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

Correct Answer: Option 3 — 6.66%
The mass by mass percentage of a solution is
Mass % = (Mass of solute / Mass of solution) × 100, where Mass of solution = Mass of solute + Mass of solvent.
⚗️ The Calculation
• Mass of solute (common salt) = 20 g; mass of solvent (water) = 280 g.
• Mass of solution = 20 + 280 = 300 g.
• Mass % = (20/300) × 100 = 2000/300 = 6.66%.
So the solution is 6.66% (by mass) common salt. Note that the solvent mass alone is never used in the denominator — the mass of the whole solution must be taken.
❌ Why the other options are incorrect
✗ 66.6% — this is ten times the right value; it comes from a decimal-point slip while dividing 2000 by 300.
✗ 0.66% — again a decimal error, ten times too small; here the ×100 has effectively been applied wrongly.
✗ 11.1% — this is (20/180) × 100 or a similar wrong denominator; the commonest trap is dividing by 280 g of water instead of the 300 g of solution — that would give 7.14%, and using an even smaller denominator gives 11.1%.
📌 SSC & Railway Exam Facts
• Mass % = (mass of solute / mass of solution) × 100; solution = solute + solvent.
• Volume % = (volume of solute / volume of solution) × 100.
• Common salt = sodium chloride = NaCl; obtained from sea water and rock salt.
• Saturated solution: no more solute dissolves at that temperature; solubility usually rises with temperature.
• Water is the universal solvent; a solution of a metal in a metal is an alloy.

Correct Answer: Option 3 — Centrifugation
A colloid is a heterogeneous mixture in which the dispersed particles are 1–1000 nm in size — far too small to be trapped by filter paper, yet large enough to scatter light (Tyndall effect). Because they pass straight through a filter, ordinary filtration cannot separate them.
Centrifugation spins the mixture at a very high speed in a centrifuge. The heavier colloidal particles experience a large centrifugal force and are thrown to the bottom of the tube as a sediment, while the lighter dispersion medium stays above and is decanted off.
Everyday uses: separating cream from milk, squeezing water out of wet clothes in a washing machine, and testing blood and urine samples in diagnostic laboratories.
❌ Why the other options are incorrect
✗ Evaporation — used to recover a dissolved non-volatile solute (e.g. common salt) from its solution; it cannot pick colloidal particles out of a mixture.
✗ Filtration — separates an insoluble solid from a liquid (a suspension); colloidal particles are too small and pass through the pores of the filter paper.
✗ Distillation — separates a volatile liquid from a non-volatile solute, or two miscible liquids with different boiling points.
📌 SSC & Railway Exam Facts
• Particle size: true solution < 1 nm, colloid 1–1000 nm, suspension > 1000 nm.
• Tyndall effect = scattering of a light beam; shown by colloids and suspensions, never by a true solution.
• Milk is a colloid (an emulsion); its cream is separated by centrifugation.
• Colloids cannot be separated by filtration but can be separated by centrifugation.
• A colloid has two parts — the dispersed phase and the dispersion medium.

Correct Answer: Option 4 — Chalk powder in water
A suspension is a heterogeneous mixture in which the solute particles are larger than 1000 nm and do not dissolve but merely remain suspended in the medium. Its tests are: the particles are visible to the naked eye, they scatter light (Tyndall effect), they settle down on standing, and they can be separated by ordinary filtration. A suspension is therefore an unstable mixture.
Chalk powder (calcium carbonate, CaCO₃) stirred into water passes every one of these tests — it makes the water turbid, it can be filtered out on filter paper, and on keeping the tumbler still the chalk settles at the bottom. Muddy water, paint and slaked lime used for whitewashing are other standard examples.
❌ Why the other options are incorrect
✗ Solution of sugar in water — a true solution: particle size < 1 nm, homogeneous, transparent, does not show the Tyndall effect and cannot be filtered.
✗ Few drops of milk in water — milk is a colloid (an emulsion); its particles are 1–1000 nm, do not settle down and pass through filter paper.
✗ Oil in water — this is an emulsion, a colloid of one liquid dispersed in another liquid, not a suspension.
📌 SSC & Railway Exam Facts
• Particle size: true solution < 1 nm < colloid < 1000 nm < suspension.
• Tyndall effect is shown by colloids and suspensions, never by a true solution.
• Suspension particles settle on standing and can be filtered; colloidal particles do neither.
• Chalk is calcium carbonate (CaCO₃); marble and limestone have the same formula.
• Mixtures are of two kinds — homogeneous (true solution, alloy) and heterogeneous (suspension, colloid).

Correct Answer: Option 2 — Milk of magnesia
A sol is the type of colloid in which a solid is the dispersed phase and a liquid is the dispersion medium — in short, solid in liquid. Milk of magnesia is finely divided solid magnesium hydroxide, Mg(OH)₂, dispersed in water, and it is the standard textbook example of a sol. (In medicine it is used as an antacid and a mild laxative.)
Every colloid is named from the pair dispersed phase in dispersion medium:
• Sol = solid in liquid — milk of magnesia, mud, ink
• Solid sol = solid in solid — coloured gemstone, milky glass
• Foam = gas in liquid — shaving cream, whipped cream, soap lather
• Emulsion = liquid in liquid — milk, face cream
❌ Why the other options are incorrect
✗ Rubber — foam rubber has a gas dispersed in a solid, which makes it a foam (solid foam), not a sol.
✗ Foam — by definition a gas dispersed in a liquid; here the dispersed phase is a gas, not a solid.
✗ Face cream — a liquid dispersed in a liquid, i.e. an emulsion, not a sol.
📌 SSC & Railway Exam Facts
• Sol = solid in liquid; gel = liquid in solid (cheese, butter, jelly).
• Aerosol = solid or liquid dispersed in a gas — smoke, fog, clouds, mist.
• Milk is an emulsion (liquid fat in water); butter is a gel.
• Milk of magnesia is Mg(OH)₂, an antacid that neutralises excess HCl in the stomach.
• Colloids show the Tyndall effect, cannot be separated by filtration, but can be separated by centrifugation.