution
Answer & solution

Answer: (a) Sand and Iron fillings

Answer: Option 1 – Sand and iron filings.
•Sand and iron filingsform a non-uniform (heterogeneous) mixture. Air, vinegar and salt solution are homogeneous.
RRB Group D 2025-26 – 13 August 2026 Shift 3 · Homogeneous and heterogeneous mixtures

Q23. Which of the following is NOT a heterogeneous mixture?

  1. (a) Lemonade (with pulp)
  2. (b) Alcohol and water
  3. (c) Soil
  4. (d) Iron filings and sulphur
Answer & solution

Answer: (b) Alcohol and water

Answer: Option 2 – Alcohol and water.
•Alcohol and watermix uniformly (homogeneous). The others are heterogeneous.
RRB Group D 2025-26 – 12 August 2026 Shift 1 · Suspensions and colloids

Q24. The correct decreasing order of the size of the particles in suspension, solution, and colloid is:

  1. (a) colloid > suspension > solution
  2. (b) solution > colloid > suspension
  3. (c) suspension > colloid > solution
  4. (d) suspension > solution > colloid
Answer & solution

Answer: (c) suspension > colloid > solution

Answer: Option 3 – suspension > colloid > solution.
• Particle size order:suspension (largest) > colloid > solution (smallest).
RRB Group D 2025-26 – 12 August 2026 Shift 2 · Homogeneous and heterogeneous mixtures

Q25. Which of the following combinations forms a homogeneous mixture when combined in water?

  1. (a) Iron filings and sulfur
  2. (b) Oil and water
  3. (c) Salt and water
  4. (d) Sugar and sand
Answer & solution

Answer: (c) Salt and water

Answer: Option 3 – Salt and water.
•Salt in waterforms a homogeneous mixture (true solution); the others are heterogeneous.
RRB Group D 2025-26 – 11 August 2026 Shift 1 · Suspensions and colloids

Q26. You are given three mixtures: muddy water, sugar solution, and milk. Which one is a suspension?

  1. (a) Muddy Sugar
  2. (b) Milk
  3. (c) Muddy water
  4. (d) Sugar solution
Answer & solution

Answer: (c) Muddy water

Answer: Option 3 – Muddy water.
•Muddy wateris a suspension (large particles settle on standing). Sugar solution is a true solution; milk is a colloid.
RRB Group D 2025-26 – 11 August 2026 Shift 3 · Homogeneous and heterogeneous mixtures

Q27. Which of the following is/are example(s) of a heterogeneous mixture?a) Colloidal solutionb) Suspensionc) Solution

  1. (a) only (a)
  2. (b) only (b)
  3. (c) (a) and (b)
  4. (d) only (c)
Answer & solution

Answer: (c) (a) and (b)

Answer: Option 3 – (a) and (b).
• Bothcolloidal solutionsandsuspensionsare heterogeneous; a true solution is homogeneous.
RRB Group D 2025-26 – 10 August 2026 Shift 1 · Suspensions and colloids

Q28. What occurs to the particles in the suspension when we shake it?

  1. (a) It dissolves permanently
  2. (b) It forms a new substance
  3. (c) It mixes temporarily and then settles
  4. (d) It remains settled
Answer & solution

Answer: (c) It mixes temporarily and then settles

Answer: Option 3 — It mixes temporarily and then settles
Shaking a suspension mixes it briefly, then particlessettleagain → Option 3.
RRB Group D 2025-26 – 10 August 2026 Shift 2 · Homogeneous and heterogeneous mixtures

Q29. Which of the following is an example of a homogeneous mixture?

  1. (a) Sugar dissolved in water
  2. (b) Oil and water
  3. (c) Iron filings and sulfur
  4. (d) Sand and water
Answer & solution

Answer: (a) Sugar dissolved in water

Answer: Option 1 – Sugar dissolved in water.
• Ahomogeneous mixturehas a uniform composition throughout (a true solution). Sugar in water is one example.
• Oil-water, iron-sulfur, sand-water areheterogeneous(non-uniform) mixtures.
RRB Group D 2025-26 – 6 August 2026 Shift 1 · Suspensions and colloids

Q30. Which of the following is a characteristic feature of a suspension?

  1. (a) It is a homogeneous mixture with solute dissolved uniformly.
  2. (b) The particles are too small to settle down on standing.
  3. (c) Suspension shows Tyndall effect but is transparent.
  4. (d) Particles can be seen with the naked eye and settle on standing.
Answer & solution

Answer: (d) Particles can be seen with the naked eye and settle on standing.

Answer: Option 4 — Particles can be seen with the naked eye and settle on standing
Asuspensionhas large particles that are visible and settle on standing → Option 4.
RRB Group D 2025-26 – 6 August 2026 Shift 2 · Suspensions and colloids

Q31. Which of the following is a property of a colloidal solution?

  1. (a) Particles are smaller than true solutions
  2. (b) Particles can be seen with the naked eye
  3. (c) Particles do not settle on standing
  4. (d) Components can be separated by filtration
Answer & solution

Answer: (c) Particles do not settle on standing

Answer: Option 3 — Particles do not settle on standing
In acolloid, particles are small enough that theydo not settleon standing → Option 3.
RRB Group D 2025-26 – 6 August 2026 Shift 3 · Homogeneous and heterogeneous mixtures

Q32. A chemistry teacher prepares a mixture by dissolving copper sulfate in water until the solution is clear blue. Why is this considered a homogeneous mixture?

  1. (a) Copper sulfate crystals float on the surface of water
  2. (b) The mixture forms two distinct layers after stirring
  3. (c) The mixture appears as a single phase with no visible separation
  4. (d) Copper sulfate can be separated by filtration easily
Answer & solution

Answer: (c) The mixture appears as a single phase with no visible separation

Answer: Option 3 — Single phase with no visible separation
A homogeneous mixture hasone uniform phasewith no visible boundaries → Option 3.
RRB Group D 2025-26 – 5 August 2026 Shift 1 · Solutions, solubility and concentration

Q33. Which of the following is NOT a property of a solution?

  1. (a) Transparent appearance
  2. (b) Separation of components by filtration
  3. (c) No settling of solute
  4. (d) Uniform composition
Answer & solution

Answer: (b) Separation of components by filtration

Answer: Option 2 — Separation of components by filtration
A true solution's particles are too small to be filtered, so "separation by filtration" isnota property of a solution → Option 2.
RRB Group D 2025-26 – 5 August 2026 Shift 2 · Suspensions and colloids

Q34. Which of the following statements correctly compares the particle sizes of a suspension and a solution?

  1. (a) The particles of a suspension are larger in size than the particles of a solution.
  2. (b) The particles of a suspension are smaller in size than the particles of a solution.
  3. (c) Both have particles of the same size.
  4. (d) Suspension particles are molecular-sized and solution particles are atomic-sized.
Answer & solution

Answer: (a) The particles of a suspension are larger in size than the particles of a solution.

Answer: Option 1 — Suspension particles are larger than solution particles
A suspension haslarge, visible particles; a solution has very small (molecular/ionic) particles. So suspension particles are larger → Option 1.
*(Translated from Bengali; solved in English.)*
RRB Group D 2025-26 – 4 August 2026 Shift 1 · Suspensions and colloids

Q35. A teacher prepares three mixtures. Sugar dissolved in water Salt dissolved in water Sand stirred in water Which of the following is a suspension?

  1. (a) The salt and water mixture
  2. (b) Both Sugar-water mixture & Sand-water mixture
  3. (c) Only the sand and water mixture
  4. (d) Only the sugar and water mixture
Answer & solution

Answer: (c) Only the sand and water mixture

Answer: Option 3 — Only the sand and water mixture
Asuspensionhas undissolved particles that settle — thesand + watermixture. (Sugar/salt dissolve → solutions.) → Option 3.
RRB Group D 2025-26 – 4 August 2026 Shift 3 · Solutions, solubility and concentration

Q36. If 25 g of salt is dissolved in 175 g of water, what is the concentration of the solution in mass percentage?

  1. (a) 15%
  2. (b) 10%
  3. (c) 18%
  4. (d) 12.5%
Answer & solution

Answer: (d) 12.5%

Answer: Option 4 — 12.5%
Solution = 25 + 175 = 200 g. Mass % = 25/200 × 100 =12.5%→ Option 4.
RRB Group D 2025-26 – 3 August 2026 Shift 1 · Solutions, solubility and concentration

Q37. A solution contains 20 grams of salt dissolved in 80 grams of water. What is the concentration of the solution in terms of mass by mass percentage?

  1. (a) 20%
  2. (b) 25%
  3. (c) 45%
  4. (d) 30%
Answer & solution

Answer: (a) 20%

Answer: Option 1 — 20%
Mass % = (mass of solute / mass of solution) × 100.
Solution mass = 20 + 80 = 100 g.
Mass % = (20/100) × 100 =20%→ Option 1.
RRB Group D 2025-26 – 3 August 2026 Shift 2 · Homogeneous and heterogeneous mixtures

Q38. Which of the following mixtures best demonstrates a heterogeneous mixture at the macroscopic level?

  1. (a) Sugar and water
  2. (b) Sand and Water
  3. (c) Salt and water
  4. (d) Nitrogen and Oxygen
Answer & solution

Answer: (b) Sand and Water

Answer: Option 2 — Sand and Water
Aheterogeneousmixture has visibly distinct parts.Sand + watershows two separate phases (sand settles) → Option 2. (Sugar/salt in water and N₂+O₂ are homogeneous.)
RRB Group D 2025-26 – 3 August 2026 Shift 3 · Homogeneous and heterogeneous mixtures

Q39. A student mixes oil and water in a beaker and notices two separate layers. Which explanation best fits this observation?

  1. (a) Oil and water form a homogeneous mixture with a single phase
  2. (b) Oil and water form a heterogeneous mixture because their components are not uniformly distributed
  3. (c) Oil dissolves completely in water to form a uniform solution
  4. (d) Oil and water chemically react to form a new compound
Answer & solution

Answer: (b) Oil and water form a heterogeneous mixture because their components are not uniformly distributed

Answer: Option 2 — Heterogeneous mixture (components not uniformly distributed)
Oil and water areimmiscibleand form two visible layers — aheterogeneous mixture→ Option 2.
RRB NTPC UG CBT-1 2025-26 – 20 June 2026 Shift 3 · Homogeneous and heterogeneous mixtures

Q40. Which type of mixture is formed when Ethyl alcohol is dissolved in water?

  1. (a) Heterogeneous
  2. (b) Compounds
  3. (c) New element
  4. (d) Homogeneous
Answer & solution

Answer: (d) Homogeneous

Topic: Types of Mixtures
When ethyl alcohol (C₂H₅OH) dissolves in water (H₂O), they mix uniformly at molecular level.
Key Points:
• Both are polar liquids and completely miscible
• Forms single phase with uniform composition throughout
• No visible boundaries between components
Homogeneous mixture= uniform composition, single phase
Heterogeneous mixture= non-uniform, multiple phases
Answer: Option 4 → Homogeneous
RRB NTPC UG CBT-1 2025-26 – 19 June 2026 Shift 1 · Separation techniques (distillation, chromatography, centrifugation, crystallisation)

Q41. In the production of crystal sugar from sugarcane juice, the sugar is separated from the liquid through the growth of solid particles (sugar crystals) from a concentrated solution (syrup). This separation technique is called _____.

  1. (a) Chromatography
  2. (b) Crystallization
  3. (c) Distillation
  4. (d) Filtration
Answer & solution

Answer: (b) Crystallization

Separation Technique in Sugar Production
Process described: Growth of solid sugar crystals from concentrated syrup solution
Crystallization:Separation method where pure solid crystals form from a saturated solution by cooling or evaporation
In sugar production: Sugarcane juice → Concentrated syrup → Crystal formation → Pure sugar crystals separate
Other options incorrect:
• Chromatography: Separates mixtures using mobile/stationary phases
• Distillation: Separates by boiling point differences
• Filtration: Separates solid particles from liquid using filter
Answer: Option 2 → Crystallization
RRB NTPC UG CBT-1 2025-26 – 14 June 2026 Shift 2 · Solutions, solubility and concentration

Q42. A solution is prepared by dissolving 15 g of salt in 585 g of water. Calculate the mass percent (w/w) of the solution.

  1. (a) 0.5%
  2. (b) 1.5%
  3. (c) 2.1%
  4. (d) 2.5%
Answer & solution

Answer: (d) 2.5%

Given: Mass of salt = 15 g, Mass of water = 585 g
Formula: Mass percent (w/w) = (Mass of solute / Total mass of solution) × 100
Total mass of solution = 15 + 585 = 600 g
Mass percent = (15/600) × 100 = 2.5%
Answer: Option 4 → 2.5%
RRB Technician Grade III 2025-26 – 10 March 2026 Shift 3 · Solutions, solubility and concentration

Q43. The solubility of gases in liquids generally:

  1. (a) Decreases with increase in temperature
  2. (b) Increases with increase in temperature
  3. (c) Is unaffected by temperature
  4. (d) First increases then decreases with increase in temperature
Answer & solution

Answer: (a) Decreases with increase in temperature

The solubility of gases in liquids decreases as temperature increases.
This happens because higher temperature gives gas molecules more energy to escape from the liquid.
So, gas is less soluble when liquid is hotter.
Hence, option 1 is correct.
RRB Technician Grade III 2025-26 – 10 March 2026 Shift 1 · Elements and compounds

Q44. Which of the following correctly represents a diatomic element?

  1. (a) H
  2. (b) He
  3. (c) H₂
  4. (d) CO
Answer & solution

Answer: (c) H₂

Diatomic elements always consist of two atoms bonded together.
Hydrogen (H₂) is a common diatomic element.
Option 1 (H) is a single atom, not diatomic.
Option 2 (He) is a noble gas and monoatomic.
Option 4 (CO) is a compound, not a pure diatomic element.
Therefore, the correct representation of a diatomic element is H₂.
RRB Technician Grade III 2025-26 – 10 March 2026 Shift 2 · Elements and compounds

Q45. Which of the following statements is correct regarding molecules of an element?

  1. (a) All elements exist as monoatomic molecules in their natural state.
  2. (b) A molecule of an element consists of only one type of atom bonded together.
  3. (c) Molecules of an element always contain two or more different types of atoms.
  4. (d) Elements do not form molecules, only compounds do.
Answer & solution

Answer: (b) A molecule of an element consists of only one type of atom bonded together.

A molecule of an element consists of atoms of the same type chemically bonded together.
For example, O2 is a molecule made of two oxygen atoms.
Monoatomic elements like noble gases do exist but not all elements are monoatomic in natural state.
Option 1 and 3 are incorrect because elements can be monoatomic or diatomic but not always multi-atom compounds.
Option 4 is wrong because elements can form molecules, such as O2 and N2.
RRB Technician Grade III 2025-26 – 9 March 2026 Shift 3 · Pure substances and mixtures

Q46. If a sample of air is compressed to half its volume, which of the following statements is correct?

  1. (a) The composition of air changes.
  2. (b) The composition of air remains same because it is a mixture.
  3. (c) Air becomes a compound on compression.
  4. (d) The proportion of gases changes.
Answer & solution

Answer: (b) The composition of air remains same because it is a mixture.

When air is compressed to half its volume, it remains a mixture of gases.
Compression changes the volume and pressure, but not the composition of the mixture.
No chemical reaction occurs that would change its components or proportions.
Hence, the composition of air remains the same because it is a mixture.
Correct answer: Option 2
RRB Technician Grade III 2025-26 – 9 March 2026 Shift 2 · Elements and compounds

Q47. Which of the following is a molecule formed by two atoms of the same element?

  1. (a) CO
  2. (b) O₃
  3. (c) H₂O
  4. (d) H₂
Answer & solution

Answer: (d) H₂

Option 4 H₂ is a molecule composed of two hydrogen atoms of the same element.
Other options are not formed by two atoms of the same element:
CO − carbon monoxide, composed of carbon and oxygen atoms.
O₃ − ozone, composed of three oxygen atoms but not two atoms only.
H₂O − water, composed of hydrogen and oxygen atoms.
Hence, H₂ is the correct answer as a molecule formed by two atoms of the same element.
RRB Technician Grade III 2025-26 – 9 March 2026 Shift 1 · Solutions, solubility and concentration

Q48. Which of the following statement about a solution is correct?

  1. (a) A solution is a heterogeneous mixture of two or more substances.
  2. (b) The solute in a solution settles down on standing.
  3. (c) A solution is a homogeneous mixture in which the solute particles are not visible to the naked eye.
  4. (d) The solute in a solution is always present in a greater amount than the solvent.
Answer & solution

Answer: (c) A solution is a homogeneous mixture in which the solute particles are not visible to the naked eye.

Option 3 is correct.
A solution is a homogeneous mixture, meaning the composition is uniform throughout.
The solute particles in a solution are dissolved and too small to be seen with the naked eye.
Option 1 is incorrect because a solution is not heterogeneous; heterogeneous mixtures have visible different components.
Option 2 is incorrect because solutes in a true solution do not settle on standing.
Option 4 is incorrect since in a solution the solute is present in a lesser amount than the solvent.
Therefore, statement 3 correctly describes a solution.
RRB Technician Grade III 2025-26 – 9 March 2026 Shift 2 · Suspensions and colloids

Q49. Which of the following statements are correct?
1. Colloids appear homogeneous because their particles are very small.
2. Colloids are truly homogeneous at the molecular level.
3. Colloidal particles scatter light.
4. Milk is a colloidal solution.

  1. (a) 1, 2, 3 and 4
  2. (b) 1, 3 and 4 only
  3. (c) 2 and 4 only
  4. (d) 1 and 2 only
Answer & solution

Answer: (b) 1, 3 and 4 only

Correct options are 1, 3 and 4 only.
1. Colloids appear homogeneous because their particles are very small, but not truly homogeneous at molecular level.
2. Colloids are heterogeneous mixtures at molecular scale, so statement 2 is incorrect.
3. Colloidal particles scatter light (Tyndall effect).
4. Milk is a widely known example of a colloidal solution.
Hence, option 2 is correct.
RRB Technician Grade III 2025-26 – 9 March 2026 Shift 3 · Pure substances and mixtures

Q50. Which of the following statements correctly differentiates a compound from a mixture?

  1. (a) Mixtures are formed through chemical reactions.
  2. (b) Compounds retain the properties of their constituents.
  3. (c) Mixtures can be separated by physical means, but compounds cannot.
  4. (d) Compounds show variable composition, mixtures do not.
Answer & solution

Answer: (c) Mixtures can be separated by physical means, but compounds cannot.

Option 3 is correct.
Mixtures are combinations of substances that can be separated by physical means because their components retain individual properties.
Compounds are chemically combined substances that cannot be separated by physical methods.
Compounds have fixed composition, while mixtures can have variable composition.
Thus, the key difference is that mixtures can be separated physically, but compounds cannot be.
RRB Technician Grade III 2025-26 – 9 March 2026 Shift 2 · Suspensions and colloids

Q51. Which of these is an example of a solid sol?

  1. (a) Jelly
  2. (b) Coloured gem
  3. (c) Cheese
  4. (d) Smoke
Answer & solution

Answer: (b) Coloured gem

Correct Answer:Coloured gem
Solution:
Solid sol= solid dispersed in a solid medium.
Coloured gem(like ruby, glass with metal particles) is an example of solid sol.
Jellyandcheeseare gels (liquid in solid).
Smokeis solid in gas.
RRB Technician Grade III 2025-26 – 6 March 2026 Shift 2 · Homogeneous and heterogeneous mixtures

Q52. A student mixes sand, salt and water in a beaker and stirs well. After some time, what will be observed?

  1. (a) Sand remains permanently suspended in water, making the mixture cloudy.
  2. (b) All components dissolve completely, forming a clear solution.
  3. (c) Sand settles at the bottom, while salt dissolves in water.
  4. (d) Sand and salt both remain undissolved, forming two distinct layers.
Answer & solution

Answer: (c) Sand settles at the bottom, while salt dissolves in water.

When sand, salt, and water are mixed:
Sand is an insoluble solid and does not dissolve in water.
Salt (sodium chloride) is soluble and dissolves completely in water.
Due to gravity, the sand particles settle down at the bottom of the beaker.
The salt dissolves uniformly, resulting in a salty solution above the sand.
Hence, sand settles at the bottom, while salt dissolves in water.
Final answer: Option 3
RRB Technician Grade III 2025-26 – 6 March 2026 Shift 1 · Solutions, solubility and concentration

Q53. If both solute and solvent masses are doubled, the concentration (mass %) of the solution will:

  1. (a) Increase by 25%
  2. (b) Become half
  3. (c) Remain the same
  4. (d) Double
Answer & solution

Answer: (c) Remain the same

Concentration (mass %) is calculated as (mass of solute / total mass of solution) × 100.
If both solute and solvent masses are doubled, the ratio of solute to total solution remains the same.
Hence, the concentration by mass percent remains unchanged.
Therefore, the concentration will remain the same.
Final answer: Option 3
RRB Technician Grade III 2025-26 – 6 March 2026 Shift 2 · Solutions, solubility and concentration

Q54. In a homogeneous mixture, the size of particles of solute is typically:

  1. (a) Less than 1 nm
  2. (b) Between 1 nm and 1000 nm
  3. (c) Larger than 1 µm
  4. (d) Visible to the naked eye
Answer & solution

Answer: (a) Less than 1 nm

In a homogeneous mixture (solution), the solute particles are molecular or ionic in size.
These particles are extremely small, typically less than 1 nanometre (nm).
Hence, the particle size is less than 1 nm, making them invisible under normal light microscopy.
This differentiates solutions from colloids and suspensions, which have larger particles.
Therefore, the correct option is Option 1: Less than 1 nm.
Answer complete.
RRB Technician Grade III 2025-26 – 6 March 2026 Shift 1 · Suspensions and colloids

Q55. Select the correct option regarding the given statements.
Statement 1: Suspensions scatter light.
Statement 2: When particles settle down, they no longer scatter light.

  1. (a) Both statements are true, and Statement 2 explains Statement 1.
  2. (b) Statement 1 is true and Statement 2 is false.
  3. (c) Both statements are true, but Statement 2 does not explain Statement 1.
  4. (d) Both statements are false.
Answer & solution

Answer: (c) Both statements are true, but Statement 2 does not explain Statement 1.

Both statements are correct: suspensions scatter light due to the presence of suspended particles.
When these particles settle down, the medium becomes clearer and no longer scatters light.
Thus, Statement 2 correctly explains why after settling, light no longer scatters.
Therefore, the correct choice is that both statements are true and Statement 2 explains Statement 1.
Final answer: Option 1
RRB JE CBT-1 2025-26 – 25 February 2026 Shift 3 · Suspensions and colloids

Q56. Which of the following everyday mixtures is best classified as a suspension?

  1. (a) Chalk powder mixed with water
  2. (b) Sugar completely dissolved in water
  3. (c) Salt solution prepared in water
  4. (d) Copper sulphate dissolved in water
Answer & solution

Answer: (a) Chalk powder mixed with water

Option 1 is correct.
A suspension is a heterogeneous mixture where particles do not dissolve but remain suspended and may settle over time.
Chalk powder in water does not dissolve and settles at the bottom, showing typical suspension behaviour.
Other options involve solutions where substances dissolve completely, forming homogeneous mixtures.
Therefore, chalk powder mixed with water is the best example of a suspension.
Answer ends here.
RRB JE CBT-1 2025-26 – 20 February 2026 Shift 1 · Solutions, solubility and concentration

Q57. Which property explains why the path of light is not visible when a beam passes through a true solution?

  1. (a) Particles can be removed by filtration
  2. (b) Particles are smaller than one nanometre
  3. (c) Particles settle down on standing still
  4. (d) Particles are heavier than solvent molecules
Answer & solution

Answer: (b) Particles are smaller than one nanometre

The path of light is not visible in a true solution because the particles in it are extremely small.
In a true solution, particles are smaller than one nanometre in size.
Such tiny particles do not scatter light, so the beam of light remains invisible when passing through the solution.
The Tyndall effect, which makes light visible, occurs only if the particles are larger, like in colloids.
Hence, the reason is the very small size of particles in a true solution.
Final answer: Option 2
RRB JE CBT-1 2025-26 – 20 February 2026 Shift 1 · Homogeneous and heterogeneous mixtures

Q58. Which statement correctly describes a heterogeneous mixture?

  1. (a) Its composition is not uniform throughout the mixture
  2. (b) Its components cannot be separated by physical methods
  3. (c) Its composition remains fixed and constant always
  4. (d) Its particles are uniformly distributed at particle level
Answer & solution

Answer: (a) Its composition is not uniform throughout the mixture

Option 1 is correct.
A heterogeneous mixture has a non-uniform composition.
Different components can be seen distinctly.
Components can be physically separated.
Other options describe properties of homogeneous mixtures or pure substances.
Thus, option 1 correctly describes a heterogeneous mixture.
Final answer: Option 1
SSC MTS 2025 – 19 February 2026 Shift 3 · Suspensions and colloids

Q59. How are colloids classified?

  1. (a) Based on the color of the colloid
  2. (b) Based on the state of the dispersing medium and the dispersed phase
  3. (c) Based on the pH of the colloid
  4. (d) Based on the temperature of the colloid
Answer & solution

Answer: (b) Based on the state of the dispersing medium and the dispersed phase

Correct Answer: Based on the state of the dispersing medium and the dispersed phase (Option 2)
Colloids are classified based on the physical state (solid, liquid, or gas) of both the dispersing medium and the dispersed phase. Examples: Sol (solid dispersed in liquid), Gel (liquid in solid), Aerosol (liquid/solid in gas), Foam (gas in liquid), Emulsion (liquid in liquid).
Why other options are wrong:
Option 1 – Color is not a basis for classifying colloids.
Option 3 – pH is a chemical property and is not used to classify colloids.
Option 4 – Temperature is not a criterion for colloid classification.
RRB JE CBT-1 2025-26 – 19 February 2026 Shift 1 · Homogeneous and heterogeneous mixtures

Q60. Which of the following everyday examples represents a homogeneous mixture?

  1. (a) Chalk powder stirred in water
  2. (b) Oil mixed thoroughly with water
  3. (c) Sand mixed uniformly with iron filings
  4. (d) Salt completely dissolved in water
Answer & solution

Answer: (d) Salt completely dissolved in water

Option 4 is correct: Salt completely dissolved in water forms a homogeneous mixture.
In this mixture, salt particles are completely dissolved, making the solution uniform throughout.
Other options represent heterogeneous mixtures where components are visibly separate or not uniformly mixed.
Therefore, salt dissolved in water is the best example of a homogeneous mixture.
Final answer: Option 4
SSC MTS 2025 – 18 February 2026 Shift 3 · Solutions, solubility and concentration

Q61. What is the amount of solute present in a given amount of solution known as?

  1. (a) Dilution
  2. (b) Saturation
  3. (c) Concentration
  4. (d) Diffusion
Answer & solution

Answer: (c) Concentration

The correct answer is option 3.
Concentration is defined as the amount of solute dissolved in a given amount of solution (or solvent). It can be expressed in various ways such as mass per volume, moles per litre (molarity), etc. A higher concentration means more solute is present in the solution.
Option 1 is incorrect: Dilution is the process of reducing the concentration of a solution by adding more solvent; it is a process, not a measure of solute amount.
Option 2 is incorrect: Saturation refers to the state when no more solute can be dissolved in a given solvent at a particular temperature; it is a condition, not a measurement of solute amount.
Option 4 is incorrect: Diffusion is the movement of molecules from a region of higher concentration to lower concentration; it is a physical process, not a measure of solute amount.
SSC MTS 2025 – 17 February 2026 Shift 1 · Suspensions and colloids

Q62. Which of the following colloids has dispersed phase as liquid and dispersing phase as solid?

  1. (a) Fog
  2. (b) Shaving cream
  3. (c) Jelly
  4. (d) Milk
Answer & solution

Answer: (c) Jelly

Jelly is a gel-type colloid where the dispersed phase is liquid and the dispersing medium (continuous phase) is solid (gelatin network). Fog is liquid dispersed in gas (aerosol); Shaving cream is gas dispersed in liquid (foam); Milk is liquid dispersed in liquid (emulsion). Only Jelly matches the given description.
SSC MTS 2025 – 13 February 2026 Shift 1 · Homogeneous and heterogeneous mixtures

Q63. Which of the following is an example of homogeneous mixture?

  1. (a) Oil and water
  2. (b) Salt dissolved in water
  3. (c) Salt and sulphur
  4. (d) Sodium chloride and iron fillings
Answer & solution

Answer: (b) Salt dissolved in water

Correct Answer: Option 2 – Salt dissolved in water
A homogeneous mixture has a uniform composition throughout, with no visible boundaries between components. When salt (NaCl) dissolves in water, it forms a uniform solution where individual ions are evenly distributed — this is a classic example of a homogeneous mixture.
Why other options are wrong:
Option 1 (Oil and water): These do not mix; oil floats on water forming two distinct layers — this is a heterogeneous mixture.
Option 3 (Salt and sulphur ): These two solids can be physically distinguished; it is a heterogeneous mixture.
Option 4 (Sodium chloride and iron fillings): These can be visually separated (and by a magnet); it is a heterogeneous mixture.
SSC MTS 2025 – 6 February 2026 Shift 3 · Suspensions and colloids

Q64. Which of the following is an example of gas-solid foam type of colloid?

  1. (a) Milk
  2. (b) Mist
  3. (c) Milk of magnesia
  4. (d) Sponge
Answer & solution

Answer: (d) Sponge

Correct Answer: 4 (Sponge)
Colloid types based on dispersed phase and dispersion medium:
- Solid foam = gas dispersed in solid medium. Example: Sponge, foam rubber, pumice stone.
Sponge has gas (air) trapped in a solid matrix, making it a gas-in-solid type of colloid (solid foam).
Option 1 (Milk) is incorrect - Milk is an emulsion (liquid dispersed in liquid: fat droplets in water).
Option 2 (Mist) is incorrect - Mist is an aerosol (liquid dispersed in gas).
Option 3 (Milk of magnesia) is incorrect - It is a sol/suspension (solid dispersed in liquid).
SSC MTS 2025 – 4 February 2026 Shift 3 · Suspensions and colloids

Q65. The solute-like component or the dispersed particles in a colloid form the _____.

  1. (a) Solvent
  2. (b) Dispersed phase
  3. (c) Continuous phase
  4. (d) Solution phase
Answer & solution

Answer: (b) Dispersed phase

Correct Answer: Dispersed phase
In a colloid, the substance dispersed as fine particles (the solute-like component) forms the DISPERSED PHASE. The medium in which the dispersed phase is spread is called the DISPERSION MEDIUM (continuous phase).
Why other options are wrong:
Solvent - Term used in true solutions, not in colloids.
Continuous phase - This is the dispersion medium, not the dispersed particles.
Solution phase - Not a standard term in colloid chemistry.
RRB NTPC UG CBT-1 2024-25 – 8 September 2025 Shift 3 · Elements and compounds

Q66. Which of the following is a compound?

  1. (a) Oxygen
  2. (b) Iron
  3. (c) Water
  4. (d) Sulphur
Answer & solution

Answer: (c) Water

RRB NTPC UG CBT-1 2024-25 – 3 September 2025 Shift 3 · Separation techniques (distillation, chromatography, centrifugation, crystallisation)

Q67. Which method is employed in the refining of petroleum to separate crude oil from high boiling point fractions like lubricating oils?

  1. (a) Fractional distillation
  2. (b) Rotary sublimation
  3. (c) Differential extraction
  4. (d) Vacuum filtration
Answer & solution

Answer: (a) Fractional distillation

RRB NTPC UG CBT-1 2024-25 – 19 August 2025 Shift 1 · Solutions, solubility and concentration

Q68. Why are soft drink and soda water bottles sealed under high pressure in beverage industries?

  1. (a) To decrease the amount of sugar
  2. (b) To increase the temperature
  3. (c) To increase the solubility of carbon dioxide
  4. (d) To prevent freezing
Answer & solution

Answer: (c) To increase the solubility of carbon dioxide

RRB NTPC UG CBT-1 2024-25 – 12 August 2025 Shift 3 · Suspensions and colloids

Q69. Although a colloid appears homogeneous, it is actually a __________.

  1. (a) suspension
  2. (b) true solution
  3. (c) homogeneous mixture
  4. (d) heterogeneous mixture
Answer & solution

Answer: (d) heterogeneous mixture

RRB Technician Grade III 2024-25 – 30 December 2024 Shift 1 · Homogeneous and heterogeneous mixtures

Q70. Which of the following is the best example of a heterogeneous mixture?

  1. (a) Sugar in water
  2. (b) Salt in water
  3. (c) Copper sulphate solution
  4. (d) Mixture of iron chloride and iron fillings
Answer & solution

Answer: (d) Mixture of iron chloride and iron fillings

Correct Answer: Option 4 — Mixture of iron chloride and iron fillings
A heterogeneous mixture is one whose composition is not uniform. Its components remain in separate phases, the boundary between them can usually be seen, and two samples taken from different parts of the mixture are not identical.
A mixture of iron chloride (a salt) and iron filings (the free metal) is exactly such a mixture. The two do not dissolve in each other, the grey metallic bits stay visibly apart from the salt, and they can be picked out by hand or lifted out with a magnet — a purely physical method, which proves it is a mixture and not a compound.
A homogeneous mixture (a true solution), by contrast, has the same composition throughout; its solute particles are smaller than 1 nm, cannot be seen even with a microscope, and never settle down.
❌ Why the other options are incorrect
✗ Sugar in water — sugar dissolves completely and spreads uniformly, giving a homogeneous solution; no separate phase is visible.
✗ Salt in water — common salt splits into Na⁺ and Cl⁻ ions that spread evenly through the water; a classic true solution.
✗ Copper sulphate solution — a uniformly blue homogeneous liquid of CuSO₄ in water; the word *solution* itself tells you it is single-phased.
📌 SSC & Railway Exam Facts
• Mixtures are of two kinds — homogeneous (solutions) and heterogeneous.
• The NCERT classic heterogeneous mixture is iron filings + sulphur powder; the iron is removed with a magnet, the sulphur with carbon disulphide.
• Particle size: true solution < 1 nm, colloid 1–1000 nm, suspension > 1000 nm.
• Air is homogeneous, while muddy water, soil and oil in water are heterogeneous.
• In a mixture the components keep their own properties and are separable by physical methods; in a compound they do not.
RRB Technician Grade III 2024-25 – 30 December 2024 Shift 3 · Solutions, solubility and concentration

Q71. Which of the following statements is correct for a homogeneous mixture of sugar in water?

  1. (a) Water is the solute.
  2. (b) Both water and sugar are solvents.
  3. (c) Water is the solvent.
  4. (d) Sugar is the solvent.
Answer & solution

Answer: (c) Water is the solvent.

Correct Answer: Option 3 — Water is the solvent.
A solution is a homogeneous mixture of two or more substances. It has two parts: the solvent, which is the component present in the larger amount and which dissolves the other, and the solute, which is present in the smaller amount and gets dissolved.
When sugar is stirred into water, the water is far greater in quantity and does the dissolving, so water is the solvent and sugar is the solute. The solution is homogeneous — the sugar particles (smaller than 1 nm) spread uniformly, the mixture stays transparent, shows no Tyndall effect, and cannot be separated by filtration.
Water dissolves so many substances that it is called the universal solvent. A solution in which water is the solvent is called an aqueous solution, marked (aq).
❌ Why the other options are incorrect
✗ Water is the solute — reversed; the solute is the dissolved substance present in the smaller amount, which here is sugar, not water.
✗ Both water and sugar are solvents — a solution can have only one solvent; the remaining components are solutes.
✗ Sugar is the solvent — sugar is the substance that gets dissolved and is present in the smaller quantity, so it is the solute.
📌 SSC & Railway Exam Facts
• Solution = solute + solvent; the solvent is the component present in the larger amount.
• Water is the universal solvent; its solutions are called aqueous solutions, (aq).
• Particle size: true solution < 1 nm, colloid 1–1000 nm, suspension > 1000 nm.
• A true solution is homogeneous, transparent, stable and shows no Tyndall effect.
• Concentration = (mass of solute / mass of solution) × 100 gives the mass percentage.
RRB Technician Grade III 2024-25 – 30 December 2024 Shift 3 · Homogeneous and heterogeneous mixtures

Q72. Which of the following is NOT an example of homogeneous mixture?

  1. (a) A mixture of copper sulphate in water
  2. (b) A mixture of salt in water
  3. (c) A mixture of iron nails in water
  4. (d) A mixture of sugar in water
Answer & solution

Answer: (c) A mixture of iron nails in water

Correct Answer: Option 3 — A mixture of iron nails in water
A homogeneous mixture has a uniform composition throughout; its components cannot be told apart and there is no visible boundary between them — a true solution such as salt in water is the standard example. A heterogeneous mixture, in contrast, has a non-uniform composition and its parts can be seen separately.
Iron nails dropped into water do not dissolve at all. The nails simply lie at the bottom and remain clearly visible, forming two distinct phases — solid iron and liquid water. The mixture is therefore heterogeneous, and it can be separated by simply picking out the nails or using a magnet. Since the question asks which is NOT homogeneous, this is the answer.
The other three all dissolve completely, giving true solutions whose particles are smaller than 1 nm, which are transparent, stable and show no Tyndall effect.
❌ Why the other options are incorrect
✗ A mixture of copper sulphate in water — CuSO₄ dissolves fully to give a clear blue solution; uniform throughout, hence homogeneous.
✗ A mixture of salt in water — the classic true solution; salt dissolves completely and is recovered by evaporation, not by filtration.
✗ A mixture of sugar in water — sugar dissolves fully and spreads uniformly, so it is also a homogeneous true solution.
📌 SSC & Railway Exam Facts
• Homogeneous: true solutions, alloys (brass, bronze), air; heterogeneous: suspensions, colloids, iron filings in sand.
• Particle size: true solution < 1 nm, colloid 1–1000 nm, suspension > 1000 nm.
• Iron filings are separated from a mixture using a magnet.
• A colloid looks homogeneous but is actually heterogeneous — e.g. milk.
• Tyndall effect is shown by colloids and suspensions, never by a true solution.
RRB Technician Grade III 2024-25 – 29 December 2024 Shift 1 · Suspensions and colloids

Q73. Which of the following is NOT a property of a colloid?

  1. (a) Colloid is a heterogeneous mixture.
  2. (b) Particles of colloids do not settle down when left undisturbed.
  3. (c) Colloid is a homogeneous mixture.
  4. (d) Colloids cannot be separated by filtration.
Answer & solution

Answer: (c) Colloid is a homogeneous mixture.

Correct Answer: Option 3 — Colloid is a homogeneous mixture.
This is a NOT-type question, so we must pick the false statement. A colloid is a heterogeneous mixture in which the dispersed particles measure 1–1000 nm. To the naked eye a colloid *looks* uniform, which is why students wrongly call it homogeneous — but under a powerful microscope two distinct parts are visible, the dispersed phase and the dispersion medium. Hence Option 3 is not a property of a colloid and is the required answer.
The other three statements are all genuine properties. Colloidal particles are too small to be trapped by filter paper, so ordinary filtration fails; they are also light enough to stay suspended, so they do not settle on standing — a colloid is therefore a stable mixture. They do, however, scatter a beam of light, giving the Tyndall effect.
❌ Why the other options are incorrect
✗ Colloid is a heterogeneous mixture — this is true; a colloid has two clearly separate phases, so it cannot be the answer to a NOT question.
✗ Particles of colloids do not settle down when left undisturbed — true; unlike a suspension, a colloid is stable and shows no sediment.
✗ Colloids cannot be separated by filtration — true; the particles pass through filter paper, and centrifugation must be used instead.
📌 SSC & Railway Exam Facts
• Particle size: true solution < 1 nm, colloid 1–1000 nm, suspension > 1000 nm.
• Tyndall effect is shown by colloids and suspensions, never by a true solution.
• A suspension settles and can be filtered; a colloid does neither — separate it by centrifugation.
• Colloid types: sol (paint), gel (jelly, butter), emulsion (milk), foam (shaving cream), aerosol (fog, smoke).
• Milk is an emulsion; smoke is a solid dispersed in a gas.
RRB Technician Grade III 2024-25 – 29 December 2024 Shift 3 · Solutions, solubility and concentration

Q74. Which of the following statements about solutions is correct?

  1. (a) The concentration of a solution can only be expressed in terms of mass per cent.
  2. (b) The solute is present in a smaller amount than the solvent.
  3. (c) A solution is a heterogeneous mixture.
  4. (d) Solutions can be separated using simple filtration.
Answer & solution

Answer: (b) The solute is present in a smaller amount than the solvent.

Correct Answer: Option 2 — The solute is present in a smaller amount than the solvent.
A solution is a homogeneous mixture of two or more substances. It has two components: the solute, which is the substance present in the smaller amount and which dissolves, and the solvent, the substance present in the larger amount which does the dissolving. In sugar solution, sugar is the solute and water the solvent; water is called the universal solvent.
Because the dissolved particles are smaller than 1 nm, a solution is perfectly uniform throughout, is transparent and stable, does not scatter light (no Tyndall effect) and the solute never settles down on standing.
❌ Why the other options are incorrect
✗ The concentration of a solution can only be expressed in terms of mass per cent — the word *only* makes it wrong; concentration can also be given as volume per cent, mass by volume per cent, molarity, molality, normality or ppm.
✗ A solution is a heterogeneous mixture — a true solution is homogeneous; heterogeneous mixtures are suspensions and colloids.
✗ Solutions can be separated using simple filtration — solute particles are far too small to be retained by filter paper; a solution is separated by evaporation, crystallisation or distillation.
📌 SSC & Railway Exam Facts
• Solution = solute + solvent; the solvent is the larger component.
• Particle size: true solution < 1 nm, colloid 1–1000 nm, suspension > 1000 nm.
• Mass per cent = (mass of solute ÷ mass of solution) × 100.
• A saturated solution can dissolve no more solute at that temperature; the amount dissolved in 100 g of solvent is its solubility.
• Alloys (brass, bronze, steel) are solid solutions; air is a gaseous solution; soda water is a gas dissolved in a liquid.
RRB Technician Grade III 2024-25 – 29 December 2024 Shift 3 · Homogeneous and heterogeneous mixtures

Q75. Which of the following best describes the mixture of air?

  1. (a) Homogeneous mixture
  2. (b) Colloids
  3. (c) Suspension
  4. (d) Heterogeneous mixture
Answer & solution

Answer: (a) Homogeneous mixture

Correct Answer: Option 1 — Homogeneous mixture
Air is a homogeneous mixture of several gases. Clean, dry air contains roughly 78% nitrogen, 21% oxygen, 0.9% argon and 0.03–0.04% carbon dioxide, together with traces of neon, helium, water vapour and dust.
It is called homogeneous because the gases are uniformly mixed right down to the molecular level — a sample of air taken from any part of a room has the same composition, and no boundary can be seen between the different gases. In fact air is often described as a gaseous solution, with nitrogen as the solvent (largest component) and the other gases as solutes.
Air is a mixture and not a compound because its components keep their own properties, are present in variable proportions, and can be separated by physical means — the fractional distillation of liquefied air, with no energy change involved in mixing.
❌ Why the other options are incorrect
✗ Colloids — a colloid needs particles of 1–1000 nm dispersed in a medium and shows the Tyndall effect; the gases of air are individual molecules far smaller than 1 nm. (Smoke and fog *are* colloids, but they are not pure air.)
✗ Suspension — a suspension has particles larger than 1000 nm that settle on standing and can be filtered; the gases of air do neither.
✗ Heterogeneous mixture — air has no visible boundary between its components and is uniform throughout, so it is not heterogeneous.
📌 SSC & Railway Exam Facts
• Composition of air: N₂ ≈ 78%, O₂ ≈ 21%, Ar ≈ 0.93%, CO₂ ≈ 0.03–0.04%.
• Air is a mixture, water (H₂O) is a compound — a very common PYQ pairing.
• Air's components are separated by the fractional distillation of liquid air; liquid nitrogen boils at −196 °C.
• Other homogeneous mixtures: sugar solution, salt solution, alloys (brass, bronze, steel), soda water.
• Heterogeneous mixtures: sand and water, oil and water, soil, muddy water.
RRB Technician Grade III 2024-25 – 28 December 2024 Shift 1 · Suspensions and colloids

Q76. Which of the following statements is INCORRECT for colloid?

  1. (a) Colloid is a homogeneous mixture.
  2. (b) Colloidal particles scatter a beam of light passing through it.
  3. (c) They do not settle down when left undisturbed.
  4. (d) Colloid is a heterogeneous mixture.
Answer & solution

Answer: (a) Colloid is a homogeneous mixture.

Correct Answer: Option 1 — Colloid is a homogeneous mixture.
A colloid is a mixture in which the dispersed particles measure 1–1000 nm. Although the mixture looks uniform to the naked eye, the particles are in fact big enough to form a separate phase, so a colloid is scientifically classified as a heterogeneous mixture. Calling it homogeneous is therefore the incorrect statement, and that is what the question asks for.
Every colloid has two parts — the dispersed phase (the particles) and the dispersion medium (the substance they float in). Milk, for example, is an emulsion: tiny fat droplets dispersed in water.
The other three statements are all true of colloids: they scatter light (Tyndall effect), they are stable and do not settle on standing, and they are indeed heterogeneous.
❌ Why the other options are incorrect
✗ Colloidal particles scatter a beam of light passing through it — this is true; it is the Tyndall effect, the standard test used to tell a colloid from a true solution.
✗ They do not settle down when left undisturbed — true; colloidal particles are kept suspended by Brownian motion, which is why a colloid is a stable mixture (unlike a suspension).
✗ Colloid is a heterogeneous mixture — true, and the very reason Option 1 is false; only one of these two opposite statements can be correct.
📌 SSC & Railway Exam Facts
• Particle size: true solution < 1 nm, colloid 1–1000 nm, suspension > 1000 nm.
• Tyndall effect is shown by colloids and suspensions, never by a true solution.
• Colloids cannot be separated by filtration, but can be separated by centrifugation.
• Types of colloid: sol (solid in liquid), gel (liquid in solid), emulsion (liquid in liquid), aerosol (liquid/solid in gas), foam (gas in liquid).
• Examples — milk, blood, ink, smoke, fog, cheese, butter, shaving cream are all colloids.
RRB Technician Grade III 2024-25 – 28 December 2024 Shift 1 · Solutions, solubility and concentration

Q77. Which of the following statements is true regarding a dilute solution?

  1. (a) It contains a large amount of solute dissolved in a small amount of solvent.
  2. (b) It exhibits a heterogeneous appearance.
  3. (c) It has a small amount of solute dissolved in a large amount of solvent.
  4. (d) It has an equal ratio of solute to solvent by mass.
Answer & solution

Answer: (c) It has a small amount of solute dissolved in a large amount of solvent.

Correct Answer: Option 3 — It has a small amount of solute dissolved in a large amount of solvent.
A solution is a homogeneous mixture of two or more substances. The component present in the smaller quantity is the solute and the one present in the larger quantity is the solvent — in a sugar solution, sugar is the solute and water the solvent.
Solutions are classified by how much solute they contain. A dilute solution holds only a small quantity of solute in a large quantity of solvent, while a concentrated solution holds a large quantity of solute. This comparison is the whole meaning of the word *dilute*, so Option 3 is the true statement.
The amount of solute is measured by concentration, commonly as mass percentage = (mass of solute ÷ mass of solution) × 100. A solution that holds the maximum solute it can dissolve at a given temperature is called saturated; its solubility rises with temperature for most solids.
❌ Why the other options are incorrect
✗ It contains a large amount of solute dissolved in a small amount of solvent — that describes a concentrated solution, the exact opposite of a dilute one; this is the most common trap in this question.
✗ It exhibits a heterogeneous appearance — every true solution is homogeneous and transparent; a heterogeneous appearance belongs to a suspension or a colloid.
✗ It has an equal ratio of solute to solvent by mass — no such definition exists; "dilute" and "concentrated" are relative comparisons, not fixed 1 : 1 ratios.
📌 SSC & Railway Exam Facts
• Solution = solute + solvent; the solvent is always the component in the greater amount.
• Dilute → less solute; Concentrated → more solute; Saturated → maximum solute at that temperature.
• Mass % of solute = (mass of solute ÷ mass of solution) × 100.
• A true solution is homogeneous, its particles are < 1 nm, and it shows no Tyndall effect.
• Alloys (brass, bronze, steel) are solid solutions; air is a gaseous solution.
RRB Technician Grade III 2024-25 – 28 December 2024 Shift 1 · Suspensions and colloids

Q78. Which of the following statements is true about suspension?

  1. (a) The solute particles cannot be separated from the mixture.
  2. (b) The solute particles don't settle down when a suspension is left undisturbed.
  3. (c) The particles of a suspension can be seen by the naked eye.
  4. (d) It is a homogeneous mixture.
Answer & solution

Answer: (c) The particles of a suspension can be seen by the naked eye.

Correct Answer: Option 3 — The particles of a suspension can be seen by the naked eye.
A suspension is a heterogeneous mixture in which the solute particles are larger than 1000 nm and do not dissolve — they merely remain suspended in the medium. Because these particles are so large, they are big enough to be seen with the naked eye, which makes the mixture look turbid or cloudy. That is exactly what Option 3 states.
The other characteristic tests of a suspension all follow from this large particle size: the particles scatter light (Tyndall effect), they settle down on standing under gravity, and they can be removed by ordinary filtration. A suspension is therefore an unstable mixture.
Standard examples are chalk powder in water, muddy water, paint, slaked lime used for whitewashing and a shaken medicine bottle marked "shake well before use".
❌ Why the other options are incorrect
✗ The solute particles cannot be separated from the mixture — false; suspension particles are easily separated by simple filtration or by just letting them settle and decanting.
✗ The solute particles don't settle down when a suspension is left undisturbed — false; they do settle under gravity, and this is the key difference from a colloid, whose particles stay suspended.
✗ It is a homogeneous mixture — false; a suspension is heterogeneous, with the particles forming a clearly separate phase.
📌 SSC & Railway Exam Facts
• Particle size: true solution < 1 nm, colloid 1–1000 nm, suspension > 1000 nm.
• Suspension particles are visible, settle on standing and can be filtered; colloidal particles do none of these.
• Tyndall effect is shown by suspensions and colloids, never by a true solution.
• Chalk is calcium carbonate (CaCO₃); marble and limestone have the same formula.
• A suspension is unstable, a colloid is stable, and a true solution is perfectly homogeneous.
RRB Technician Grade III 2024-25 – 28 December 2024 Shift 1 · Homogeneous and heterogeneous mixtures

Q79. Which of the following is an example of a heterogeneous solution?

  1. (a) Tincture of iodine
  2. (b) Lemonade
  3. (c) Mixture of ethanol in water
  4. (d) Mixture of oil in water
Answer & solution

Answer: (d) Mixture of oil in water

Correct Answer: Option 4 — Mixture of oil in water
A homogeneous mixture has a uniform composition throughout with no visible boundary between its parts, while a heterogeneous mixture has a non-uniform composition and shows a clear boundary of separation.
Oil and water are immiscible — oil is non-polar while water is polar, and "like dissolves like". However hard the two are shaken, the oil soon separates and floats as a distinct layer on top, because oil is less dense than water. Since two separate phases with a visible boundary are present, the mixture is heterogeneous.
The other three choices are all true solutions — uniform, transparent, with particles smaller than 1 nm that never settle and cannot be filtered out.
❌ Why the other options are incorrect
✗ Tincture of iodine — a homogeneous solution of iodine dissolved in alcohol; a solid-in-liquid solution used as an antiseptic.
✗ Lemonade — a homogeneous solution of sugar, salt and lemon juice in water; the dissolved substances are uniformly spread and invisible.
✗ Mixture of ethanol in water — ethanol and water are completely miscible in all proportions (both are polar and form hydrogen bonds), giving a single uniform liquid.
📌 SSC & Railway Exam Facts
• Homogeneous: true solutions, alloys, air. Heterogeneous: suspensions, colloids, oil + water, sand + water.
• Tincture of iodine = iodine in alcohol; aerated drink = CO₂ gas in water; air = gas in gas.
• Immiscible liquids (oil–water, kerosene–water) are separated using a separating funnel.
• "Like dissolves like" — polar solutes dissolve in polar solvents, non-polar in non-polar.
• Oil floats on water because its density is lower; a soap or detergent forms an emulsion between the two.
RRB Technician Grade III 2024-25 – 28 December 2024 Shift 1 · Homogeneous and heterogeneous mixtures

Q80. Which of the following is NOT an example of a homogeneous mixture?

  1. (a) Orange juice with pulp
  2. (b) Sugar solution
  3. (c) Vinegar
  4. (d) Pure Air
Answer & solution

Answer: (a) Orange juice with pulp

Correct Answer: Option 1 — Orange juice with pulp
A homogeneous mixture has the same composition and properties throughout, and its components cannot be distinguished by the eye. A heterogeneous mixture does not — its components form visibly separate parts.
Orange juice with pulp contains solid pulp particles that are clearly visible, do not dissolve in the juice, and settle to the bottom if the glass is left standing. Two distinct phases are present, so it is a heterogeneous mixture (a suspension) — hence the odd one out.
The other three are all homogeneous: their components are mixed at the molecular level and no boundary can be seen.
❌ Why the other options are incorrect
✗ Sugar solution — a classic true solution; sugar dissolves completely in water, giving particles < 1 nm that never settle, so it is homogeneous.
✗ Vinegar — a homogeneous solution of about 5–8% acetic acid (ethanoic acid, CH₃COOH) in water; it is uniform and transparent throughout.
✗ Pure Air — a homogeneous mixture of gases (N₂ ≈ 78%, O₂ ≈ 21%, Ar ≈ 0.93%, CO₂ ≈ 0.04%); pure, dust-free air has a uniform composition. (Air carrying smoke or dust would be heterogeneous — hence the word "Pure".)
📌 SSC & Railway Exam Facts
• Homogeneous: sugar/salt solution, vinegar, air, alloys (brass, bronze, steel).
• Heterogeneous: pulpy juice, muddy water, sand + salt, oil + water, milk and other colloids.
• Vinegar = dilute ethanoic acid, CH₃COOH; glacial acetic acid freezes at about 17 °C.
• Composition of air: nitrogen ≈ 78%, oxygen ≈ 21%, argon ≈ 0.93%, carbon dioxide ≈ 0.04%.
• All solutions are homogeneous, but a mixture may be either homogeneous or heterogeneous.
RRB Technician Grade III 2024-25 – 28 December 2024 Shift 2 · Solutions, solubility and concentration

Q81. Which of the following is NOT a characteristic of a solution?

  1. (a) The particles of a solution are smaller than 1 nm (10⁻⁹ metre) in diameter.
  2. (b) A solution is a homogenous mixture.
  3. (c) Because of very small particle size, particles do not scatter a beam of light passing through the solution.
  4. (d) The solute particles can be separated from the mixture by the process of filtration.
Answer & solution

Answer: (d) The solute particles can be separated from the mixture by the process of filtration.

Correct Answer: Option 4 — The solute particles can be separated from the mixture by the process of filtration.
A true solution is a homogeneous mixture of two or more substances, made of a solute dissolved in a solvent. Its particles are smaller than 1 nm (10⁻⁹ m), which is far smaller than the pores of a filter paper.
Because of this extremely small size, the solute particles pass straight through filter paper and therefore cannot be separated by filtration. They also never settle down on standing, and they do not scatter light, so a true solution does not show the Tyndall effect and looks perfectly transparent.
A solute is recovered from a solution by evaporation, crystallisation or distillation, not by filtration. Hence statement 4 is false and is the required answer.
❌ Why the other options are incorrect
✗ The particles of a solution are smaller than 1 nm — true; this is the defining size limit of a true solution (colloid 1–1000 nm, suspension > 1000 nm).
✗ A solution is a homogenous mixture — true; the composition is uniform throughout and the solute cannot be seen as a separate phase.
✗ Because of very small particle size, particles do not scatter a beam of light — true; this is exactly why a true solution shows no Tyndall effect.
📌 SSC & Railway Exam Facts
• Solution = solute + solvent; water is the universal solvent.
• Particle size: true solution < 1 nm, colloid 1–1000 nm, suspension > 1000 nm.
• Tyndall effect is shown by colloids and suspensions, never by a true solution.
• Filtration separates a suspension; centrifugation separates a colloid; evaporation/crystallisation recovers a dissolved solute.
• Alloys (brass, bronze, steel) are solid solutions; air is a gaseous solution.
RRB Technician Grade III 2024-25 – 28 December 2024 Shift 3 · Suspensions and colloids

Q82. Which of the following statements about a colloidal solution is INCORRECT?

  1. (a) Colloidal solutions are a heterogeneous mixture.
  2. (b) Milk is not a colloidal solution.
  3. (c) Fog is an example of a colloidal solution.
  4. (d) Components cannot be separated by the process of filtration.
Answer & solution

Answer: (b) Milk is not a colloidal solution.

Correct Answer: Option 2 — Milk is not a colloidal solution.
Milk is one of the best-known colloids. It is an emulsion — tiny droplets of liquid fat (the dispersed phase) spread through water (the dispersion medium). The fat droplets are 1–1000 nm across, so they make milk look opaque and white, they scatter light (Tyndall effect), and they do not settle on standing. Saying that milk is not a colloidal solution is therefore false, which is exactly what the question asks for.
The cream of milk can be separated only by centrifugation, never by filtration — a further proof that milk is a colloid and not a suspension.
❌ Why the other options are incorrect
✗ Colloidal solutions are a heterogeneous mixture — true; although a colloid looks uniform, its particles form a separate phase, so it is classified as heterogeneous.
✗ Fog is an example of a colloidal solution — true; fog is an aerosol, with tiny water droplets dispersed in air (liquid in gas).
✗ Components cannot be separated by the process of filtration — true; colloidal particles are far too small to be held back by filter paper and pass straight through, so centrifugation must be used.
📌 SSC & Railway Exam Facts
• Milk = emulsion (liquid in liquid); fog/cloud = aerosol (liquid in gas); smoke = solid in gas.
• Butter and cheese = gel (liquid in solid); shaving/whipped cream = foam (gas in liquid); ink and blood = sol.
• Particle size: true solution < 1 nm, colloid 1–1000 nm, suspension > 1000 nm.
• Colloids are separated by centrifugation, suspensions by filtration.
• Tyndall effect is shown by colloids and suspensions; a true solution never shows it.
RRB Technician Grade III 2024-25 – 28 December 2024 Shift 3 · Suspensions and colloids

Q83. Which of the following shows Tyndall effect?

  1. (a) Colloids
  2. (b) Gases
  3. (c) Solutions
  4. (d) Metals
Answer & solution

Answer: (a) Colloids

Correct Answer: Option 1 — Colloids
The Tyndall effect is the scattering of a beam of light by the particles of a mixture, which makes the path of the beam visible. A mixture can show it only if its particles are large enough to intercept light, that is 1 nm and above.
Colloidal particles measure 1–1000 nm, comparable to the wavelength of visible light (400–700 nm), so they scatter the beam strongly and the path of light can be seen from the side. This is the standard laboratory test used to distinguish a colloid from a true solution. Suspensions, whose particles are even bigger, show it as well.
Familiar sights: a beam of sunlight in a dusty room, car head-lights in fog, the light of a cinema projector in a smoky hall, and sunlight filtering through the mist in a dense forest.
❌ Why the other options are incorrect
✗ Gases — a pure gas is homogeneous and its molecules are far smaller than 1 nm, so it does not scatter a beam; only a gas carrying dust or water droplets (which is then a colloid — smoke or fog) appears to do so.
✗ Solutions — in a true solution the particles are smaller than 1 nm; they cannot scatter light, so a true solution shows no Tyndall effect and stays perfectly transparent.
✗ Metals — metals are pure substances (elements), not mixtures with dispersed particles; light is reflected from a polished metal surface, which is a different phenomenon.
📌 SSC & Railway Exam Facts
• Tyndall effect = scattering of light by colloidal (and suspension) particles; named after John Tyndall.
• Colloid → shows it; true solution → never shows it. This is the standard test to tell the two apart.
• Particle size: true solution < 1 nm, colloid 1–1000 nm, suspension > 1000 nm.
• Scattering also makes the sky blue and the setting Sun red (Rayleigh scattering ∝ 1/λ⁴).
• Milk, fog, smoke, ink, blood, cheese, butter are everyday colloids; all show the Tyndall effect.
RRB Technician Grade III 2024-25 – 27 December 2024 Shift 1 · Elements and compounds

Q84. Which of the following statements is true about a compound?

  1. (a) The new product formed has totally different properties.
  2. (b) The new product formed has similar properties to the constituent substance.
  3. (c) It is a physical change.
  4. (d) The constituents can be separated by physical process.
Answer & solution

Answer: (a) The new product formed has totally different properties.

Correct Answer: Option 1 — The new product formed has totally different properties.
A compound is a pure substance formed when two or more elements combine chemically in a fixed proportion by mass. Because a genuine chemical bond is made, the compound is a brand-new substance whose properties are completely different from those of the elements that formed it.
The classic illustration is iron + sulphur. A mere mixture of iron filings and sulphur powder still shows both sets of properties — the iron is still attracted by a magnet and still dissolves in dilute acid. But on heating the two combine to give iron sulphide (FeS), a compound that is not attracted by a magnet at all and behaves quite differently.
Compound formation is therefore a chemical change, it is accompanied by a change in energy (heat or light), and its constituents can be separated only by chemical means (electrolysis, heating) — never by filtration or by using a magnet.
❌ Why the other options are incorrect
✗ The new product formed has similar properties to the constituent substance — that is true of a mixture, not a compound; in a mixture every component keeps its own properties.
✗ It is a physical change — the formation of a compound is a chemical change, because new bonds are made and a new substance appears.
✗ The constituents can be separated by physical process — only a mixture can be separated physically, by filtration, distillation, magnet, sublimation or chromatography.
📌 SSC & Railway Exam Facts
• Elements — made of one kind of atom; Compounds — elements combined in a fixed ratio; Mixtures — any ratio, no chemical bond.
• Water (H₂O) is a compound of H and O in a fixed 1 : 8 ratio by mass, yet neither H nor O puts out a fire the way water does.
• Air is a mixture, brass is a mixture (alloy), but common salt and water are compounds.
• A mixture is either homogeneous (solution) or heterogeneous (suspension, colloid).
• Law of constant proportions (Proust) — a compound always contains the same elements in the same fixed ratio by mass.
RRB Technician Grade III 2024-25 – 27 December 2024 Shift 1 · Solutions, solubility and concentration

Q85. While dissolving solute in a solution, when no more solute can be dissolved in a solution at a given temperature, it is called ________.

  1. (a) a saturated solution
  2. (b) an unsaturated solution
  3. (c) a suspension
  4. (d) a colloidal solution
Answer & solution

Answer: (a) a saturated solution

Correct Answer: Option 1 — a saturated solution
A saturated solution is a solution in which no more solute can be dissolved at a given temperature. Once this point is reached, any further solute added simply lies at the bottom as undissolved solid.
The quantity of solute needed to saturate 100 g of solvent at a stated temperature is called the solubility of that solute. Solubility must always be quoted at a particular temperature, because heating a solution generally lets it hold more solute.
For most solids the solubility increases with temperature, so a saturated solution becomes unsaturated on heating; when it is cooled again, the surplus solute separates out as crystals — the principle behind the purification technique of crystallisation. For gases, in contrast, solubility decreases as temperature rises and increases with pressure.
❌ Why the other options are incorrect
✗ an unsaturated solution — exactly the opposite; it is a solution that can still dissolve more solute at that temperature.
✗ a suspension — a heterogeneous mixture whose solid particles never dissolve at all; they stay visible and settle down on standing.
✗ a colloidal solution — a heterogeneous mixture with particles of 1–100 nm that show the Tyndall effect; it is not a question of dissolving a solute to its limit.
📌 SSC & Railway Exam Facts
• Solubility = mass of solute in grams that saturates 100 g of solvent at a given temperature.
• Solution = solute + solvent; the solvent is the component present in the greater amount.
• Concentration = (mass of solute / mass of solution) × 100 for mass percentage.
• Gases dissolve better at low temperature and high pressure — Henry's law; that is why a cold soft drink fizzes when opened.
• Crystallisation is used to purify alum, copper sulphate and salt obtained from sea water.
RRB Technician Grade III 2024-25 – 27 December 2024 Shift 1 · Solutions, solubility and concentration

Q86. Which of the following statements is NOT true about a solution?

  1. (a) Particles of the solution can be seen by the naked eye.
  2. (b) The solute particles cannot be separated by filtration.
  3. (c) A solution is a homogeneous mixture.
  4. (d) The solute particles do not settle down when left undisturbed.
Answer & solution

Answer: (a) Particles of the solution can be seen by the naked eye.

Correct Answer: Option 1 — Particles of the solution can be seen by the naked eye.
A true solution is a homogeneous mixture of two or more substances, in which the solute particles are smaller than 1 nm (10⁻⁹ m). Particles that small are far too tiny to be seen with the naked eye, or even with a microscope. That is why the statement in Option 1 is not true, and it is the required answer.
Because of this extremely small particle size, a true solution also does not scatter a beam of light — it shows no Tyndall effect — and it is completely stable.
Sugar or salt dissolved in water, air, brass and tincture iodine are common examples of true solutions.
❌ Why the other options are incorrect (they ARE true)
✗ The solute particles cannot be separated by filtration — true; the particles are so small that they pass straight through filter paper. A solution is separated by evaporation, distillation or crystallisation.
✗ A solution is a homogeneous mixture — true; its composition is uniform throughout, and no boundary can be seen between solute and solvent.
✗ The solute particles do not settle down when left undisturbed — true; a true solution is stable and never shows sedimentation, unlike a suspension.
📌 SSC & Railway Exam Facts
• Particle size: solution < 1 nm, colloid 1–100 nm, suspension > 100 nm.
• Solution — homogeneous, stable, no Tyndall effect, passes through filter paper.
• Colloid — appears homogeneous but is heterogeneous, stable, shows Tyndall effect, separated by centrifugation.
• Suspension — heterogeneous, unstable, shows Tyndall effect, separated by filtration.
• Alloys such as brass (Cu + Zn) are solid solutions, and air is a gaseous solution.
RRB Technician Grade III 2024-25 – 27 December 2024 Shift 1 · Suspensions and colloids

Q87. Which of the following is the dispersed phase of automobile exhaust?

  1. (a) Liquid phase
  2. (b) Plasma phase
  3. (c) Gaseous phase
  4. (d) Solid phase
Answer & solution

Answer: (d) Solid phase

Correct Answer: Option 4 — Solid phase
Every colloid has two parts: the dispersed phase, which is the substance present in small quantity and spread through the mixture, and the dispersion medium, which is the substance in which it is spread.
Automobile exhaust is a form of smoke. Smoke consists of fine solid particles — carbon (soot), unburnt hydrocarbons and metallic particulates — suspended in air. Therefore:
• Dispersed phase = solid (the carbon and particulate matter)
• Dispersion medium = gas (the air / exhaust gases)
• This type of colloid is called an aerosol.
That is why the answer is the solid phase. These suspended solid particles are the particulate matter (PM₂.₅ and PM₁₀) that causes serious respiratory disease and smog in cities.
❌ Why the other options are incorrect
✗ Liquid phase — a liquid dispersed in a gas gives fog, mist and clouds, not the sooty smoke of an exhaust pipe.
✗ Plasma phase — plasma is the ionised fourth state of matter found in stars and fluorescent tubes; it is never the dispersed phase of a colloid.
✗ Gaseous phase — a gas dispersed in a gas would simply form a homogeneous mixture like air, not a colloid at all, since gases mix completely.
📌 SSC & Railway Exam Facts
• Aerosol — solid or liquid in gas: smoke (solid in gas), fog, mist, clouds (liquid in gas).
• Foam — gas in liquid (shaving cream); solid foam — gas in solid (sponge, pumice).
• Emulsion — liquid in liquid (milk, cream); Gel — liquid in solid (cheese, butter, jelly).
• Sol — solid in liquid (ink, paint, muddy water, starch solution).
• Smoke is removed from factory chimneys by an electrostatic precipitator (Cottrell precipitator).
RRB Technician Grade III 2024-25 – 26 December 2024 Shift 1 · Suspensions and colloids

Q88. From the four possibilities, suspension has which property?

  1. (a) Heterogeneous mixture
  2. (b) Solid mixture
  3. (c) Plasma mixture
  4. (d) Homogenous mixture
Answer & solution

Answer: (a) Heterogeneous mixture

Correct Answer: Option 1 — Heterogeneous mixture
A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium. Its particles are larger than 1000 nm (1 μm), big enough to be seen with the naked eye.
Key properties of a suspension:
• It is heterogeneous — the particles and the medium form two visible phases.
• The particles settle down when left undisturbed, so it is unstable.
• The particles are retained by filter paper, so a suspension can be separated by simple filtration.
• It scatters a beam of light, showing the Tyndall effect.
Common examples: muddy water, chalk powder in water, paints, slaked lime in water and a flour–water mixture.
❌ Why the other options are incorrect
✗ Solid mixture — a suspension always needs a liquid or gaseous medium; a mixture of two solids is not a suspension.
✗ Plasma mixture — plasma is the fourth state of matter, a super-energetic ionised gas found in stars and fluorescent tubes; it has nothing to do with suspensions.
✗ Homogenous mixture — a homogeneous mixture is a true solution, whose particles are smaller than 1 nm and never settle; a suspension is the opposite.
📌 SSC & Railway Exam Facts
• Particle size: true solution < 1 nm, colloid 1–1000 nm, suspension > 1000 nm.
• Solution — homogeneous, stable, no Tyndall effect; colloid and suspension — heterogeneous, show Tyndall effect.
• A suspension is separated by filtration; a colloid needs centrifugation.
• States of matter: solid, liquid, gas, plasma and Bose–Einstein condensate (BEC).
• Milk, blood, fog, smoke are colloids, not suspensions.
RRB Technician Grade III 2024-25 – 26 December 2024 Shift 3 · Suspensions and colloids

Q89. Which of the following statements is true regarding a suspension?

  1. (a) Particles are smaller than those in a solution.
  2. (b) Particles cannot be separated by filtration.
  3. (c) Particles settle down after some time.
  4. (d) Particles do not scatter light.
Answer & solution

Answer: (c) Particles settle down after some time.

Correct Answer: Option 3 — Particles settle down after some time.
A suspension is a heterogeneous mixture in which the solute particles are large enough to be seen with the naked eye — bigger than 100 nm (10⁻⁷ m) — and remain merely dispersed, not dissolved, in the liquid.
Because these particles are heavy, gravity pulls them down and, once the mixture is left undisturbed, they settle at the bottom as a separate layer. This settling is called sedimentation, and it is the single most characteristic property of a suspension. Chalk powder or sand shaken with water, muddy river water, and a bottle of paint are the standard examples.
Suspension particles are large enough to be retained by filter paper, and they do scatter a beam of light, making its path visible — the Tyndall effect.
❌ Why the other options are incorrect
✗ Particles are smaller than those in a solution — the reverse is true. Solution particles are less than 1 nm, colloid particles 1–100 nm, and suspension particles greater than 100 nm, the largest of the three.
✗ Particles cannot be separated by filtration — suspension particles are the only ones that are removed by ordinary filter paper; it is solution particles that pass straight through.
✗ Particles do not scatter light — suspensions do scatter light and show the Tyndall effect; it is a true solution that does not scatter light.
📌 SSC & Railway Exam Facts
• Particle size: true solution < 1 nm, colloid 1–100 nm, suspension > 100 nm.
• Solution — homogeneous, stable, no Tyndall effect; colloid — heterogeneous, stable, shows Tyndall effect; suspension — heterogeneous, unstable, shows Tyndall effect.
• Colloids are separated by centrifugation, suspensions by filtration.
• The Tyndall effect explains the visible beam of a torch in a misty forest and light entering a dusty room.
• Milk is an emulsion (colloid), muddy water is a suspension, and sugar in water is a true solution.
RRB Technician Grade III 2024-25 – 26 December 2024 Shift 3 · Solutions, solubility and concentration

Q90. What is the term used to describe a solution that cannot dissolve any more solute at a given temperature?

  1. (a) Dilute solution
  2. (b) Unsaturated solution
  3. (c) Heterogeneous solution
  4. (d) Saturated solution
Answer & solution

Answer: (d) Saturated solution

Correct Answer: Option 4 — Saturated solution
A saturated solution is one in which no more solute can be dissolved at a given temperature. Any extra solute added simply settles at the bottom as undissolved solid.
The amount of solute needed to saturate 100 g of the solvent at that temperature is called the solubility of the solute. Solubility is therefore a property that is always quoted at a stated temperature, because it changes with temperature.
For most solids the solubility increases on heating — a saturated solution becomes unsaturated when warmed, and more solute can then be dissolved. On cooling the solution back down, the excess solute separates out as crystals, which is exactly how the technique of crystallisation purifies a solid. For gases, however, solubility decreases with a rise in temperature.
❌ Why the other options are incorrect
✗ Dilute solution — this only means the solution contains a relatively small quantity of solute; it can still dissolve a great deal more.
✗ Unsaturated solution — this is the exact opposite: a solution that can still dissolve more solute at that temperature.
✗ Heterogeneous solution — a contradiction in terms. A true solution is always homogeneous; heterogeneous mixtures are suspensions and colloids.
📌 SSC & Railway Exam Facts
• Solubility = grams of solute that saturate 100 g of solvent at a given temperature.
• Solution = solute + solvent; the solvent is present in the larger amount.
• Solubility of solids increases with temperature; solubility of gases decreases with temperature and increases with pressure.
• Fizz escapes from a cold drink bottle when opened because lowering the pressure lowers CO₂ solubility — Henry's law.
• Crystallisation is used to purify copper sulphate, alum and common salt.
RRB Technician Grade III 2024-25 – 26 December 2024 Shift 3 · Suspensions and colloids

Q91. Which of the following is a suspension?

  1. (a) Solids dispersed in solids
  2. (b) Gases dispersed in solids
  3. (c) Gases dispersed in liquids
  4. (d) Solids dispersed in liquids
Answer & solution

Answer: (d) Solids dispersed in liquids

Correct Answer: Option 4 — Solids dispersed in liquids
A suspension is a heterogeneous mixture in which insoluble solid particles are spread throughout a liquid without dissolving. The solid is the dispersed phase and the liquid is the dispersion medium, and the particles are larger than 100 nm, big enough to be seen and to settle out on standing.
Everyday examples are chalk powder in water, sand in water, muddy river water, paint, and a bottle of medicine marked "shake well before use" — the instruction itself exists because the solid keeps settling.
Suspension particles scatter light (Tyndall effect), are retained by filter paper, and the mixture is unstable.
❌ Why the other options are incorrect
✗ Solids dispersed in solids — this is a solid sol colloid, as in coloured gemstones and alloys; it is not a suspension.
✗ Gases dispersed in solids — this is a solid foam, as in sponge, foam rubber and pumice stone.
✗ Gases dispersed in liquids — this is a foam, as in soap lather, shaving cream and whipped cream.
📌 SSC & Railway Exam Facts
• Colloid types: sol (solid in liquid — ink, paints), gel (liquid in solid — cheese, butter, jelly), emulsion (liquid in liquid — milk, cream), aerosol (solid/liquid in gas — smoke, fog, clouds), foam (gas in liquid — shaving cream).
• Smoke = solid in gas; fog and clouds = liquid in gas; milk = liquid in liquid.
• Suspension particles settle on standing; colloid particles do not settle.
• Suspensions are separated by filtration or sedimentation and decantation.
• Butter and cheese are gels; milk and cream are emulsions.
RRB Technician Grade III 2024-25 – 24 December 2024 Shift 1 · Suspensions and colloids

Q92. Which of the following is the dispersed phase of butter?

  1. (a) Only solid
  2. (b) Both solid and liquid
  3. (c) Only liquid
  4. (d) Only gas
Answer & solution

Answer: (c) Only liquid

Correct Answer: Option 3 — Only liquid
Every colloid has two parts: the dispersed phase (the substance present in small particles, like the solute) and the dispersing medium (the substance in which it is spread, like the solvent). Colloids are named from the physical states of these two parts.
Butter is a gel — a liquid dispersed in a solid. Tiny droplets of water (liquid) are trapped inside a continuous network of solid fat. Therefore the dispersed phase of butter is a liquid and its dispersing medium is a solid. Cheese and table jelly belong to the same class.
❌ Why the other options are incorrect
✗ Only solid — a solid dispersed in a liquid gives a sol (paint, ink, blood); butter is not a sol.
✗ Both solid and liquid — a colloid has only one dispersed phase, never two at the same time.
✗ Only gas — a gas dispersed in a liquid gives a foam (shaving cream, soap lather), and a gas in a solid gives a solid foam (sponge, pumice stone).
📌 SSC & Railway Exam Facts
• Butter, cheese, jelly = gel → liquid in solid.
• Milk, face cream = emulsion → liquid in liquid; paint, ink, blood = sol → solid in liquid.
• Shaving cream, whipped cream = foam → gas in liquid; fog, mist, clouds = aerosol → liquid in gas.
• Smoke = aerosol → solid in gas; sponge, pumice = solid foam → gas in solid.
• Colloid particle size 1–1000 nm; they show the Tyndall effect and are separated by centrifugation.
RRB Technician Grade III 2024-25 – 24 December 2024 Shift 2 · Separation techniques (distillation, chromatography, centrifugation, crystallisation)

Q93. Which technique is useful for the separation of colloidal particles?

  1. (a) Sublimation technique
  2. (b) Filtration technique
  3. (c) Centrifugation technique
  4. (d) Recrystallisation technique
Answer & solution

Answer: (c) Centrifugation technique

Correct Answer: Option 3 — Centrifugation technique
In a colloidal solution the particle size lies between 1 nm and 1000 nm. Such particles are too small to be held back by filter paper and they never settle down under gravity, so ordinary filtration and sedimentation both fail.
They are separated by centrifugation. The mixture is placed in a centrifuging machine and rotated at a very high speed. The denser colloidal particles are thrown outwards and collect at the bottom of the tube, while the lighter dispersion medium stays above and is decanted off.
Everyday and laboratory uses: the milk cream separator, the spin-drier of a washing machine, separating butter from cream, and testing blood and urine samples in pathology laboratories.
❌ Why the other options are incorrect
✗ Sublimation technique — used to separate a solid that sublimes (camphor, naphthalene, NH₄Cl, iodine) from a non-subliming solid; colloidal particles do not sublime.
✗ Filtration technique — the pores of filter paper are much larger than colloidal particles, so the whole colloid simply passes through; filtration works for a suspension, not a colloid.
✗ Recrystallisation technique — purifies an impure solid by dissolving it and letting pure crystals form again; it has nothing to do with colloids.
📌 SSC & Railway Exam Facts
• Particle size: true solution < 1 nm, colloid 1–1000 nm, suspension > 1000 nm.
• A colloid scatters a beam of light — the Tyndall effect; a true solution does not.
• A colloid is heterogeneous but looks homogeneous; a suspension settles on standing.
• Milk, blood, fog, smoke, ink, butter, cheese are common colloids.
• Centrifuge works on the principle that denser particles are pushed outward during fast rotation.
RRB Technician Grade III 2024-25 – 24 December 2024 Shift 2 · Suspensions and colloids

Q94. Which of the following is NOT a property of a colloidal solution?

  1. (a) The components of a colloidal solution are only the dispersed phase and the dispersion medium.
  2. (b) The size of the particles of a colloid is very large.
  3. (c) A colloid is a heterogeneous mixture.
  4. (d) A colloid is big enough to scatter a beam of light passing through it and make its path visible.
Answer & solution

Answer: (b) The size of the particles of a colloid is very large.

Correct Answer: Option 2 — The size of the particles of a colloid is very large.
In a colloidal solution the particles of the dispersed phase are of size 1 nm to 1000 nm. Such particles are too small to be seen with the naked eye and they pass straight through ordinary filter paper, which is why a colloid looks homogeneous to the eye.
So describing colloidal particles as "very large" is wrong — that description belongs to a suspension, whose particles are bigger than 1000 nm, are visible to the eye, are retained by filter paper and settle down on standing. Colloidal particles, in contrast, are stable and never settle.
❌ Why the other options are incorrect
✗ The components of a colloidal solution are only the dispersed phase and the dispersion medium. — correct; the solute-like part is the dispersed phase and the solvent-like part is the dispersion medium.
✗ A colloid is a heterogeneous mixture. — correct; a colloid is heterogeneous, though it appears homogeneous to the naked eye.
✗ A colloid is big enough to scatter a beam of light passing through it and make its path visible. — correct; this scattering is the Tyndall effect, the standard test for a colloid.
📌 SSC & Railway Exam Facts
• Particle size: true solution < 1 nm, colloid 1–1000 nm, suspension > 1000 nm.
• Milk, blood, fog, smoke, ink, cheese, butter, shaving cream are common colloids.
• Milk = liquid fat in water → emulsion; fog = liquid in gas → aerosol; smoke = solid in gas → aerosol.
• A colloid cannot be separated by filtration but can be by centrifugation.
• The Tyndall effect is seen when sunlight enters a dark room through a small hole, or through the canopy of a dense forest.
RRB Technician Grade III 2024-25 – 24 December 2024 Shift 3 · Homogeneous and heterogeneous mixtures

Q95. Which of the following statements is correct for a heterogeneous mixture?

  1. (a) A clear solution is called a heterogeneous mixture.
  2. (b) The composition of the mixture is uniform.
  3. (c) The composition of the mixture is non-uniform.
  4. (d) Mixtures don't contain physically distinct parts.
Answer & solution

Answer: (c) The composition of the mixture is non-uniform.

Correct Answer: Option 3 — The composition of the mixture is non-uniform.
A heterogeneous mixture is one whose composition is not uniform throughout. Different portions of the mixture have different properties, and the components can usually be seen as physically distinct parts, separated by a visible boundary.
Common examples: sand and salt, oil and water, iron filings and sulphur powder, soil, muddy water and a suspension of chalk in water. Such mixtures can normally be separated by simple physical methods like handpicking, sieving, filtration, magnetic separation or a separating funnel.
In contrast, a homogeneous mixture — a true solution such as salt in water, sugar syrup or brass — has a uniform composition throughout and shows no visible boundary between its parts.
❌ Why the other options are incorrect
✗ A clear solution is called a heterogeneous mixture. — a clear solution is homogeneous; its particles are smaller than 1 nm and never settle.
✗ The composition of the mixture is uniform. — uniform composition is the defining property of a homogeneous mixture, the exact opposite of heterogeneous.
✗ Mixtures don't contain physically distinct parts. — heterogeneous mixtures do contain physically distinct parts; that is exactly how they are recognised.
📌 SSC & Railway Exam Facts
• Homogeneous — uniform composition (salt solution, air, brass, alloys); Heterogeneous — non-uniform (oil + water, sand + salt, soil).
• Colloids and suspensions are heterogeneous; a true solution is homogeneous.
• Particle size: solution < 1 nm, colloid 1–1000 nm, suspension > 1000 nm.
• Mixtures have no fixed composition; compounds have a fixed ratio by mass.
• Components of a mixture keep their own properties and are separated by physical methods.
RRB Technician Grade III 2024-25 – 23 December 2024 Shift 1 · Pure substances and mixtures

Q96. Which of the following is an example of a mixture?

  1. (a) Air
  2. (b) Water (H₂O)
  3. (c) Salt (NaCl)
  4. (d) Iron (Fe)
Answer & solution

Answer: (a) Air

Correct Answer: Option 1 — Air
A mixture contains two or more substances mixed in any proportion, with no chemical bond between them. Each component keeps its own properties, and the components can be separated by physical methods such as filtration, distillation or evaporation.
Air is the standard example of a homogeneous mixture of gases — roughly 78% nitrogen, 21% oxygen, about 0.93% argon and 0.03–0.04% carbon dioxide, together with water vapour, dust and traces of other gases. These are not chemically combined: the proportion varies from place to place, each gas keeps its own properties, and the gases are separated industrially by the fractional distillation of liquid air.
❌ Why the other options are incorrect
✗ Water (H₂O) — a compound: hydrogen and oxygen are chemically combined in a fixed ratio (2 : 1 by atoms, 1 : 8 by mass) and can be separated only by electrolysis, a chemical method.
✗ Salt (NaCl) — also a compound, formed by an ionic bond between Na⁺ and Cl⁻ in a fixed ratio; it shows neither the properties of sodium metal nor those of chlorine gas.
✗ Iron (Fe) — a pure element, made of only one kind of atom; an element can be neither a mixture nor a compound.
📌 SSC & Railway Exam Facts
• Matter is classified into pure substances (elements and compounds) and mixtures (homogeneous and heterogeneous).
• Air is a homogeneous mixture: N₂ ≈ 78%, O₂ ≈ 21%, Ar ≈ 0.93%, CO₂ ≈ 0.03–0.04%.
• A mixture's components keep their own properties and are separated physically; a compound needs chemical separation.
• Brass (Cu + Zn), bronze (Cu + Sn) and steel (Fe + C) are alloys, i.e. homogeneous mixtures.
• Liquid air is separated by fractional distillation; the most abundant gas in the atmosphere is nitrogen.
RRB Technician Grade III 2024-25 – 23 December 2024 Shift 2 · Pure substances and mixtures

Q97. Consider the following statements about a mixture and determine which of the statements is/are correct.
1. A mixture has a variable composition.
2. In compounds, the composition of each new substance is always fixed.
3. A mixture shows the properties of the constituent substances.

  1. (a) Only 2 and 3
  2. (b) 1, 2 and 3
  3. (c) Only 1
  4. (d) Only 1 and 2
Answer & solution

Answer: (b) 1, 2 and 3

Correct Answer: Option 2 — 1, 2 and 3
A mixture is formed when two or more substances are simply mixed together in any proportion without any chemical reaction. A compound is formed when elements combine chemically in a fixed ratio by mass.
Statement 1 is correct — the constituents of a mixture can be taken in any ratio, so its composition is variable. Sugar solution may be weak or strong; air contains varying amounts of water vapour.
Statement 2 is correct — this is the Law of Constant Proportions (Proust's Law). Water is always H₂O, i.e. H : O = 1 : 8 by mass, whatever its source.
Statement 3 is correct — since no new substance is produced, every component of a mixture retains its own properties. In a mixture of iron filings and sulphur, the iron is still attracted by a magnet; in iron sulphide (FeS), formed by heating them, it is not.
❌ Why the other options are incorrect
✗ Only 2 and 3 — it wrongly rejects statement 1, yet variable composition is the very definition of a mixture.
✗ Only 1 — it rejects the Law of Constant Proportions and the retention of properties, both of which are true.
✗ Only 1 and 2 — it wrongly rejects statement 3; a mixture certainly shows its constituents' properties.
📌 SSC & Railway Exam Facts
• Mixture: variable composition, no energy change, separable by physical methods.
• Compound: fixed composition, energy change occurs, separable only by chemical methods.
• Law of Constant Proportions was given by Joseph Proust.
• Air is a homogeneous mixture; water (H₂O) is a compound — a classic exam pair.
• A homogeneous mixture of two or more metals is an alloy, e.g. brass = Cu + Zn.
RRB Technician Grade III 2024-25 – 23 December 2024 Shift 2 · Solutions, solubility and concentration

Q98. Which of the following expressions gives the correct formula for mass by mass percentage of a solution?

  1. (a) ((Mass of solute)/(Volume of solution)) × 100
  2. (b) ((Mass of solvent)/(Mass of solution)) × 100
  3. (c) ((Volume of solute)/(Mass of solution)) × 100
  4. (d) ((Mass of solute)/(Mass of solution)) × 100
Answer & solution

Answer: (d) ((Mass of solute)/(Mass of solution)) × 100

Correct Answer: Option 4 — ((Mass of solute)/(Mass of solution)) × 100
A solution is a homogeneous mixture of a solute (the dissolved substance, present in smaller amount) and a solvent (the dissolving medium, present in larger amount). Concentration tells us how much solute is present in a given amount of solution.
The mass by mass percentage is defined as
Mass % = (Mass of solute / Mass of solution) × 100, where Mass of solution = Mass of solute + Mass of solvent.
For example, dissolving 20 g of salt in 80 g of water gives a solution of mass 100 g, so the concentration is (20/100) × 100 = 20%. Notice that both quantities in the ratio are masses — that is what makes it *mass by mass*. Similarly, volume by volume % = (Volume of solute / Volume of solution) × 100 and mass by volume % = (Mass of solute / Volume of solution) × 100.
❌ Why the other options are incorrect
✗ ((Mass of solute)/(Volume of solution)) × 100 — this is mass by volume percentage, a different expression of concentration.
✗ ((Mass of solvent)/(Mass of solution)) × 100 — it uses the solvent instead of the solute; concentration always refers to the solute.
✗ ((Volume of solute)/(Mass of solution)) × 100 — mixing volume on top with mass below is dimensionally meaningless.
📌 SSC & Railway Exam Facts
• Mass % = (mass of solute / mass of solution) × 100; mass of solution = solute + solvent.
• Solute + Solvent = Solution; in a sugar solution, sugar is the solute and water the solvent.
• Water is called the universal solvent.
• True solution particle size < 1 nm; colloid 1–1000 nm; suspension > 1000 nm.
• Colloids scatter light and show the Tyndall effect; true solutions do not.
RRB Technician Grade III 2024-25 – 23 December 2024 Shift 3 · Suspensions and colloids

Q99. Which of the following statements is/are correct?
(i) Suspension is a heterogeneous mixture.
(ii) The particles of a suspension scatter a beam of light passing through it and make its path visible.
(iii) The solute particles settle down when a suspension is left undisturbed, that is, a suspension is unstable.

  1. (a) Only ii and iii
  2. (b) i, ii and iii
  3. (c) Only i
  4. (d) Only i and ii
Answer & solution

Answer: (b) i, ii and iii

Correct Answer: Option 2 — i, ii and iii
A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium. Its particles are large — bigger than 1000 nm (100 nm in some texts) — and can be seen with the naked eye. Chalk powder in water, muddy water and paints are the standard examples. NCERT lists exactly the three properties given here.
(i) is correct — the particles stay as a separate visible phase, so the mixture is heterogeneous.
(ii) is correct — the large particles scatter a beam of light and make its path visible; this is the Tyndall effect, shown by suspensions and colloids but not by true solutions.
(iii) is correct — on standing undisturbed, the heavy particles settle down under gravity, so a suspension is unstable and can be separated by filtration.
❌ Why the other options are incorrect
✗ Only ii and iii — it wrongly rejects (i), yet heterogeneity is the very definition of a suspension.
✗ Only i — it rejects both the Tyndall effect and the settling of particles, which are the two most tested properties.
✗ Only i and ii — it wrongly rejects (iii); unlike a colloid, a suspension is unstable and does settle.
📌 SSC & Railway Exam Facts
• Particle size: true solution < 1 nm, colloid 1–1000 nm, suspension > 1000 nm.
• Tyndall effect is shown by colloids and suspensions, not by true solutions.
• A suspension can be separated by filtration; a colloid cannot — it needs centrifugation.
• Colloids are stable and do not settle; suspensions are unstable and settle on standing.
• Everyday Tyndall effect: sunbeam through a dense forest or through a slit into a dusty room.
RRB Technician Grade III 2024-25 – 23 December 2024 Shift 3 · Suspensions and colloids

Q100. Face cream belongs to which type of colloid form?

  1. (a) Gel
  2. (b) Emulsion
  3. (c) Solid
  4. (d) Foam
Answer & solution

Answer: (b) Emulsion

Correct Answer: Option 2 — Emulsion
A colloid is a heterogeneous mixture whose particles are 1–1000 nm in size — too small to settle, but large enough to scatter light (Tyndall effect). Colloids are classified from the physical state of the dispersed phase and the dispersing medium.
When both the dispersed phase and the dispersing medium are liquids, the colloid is called an emulsion. Face cream, like milk, butter and cold cream, is a liquid dispersed in another liquid, so it is an emulsion. A substance added to keep such a mixture from separating is called an emulsifying agent — soap acts as one when it cleans oily dirt.
❌ Why the other options are incorrect
✗ Gel — a gel is a liquid dispersed in a solid, such as cheese, butter-jelly, curd or table jelly; it is semi-solid and holds its shape.
✗ Solid — "solid" is a state of matter, not a class of colloid; a solid-in-solid colloid would be a solid sol (e.g. coloured gemstone).
✗ Foam — a foam is a gas dispersed in a liquid, such as shaving cream, soap lather or whipped cream.
📌 SSC & Railway Exam Facts
• Emulsion = liquid in liquid — milk, face cream, cold cream.
• Gel = liquid in solid — cheese, butter, jelly; Foam = gas in liquid — shaving cream, soda froth.
• Aerosol = liquid or solid in gas — fog, clouds, mist, smoke; Sol = solid in liquid — paint, ink, blood.
• Milk is an emulsion of liquid fat in water; soap acts as an emulsifying agent.
• Colloid particle size 1–1000 nm; colloids show the Tyndall effect and are separated by centrifugation.
12

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