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Carbon and its Compounds PYQs – Chemistry, SSC & Railway

167 Carbon and its Compounds (Chemistry) questions asked in recent SSC and Railway exams, newest first, with exam, date & shift, answers and solutions – page 2 of 2.

📚 167 questionsSSC 4 · Railway 163
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RRB JE CBT-1 2025-26 – 19 February 2026 Shift 3 · Hydrocarbons - alkanes, alkenes, alkynes

Q101. Which of the following is the correct relationship between the number of carbon and hydrogen atoms in alkenes?

  1. (a) CnH2n+4
  2. (b) CnH2n+2
  3. (c) CnH2n
  4. (d) CnH2n-2
Answer & solution

Answer: (c) CnH2n

Alkenes are hydrocarbons with at least one carbon-carbon double bond.
The general formula for alkenes is CnH2n, where n is the number of carbon atoms.
This formula indicates there are twice as many hydrogen atoms as carbon atoms.
Hence, the correct relation is CnH2n.
Final answer: Option 3
RRB JE CBT-1 2025-26 – 19 February 2026 Shift 1 · Chemical properties of carbon compounds (combustion, oxidation, addition, substitution)

Q102. Which statement best explains why unsaturated carbon compounds are generally more reactive than saturated carbon compounds?

  1. (a) Unsaturated compounds have fewer hydrogen atoms attached to carbon
  2. (b) Unsaturated compounds contain only single bonds between carbon atoms
  3. (c) Unsaturated compounds have double or triple bonds between carbon atoms
  4. (d) Unsaturated compounds form only straight carbon chains
Answer & solution

Answer: (c) Unsaturated compounds have double or triple bonds between carbon atoms

Unsaturated carbon compounds have double or triple bonds between carbon atoms.
These multiple bonds are areas of higher electron density and are more reactive than single bonds.
Saturated compounds contain only single bonds, making them less reactive.
Hence, the presence of double or triple bonds increases the reactivity of unsaturated compounds.
Final answer: Option 3
SSC MTS 2025 – 18 February 2026 Shift 1 · Ethanol and ethanoic acid

Q103. Which of the following is crop is primarily used in the production of Ethanol?

  1. (a) Sugarcane and Maize
  2. (b) Tea leaves and Coffee beans
  3. (c) Potato and Tomato
  4. (d) Tomato and Tea leaves
Answer & solution

Answer: (a) Sugarcane and Maize

Sugarcane and Maize are the primary crops used in the production of Ethanol. Sugarcane is fermented to produce ethanol from its sucrose content, while maize (corn) provides starch that is converted to sugar and then fermented. Tea leaves, coffee beans, potatoes, and tomatoes are not primary ethanol crops.
SSC MTS 2025 – 17 February 2026 Shift 1 · Biomolecules - chemistry (carbohydrates, proteins, fats)

Q104. How many carbon atoms are present in one molecule of glucose?

  1. (a) 3
  2. (b) 4
  3. (c) 5
  4. (d) 6
Answer & solution

Answer: (d) 6

Glucose has the molecular formula C₆H₁₂O₆, which contains 6 carbon atoms. It is the most common monosaccharide and a primary energy source for living organisms. Options 3, 4, and 5 are incorrect counts of carbon in glucose.
RRB NTPC UG CBT-1 2024-25 – 3 September 2025 Shift 1 · Chemical properties of carbon compounds (combustion, oxidation, addition, substitution)

Q105. Which reaction occurs when an alkoxide ion attacks a primary alkyl halide via SN2 mechanism?

  1. (a) Williamson synthesis
  2. (b) Coumarin synthesis
  3. (c) Arndt-Eistert synthesis
  4. (d) Friedlaender synthesis
Answer & solution

Answer: (a) Williamson synthesis

RRB NTPC UG CBT-1 2024-25 – 2 September 2025 Shift 3 · Ethanol and ethanoic acid

Q106. Industrially, which acid is mainly produced by the carbonylation of methanol using carbon monoxide?

  1. (a) Acetic acid
  2. (b) Hydrochloric acid
  3. (c) Nitric acid
  4. (d) Citric acid
Answer & solution

Answer: (a) Acetic acid

RRB NTPC UG CBT-1 2024-25 – 1 September 2025 Shift 1 · Polymers and plastics

Q107. Which of the following substances is classified as an elastomer?

  1. (a) Tissue in Kidney
  2. (b) Tissue in Aorta
  3. (c) Tissue in Veins
  4. (d) Tissue in Pancreas
Answer & solution

Answer: (b) Tissue in Aorta

RRB NTPC UG CBT-1 2024-25 – 20 August 2025 Shift 3 · Polymers and plastics

Q108. Which of the following is an example of a synthetic polymer used in industry?

  1. (a) Starch
  2. (b) Nylon-6,6
  3. (c) Cellulose
  4. (d) Rubber
Answer & solution

Answer: (b) Nylon-6,6

RRB NTPC UG CBT-1 2024-25 – 14 August 2025 Shift 3 · Chemical properties of carbon compounds (combustion, oxidation, addition, substitution)

Q109. Which chemical reaction produces ortho-hydroxybenzoic acid, the primary reaction product used as a raw material for medications?

  1. (a) Friedel-Crafts reaction
  2. (b) Stephen reaction
  3. (c) Reimer-Tiemann reaction
  4. (d) Kolbe’s reaction
Answer & solution

Answer: (d) Kolbe’s reaction

RRB NTPC UG CBT-1 2024-25 – 11 August 2025 Shift 1 · Biomolecules - chemistry (carbohydrates, proteins, fats)

Q110. Amylopectin is a branched-chain polysaccharide composed of glucose units linked primarily by ________.

  1. (a) β-1,6-glycosidic bonds
  2. (b) β-1,4-glycosidic bonds
  3. (c) α-1,4-glycosidic bonds
  4. (d) α-1,2-glycosidic bonds
Answer & solution

Answer: (c) α-1,4-glycosidic bonds

RRB NTPC UG CBT-1 2024-25 – 11 August 2025 Shift 1 · Functional groups

Q111. Which general molecular structure characterises the class of organic compounds known as diazonium compounds or diazonium salts?

  1. (a) R-C≡N
  2. (b) R-N₂⁺ X ⁻
  3. (c) R-CO-NR'R"
  4. (d) R₂C=NR'
Answer & solution

Answer: (b) R-N₂⁺ X ⁻

RRB NTPC UG CBT-1 2024-25 – 8 August 2025 Shift 2 · Polymers and plastics

Q112. Which resilient and adaptable polymer is frequently found in industrial brushes, toothbrushes, and other cleaning supplies?

  1. (a) Terylene
  2. (b) Nylon 6, 6
  3. (c) Teflon
  4. (d) Neoprene
Answer & solution

Answer: (b) Nylon 6, 6

RRB NTPC UG CBT-1 2024-25 – 7 August 2025 Shift 1 · Soaps and detergents

Q113. Which acid is used in non-ionic detergent reactions to improve cleaning qualities and thicken detergents?

  1. (a) Stearic acid
  2. (b) Propanoic acid
  3. (c) Oxalic acid
  4. (d) Barbituric acid
Answer & solution

Answer: (a) Stearic acid

RRB NTPC GL CBT-1 2024-25 – 19 June 2025 Shift 3 · Biomolecules - chemistry (carbohydrates, proteins, fats)

Q114. In lipid biochemistry, why do unsaturated fatty acids tend to be liquid at room temperature?

  1. (a) They form ionic bonds that prevent solidification.
  2. (b) Their double bonds increase molecular symmetry leading to less crystal packing.
  3. (c) They have lower molecular weights than saturated fatty acids.
  4. (d) The cis-double bonds introduce kinks, reducing van der Waals interactions.
Answer & solution

Answer: (d) The cis-double bonds introduce kinks, reducing van der Waals interactions.

RRB NTPC GL CBT-1 2024-25 – 18 June 2025 Shift 2 · Chemical properties of carbon compounds (combustion, oxidation, addition, substitution)

Q115. In industrial settings, Freon-12 is most commonly manufactured by which chemical reaction?

  1. (a) Elimination reaction
  2. (b) Swarts reaction
  3. (c) Wurtz reaction
  4. (d) Substitution reaction
Answer & solution

Answer: (b) Swarts reaction

RRB NTPC GL CBT-1 2024-25 – 6 June 2025 Shift 3 · Biomolecules - chemistry (carbohydrates, proteins, fats)

Q116. Which functional groups in amino acids are directly involved in forming peptide bonds?

  1. (a) Carbonyl and phosphate groups
  2. (b) Methyl and acetyl groups
  3. (c) Hydroxyl and sulfhydryl groups
  4. (d) Carboxyl and amino groups
Answer & solution

Answer: (d) Carboxyl and amino groups

RRB Technician Grade III 2024-25 – 30 December 2024 Shift 1 · Ethanol and ethanoic acid

Q117. Heating of ethanol at 443 K with excess concentrated sulphuric acid yields which of the following?

  1. (a) Ether
  2. (b) Acetic acid
  3. (c) Ethane
  4. (d) Ethene
Answer & solution

Answer: (d) Ethene

Correct Answer: Option 4 — Ethene
When ethanol (C₂H₅OH) is heated at 443 K (170 °C) with excess concentrated sulphuric acid, it undergoes dehydration — the removal of a water molecule — and is converted into the unsaturated hydrocarbon ethene (C₂H₄).
The concentrated H₂SO₄ does not appear in the products: it acts as a dehydrating agent, pulling out one H and one OH from adjacent carbon atoms of the alcohol. The two carbons are then left with a spare electron pair each and join through a double bond, giving an alkene. Because the acid is recovered unchanged, it is counted as a catalyst/dehydrating agent and written over the arrow.
⚗️ The Reaction
C₂H₅OH → (conc. H₂SO₄, 443 K) C₂H₄ + H₂O
• H and OH are removed from neighbouring carbons, so a C=C double bond forms.
• Ethene is an unsaturated hydrocarbon (alkene, general formula CₙH₂ₙ).
• At the lower temperature of 413 K, excess ethanol instead gives diethyl ether, not ethene.
❌ Why the other options are incorrect
✗ Ether — diethyl ether (C₂H₅OC₂H₅) is formed when excess ethanol is heated with conc. H₂SO₄ at the lower temperature of about 413 K (140 °C); at 443 K with excess acid the product is the alkene.
✗ Acetic acid — ethanoic acid (CH₃COOH) comes from the oxidation of ethanol by alkaline KMnO₄ or acidified K₂Cr₂O₇, not from dehydration.
✗ Ethane — C₂H₆ would need hydrogenation; in fact it is obtained by adding H₂ to ethene over a nickel catalyst, so it is the reverse direction.
📌 SSC & Railway Exam Facts
• Dehydration of ethanol at 443 K with conc. H₂SO₄ → ethene; conc. H₂SO₄ is the dehydrating agent.
• Oxidation of ethanol gives ethanoic acid; alkaline KMnO₄ and acidified K₂Cr₂O₇ are the oxidising agents.
• Ethanol + Na → sodium ethoxide + H₂; 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂.
• Ethene is the plant hormone used for the artificial ripening of fruits.
• Ethanol is the alcohol of alcoholic drinks; methanol is poisonous and causes blindness.
RRB Technician Grade III 2024-25 – 30 December 2024 Shift 1 · Ethanol and ethanoic acid

Q118. The process of fermentation involves the conversion of sugars into which of the following compounds?

  1. (a) Ethanoic acid
  2. (b) Ethene
  3. (c) Ethanol
  4. (d) Methane
Answer & solution

Answer: (c) Ethanol

Correct Answer: Option 3 — Ethanol
Fermentation is the slow anaerobic (oxygen-free) breakdown of sugars by enzymes present in yeast, in which the sugar is converted into ethanol (C₂H₅OH) and carbon dioxide. It is the oldest industrial method of making alcohol, carried out at about 303 K (30 °C).
The change takes place in two enzyme-controlled steps. Invertase first splits sucrose (C₁₂H₂₂O₁₁) into glucose and fructose; then zymase, also from yeast, breaks each C₆H₁₂O₆ unit into two molecules of ethanol and two of CO₂. Molasses, grains and fruit juices are the usual raw materials.
⚗️ The Reaction
C₁₂H₂₂O₁₁ + H₂O → (invertase) C₆H₁₂O₆ + C₆H₁₂O₆
C₆H₁₂O₆ → (zymase) 2C₂H₅OH + 2CO₂
• Yeast supplies the enzymes invertase and zymase; the process needs no oxygen.
• The escaping CO₂ is what makes bread and dough rise.
• The alcohol so obtained is concentrated afterwards by fractional distillation.
❌ Why the other options are incorrect
✗ Ethanoic acid — CH₃COOH is the next stage: it forms when the ethanol is further oxidised by acetic acid bacteria in air, which is how vinegar is produced. Fermentation of sugar itself stops at ethanol.
✗ Ethene — C₂H₄ is obtained by dehydrating ethanol with conc. H₂SO₄ at 443 K, a laboratory reaction and not a biological one.
✗ Methane — CH₄ is the main gas of biogas/marsh gas, produced by the anaerobic decay of animal and plant waste by bacteria, not by the fermentation of sugar with yeast.
📌 SSC & Railway Exam Facts
• Fermentation: sugar → ethanol + CO₂, by the enzymes of yeast, at about 303 K.
• Enzymes: invertase (sucrose → glucose + fructose) and zymase (glucose → ethanol + CO₂).
• Ethanol is also called grain alcohol; spirit and wine are made this way.
• Ethanol + O₂ (bacteria) → ethanoic acid; a 5–8% solution of ethanoic acid in water is vinegar.
• Pure ethanoic acid freezes at 290 K (16.6 °C) and is therefore called glacial acetic acid.
RRB Technician Grade III 2024-25 – 30 December 2024 Shift 3 · Homologous series

Q119. By which unit do the successive members of the homogenous series CH₄, C₂H₆, and C₃H₈ differ from each other?

  1. (a) CH₂
  2. (b) CH
  3. (c) CH₃
  4. (d) CH₄
Answer & solution

Answer: (a) CH₂

Correct Answer: Option 1 — CH₂
A homologous series is a family of organic compounds that have the same general formula and the same functional group, in which each member differs from the next by one −CH₂− unit. The compounds given — CH₄ (methane), C₂H₆ (ethane) and C₃H₈ (propane) — are the first three alkanes, general formula CₙH₂ₙ₊₂.
Counting the atoms proves it: going from CH₄ to C₂H₆ adds 1 carbon and 2 hydrogens, and from C₂H₆ to C₃H₈ again adds 1 carbon and 2 hydrogens. That added unit is exactly CH₂.
Each step also raises the molecular mass by 14 u (C = 12, H₂ = 2). Because the members differ only in chain length, they show similar chemical properties but a gradual change in physical properties — melting point, boiling point and density rise steadily as the chain grows.
❌ Why the other options are incorrect
✗ CH — adding only CH would give C₂H₅ from CH₄, which is not a stable molecule; the hydrogen count would not fit CₙH₂ₙ₊₂.
✗ CH₃ — this is the methyl group, an alkyl substituent, not the difference between two successive members; adding CH₃ to CH₄ would give C₂H₇, which does not exist.
✗ CH₄ — methane itself is the first member of the series; the difference between CH₄ and C₂H₆ is only CH₂, not a whole CH₄.
📌 SSC & Railway Exam Facts
• Successive members of a homologous series differ by −CH₂−, i.e. by 14 u in molecular mass.
• Alkane = CₙH₂ₙ₊₂ (saturated), alkene = CₙH₂ₙ, alkyne = CₙH₂ₙ₋₂ (both unsaturated).
• First four alkanes: methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀.
• Methane (CH₄) is the main constituent of CNG and biogas; it is also called marsh gas.
• Members of a homologous series have similar chemical but graded physical properties.
RRB Technician Grade III 2024-25 – 30 December 2024 Shift 3 · Bonding in carbon and catenation

Q120. The term 'Catenation' is used to define a unique property of carbon. Which of the following gives the correct definition of catenation?

  1. (a) Unique ability to form bonds with other atoms of nitrogen.
  2. (b) Unique ability to form bonds with other atoms of oxygen.
  3. (c) Unique ability to form bonds with other atoms of hydrogen.
  4. (d) Unique ability to form bonds with other atoms of carbon.
Answer & solution

Answer: (d) Unique ability to form bonds with other atoms of carbon.

Correct Answer: Option 4 — Unique ability to form bonds with other atoms of carbon.
Catenation is the property of an element to link its own atoms together to form long chains, branched chains and closed rings. Carbon shows catenation to a far greater extent than any other element, and this single property is the reason organic chemistry exists as a separate branch with millions of carbon compounds.
Carbon can catenate so well because its C–C bond is exceptionally strong and stable (carbon is small, so the shared electrons are held tightly by the nucleus). Carbon also has a valency of 4 (tetravalency), so after joining other carbon atoms it still has spare bonds for hydrogen, oxygen, nitrogen, sulphur or halogens.
The bonds formed can be single (C–C), double (C=C) or triple (C≡C), which allows chains, branches and rings such as benzene.
❌ Why the other options are incorrect
✗ Bonds with other atoms of nitrogen — nitrogen does catenate, but only very weakly; in any case catenation is defined as bonding with atoms of the same element, and here that element is carbon.
✗ Bonds with other atoms of oxygen — oxygen's catenation is extremely limited (only H₂O₂-type peroxide links); this is not the definition of catenation for carbon.
✗ Bonds with other atoms of hydrogen — hydrogen is monovalent and can form only one bond, so it cannot form chains at all; C–H bonding is not catenation.
📌 SSC & Railway Exam Facts
• Catenation = self-linking of atoms of the same element; shown best by carbon, then silicon and sulphur.
• Carbon is tetravalent (valency 4) — configuration 2, 4 — and forms covalent bonds.
• Carbon's two special properties — catenation and tetravalency — give rise to millions of compounds.
• Allotropes of carbon: diamond (hardest natural substance), graphite (conducts electricity), fullerene (C₆₀, buckminsterfullerene).
• Graphite is soft and slippery; diamond has no free electron, so it does not conduct electricity.
RRB Technician Grade III 2024-25 – 29 December 2024 Shift 1 · Soaps and detergents

Q121. Sodium or potassium salt of long chain carboxylic acid is known as which of the following?

  1. (a) Both detergent and soap
  2. (b) Soap
  3. (c) Micelle
  4. (d) Detergent
Answer & solution

Answer: (b) Soap

Correct Answer: Option 2 — Soap
A soap is defined as the sodium or potassium salt of a long chain carboxylic acid (fatty acid). A typical example is sodium stearate, C₁₇H₃₅COONa, obtained from stearic acid.
A soap molecule has two ends of opposite nature. The long hydrocarbon chain is the hydrophobic (water-hating), oil-loving tail, while the –COO⁻Na⁺ group is the hydrophilic (water-loving) head. The tails bury themselves in the grease while the heads stay in the water, so the dirt is lifted off the cloth and carried away.
Soap is manufactured by saponification — boiling an oil or fat with sodium hydroxide. Sodium soaps are hard soaps (bathing and washing bars) while potassium soaps are soft soaps (shaving creams, liquid soaps).
⚗️ The Reaction — Saponification
(C₁₇H₃₅COO)₃C₃H₅ + 3NaOH → 3C₁₇H₃₅COONa + C₃H₅(OH)₃
• An ester (fat/oil) is hydrolysed by alkali to give soap plus glycerol.
• The by-product glycerol (C₃H₅(OH)₃) is used in medicines, cosmetics and explosives.
• Common salt is added at the end — salting out — to make the soap separate and float.
❌ Why the other options are incorrect
✗ Both detergent and soap — a detergent is not a salt of a carboxylic acid, so the two cannot be grouped together here.
✗ Micelle — a micelle is the spherical cluster that soap molecules form in water with their tails pointing inwards; it is a structure, not a chemical class.
✗ Detergent — a detergent is the ammonium or sulphonate salt of a long chain benzene sulphonic acid or sulphate, e.g. sodium alkyl benzene sulphonate.
📌 SSC & Railway Exam Facts
• Soap = sodium/potassium salt of fatty acid; detergent = salt of sulphonic acid/sulphate.
• Soap does not lather in hard water — Ca²⁺ and Mg²⁺ ions form an insoluble scum; detergents work in hard water.
• Soap making is saponification; the by-product is glycerol.
• Micelle formation is the reason soap cleans; micelles scatter light and show the Tyndall effect.
• Detergents with branched chains are non-biodegradable and cause water pollution; straight-chain ones are biodegradable.
RRB Technician Grade III 2024-25 – 29 December 2024 Shift 1 · Bonding in carbon and catenation

Q122. Carbon has the unique ability to form bonds with other atoms of carbon, giving rise to large molecules. This property is called _______.

  1. (a) Catenation
  2. (b) Cyclisation
  3. (c) Metallic bonding
  4. (d) Polymerisation
Answer & solution

Answer: (a) Catenation

Correct Answer: Option 1 — Catenation
Catenation is the ability of an element to form bonds with other atoms of the same element, producing long chains, branched chains and closed rings. Carbon shows catenation to a far greater extent than any other element, and this single property is the reason an entire branch — organic chemistry — exists.
Two factors make carbon so good at it. First, carbon is a very small atom, so the C–C bond is short and exceptionally strong (bond energy about 348 kJ/mol), making the chains stable. Second, carbon is tetravalent — each atom can form four covalent bonds, so after joining two carbons there are still bonds left for hydrogen or other atoms.
Together, catenation + tetravalency explain why over ten million carbon compounds are known — more than the compounds of all other elements put together. The chains may be straight (n-butane), branched (isobutane) or cyclic (cyclohexane, benzene).
❌ Why the other options are incorrect
✗ Cyclisation — this is only the closing of a chain into a ring; it is one *outcome* of catenation, not the name of the general self-linking property.
✗ Metallic bonding — the attraction between positive metal kernels and a sea of delocalised electrons; carbon is a non-metal and bonds covalently.
✗ Polymerisation — the joining of many small monomer units into a giant molecule (polythene, PVC); it needs repeating monomers and is not carbon's basic atom-to-atom bonding ability.
📌 SSC & Railway Exam Facts
• Catenation + tetravalency are the two reasons for the huge number of carbon compounds.
• Order of catenation: C >> Si > Ge ≈ Sn; silicon catenates only up to about 7–8 atoms.
• C–C bond energy ≈ 348 kJ/mol; carbon's atomic number is 6, electronic configuration 2, 4.
• Carbon almost always forms covalent bonds because losing or gaining 4 electrons needs too much energy.
• Allotropes of carbon: diamond (hardest natural substance), graphite (conducts electricity), fullerene (C₆₀, buckminsterfullerene).
RRB Technician Grade III 2024-25 – 29 December 2024 Shift 1 · Functional groups

Q123. Match the class of organic compounds (Column I) with the suffix used (Column II).
(i) Ketone — (a) -al (-CHO)
(ii) Alkene — (b) -one (-C=O)
(iii) Aldehyde — (c) -ene (-C=C-)
(iv) Carboxylic acid — (d) -oic acid (-COOH)

  1. (a) (i)-(c); (ii)-(d); (iii)-(b); (iv)-(a)
  2. (b) (i)-(c); (ii)-(b); (iii)-(a); (iv)-(d)
  3. (c) (i)-(b); (ii)-(c); (iii)-(d); (iv)-(a)
  4. (d) (i)-(b); (ii)-(c); (iii)-(a); (iv)-(d)
Answer & solution

Answer: (d) (i)-(b); (ii)-(c); (iii)-(a); (iv)-(d)

Correct Answer: Option 4 — (i)-(b); (ii)-(c); (iii)-(a); (iv)-(d)
In the IUPAC system the suffix of a name announces the functional group present, while the prefix counts the carbon atoms (meth-, eth-, prop-, but-).
• (i) Ketone → (b) -one (–C=O). The carbonyl group sits inside the chain, between two carbon atoms. Example: propanone, CH₃COCH₃ (acetone) — the smallest possible ketone, since it needs at least 3 carbons.
• (ii) Alkene → (c) -ene (–C=C–). An unsaturated hydrocarbon with a carbon–carbon double bond. Example: ethene, C₂H₄; general formula CₙH₂ₙ.
• (iii) Aldehyde → (a) -al (–CHO). Here the carbonyl group is at the end of the chain and carries a hydrogen. Example: ethanal, CH₃CHO.
• (iv) Carboxylic acid → (d) -oic acid (–COOH). Example: ethanoic acid, CH₃COOH (acetic acid), the acid in vinegar.
The usual trap is swapping -al and -one, so remember: aldehyde → -al (both begin with *al*), and ketone → -one (both contain *one*).
❌ Why the other options are incorrect
✗ (i)-(c); (ii)-(d); (iii)-(b); (iv)-(a) — it calls a ketone an alkene and a carboxylic acid an aldehyde; every pair is misplaced.
✗ (i)-(c); (ii)-(b); (iii)-(a); (iv)-(d) — only (iii) and (iv) are right; ketone and alkene have been interchanged.
✗ (i)-(b); (ii)-(c); (iii)-(d); (iv)-(a) — ketone and alkene are right, but aldehyde and carboxylic acid have been swapped.
📌 SSC & Railway Exam Facts
• Suffixes: alkane → -ane, alkene → -ene, alkyne → -yne, alcohol → -ol, aldehyde → -al, ketone → -one, acid → -oic acid.
• General formulas: alkane CₙH₂ₙ₊₂, alkene CₙH₂ₙ, alkyne CₙH₂ₙ₋₂.
• Ethanol (C₂H₅OH) is the alcohol in alcoholic drinks; ethanoic acid (CH₃COOH) is vinegar (5–8% solution).
• Glacial acetic acid is pure ethanoic acid, which freezes at 16.6 °C.
• Consecutive members of a homologous series differ by –CH₂–, i.e. by 14 u.
RRB Technician Grade III 2024-25 – 29 December 2024 Shift 2 · Bonding in carbon and catenation

Q124. Which of the following is the valency of carbon in ethene?

  1. (a) Three
  2. (b) Six
  3. (c) Two
  4. (d) Four
Answer & solution

Answer: (d) Four

Correct Answer: Option 4 — Four
Carbon has the atomic number 6 and the electronic configuration 2, 4. With four valence electrons it can neither lose four electrons (too much energy needed) nor gain four, so it shares all four and forms four covalent bonds. Its valency is therefore always four — a property called tetravalency, and it does not change from one compound to another.
In ethene, C₂H₄ (H₂C=CH₂), each carbon forms one double bond (2 bonds) with the other carbon and one single bond with each of two hydrogen atoms — that is 2 + 1 + 1 = 4 bonds. The valency is still four even though ethene is unsaturated; unsaturation means a double bond, not a reduced valency.
❌ Why the other options are incorrect
✗ Three — no neutral carbon compound leaves carbon with three bonds; a species with three bonds would be an unstable ion or free radical.
✗ Six — six is carbon's atomic number, not its valency; a common trap that confuses the two.
✗ Two — two is the number of hydrogen atoms bonded to each carbon in ethene, or the bond order of the C=C bond, but not carbon's valency.
📌 SSC & Railway Exam Facts
• Tetravalency + catenation give carbon its huge number of compounds (over 10 million).
• Ethane C₂H₆ (C–C), ethene C₂H₄ (C=C), ethyne C₂H₂ (C≡C) — carbon's valency is 4 in all three.
• Carbon's electronic configuration is 2, 4; it forms covalent bonds only.
• Catenation is the self-linking of carbon atoms into chains, branches and rings.
• Valency: H = 1, O = 2, N = 3, C = 4.
RRB Technician Grade III 2024-25 – 29 December 2024 Shift 2 · Hydrocarbons - alkanes, alkenes, alkynes

Q125. Which of the following compounds is/are saturated?
(i) C₂H₆
(ii) C₃H₈
(iii) C₂H₄
(iv) C₄H₁₀

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

Answer: (d) i, ii and iv

Correct Answer: Option 4 — i, ii and iv
A saturated hydrocarbon (alkane) contains only single covalent bonds between its carbon atoms and therefore carries the maximum possible number of hydrogen atoms. Its general formula is CₙH₂ₙ₊₂, so the quickest test in the exam is to put the number of carbons in 2n + 2 and compare with the hydrogens printed.
⚗️ Applying the CₙH₂ₙ₊₂ test
• (i) C₂H₆ — n = 2, 2n + 2 = 6 ✔ → ethane, saturated.
• (ii) C₃H₈ — n = 3, 2n + 2 = 8 ✔ → propane, saturated.
• (iii) C₂H₄ — n = 2 needs 6 H but only 4 are present → a double bond (C=C) is present → ethene, unsaturated.
• (iv) C₄H₁₀ — n = 4, 2n + 2 = 10 ✔ → butane, saturated.
• Hence (i), (ii) and (iv) are saturated.
❌ Why the other options are incorrect
✗ i and ii — correct as far as it goes, but it leaves out C₄H₁₀ (butane), which also fits CₙH₂ₙ₊₂ and is saturated.
✗ i and iv — omits C₃H₈ (propane), a perfectly saturated alkane.
✗ i, ii and iii — wrongly includes C₂H₄ (ethene), which has a double bond and is therefore unsaturated; this is the classic mistake of counting only carbons and ignoring the hydrogen count.
📌 SSC & Railway Exam Facts
• Alkane CₙH₂ₙ₊₂ (single bond), alkene CₙH₂ₙ (double bond), alkyne CₙH₂ₙ₋₂ (triple bond).
• Saturated compounds burn with a clean blue flame; unsaturated ones with a sooty yellow flame.
• Unsaturated hydrocarbons decolourise bromine water; saturated ones do not — the standard laboratory test.
• Alkanes give substitution reactions; alkenes and alkynes give addition reactions.
• C₂H₆ ethane, C₃H₈ propane, C₄H₁₀ butane (in LPG), C₂H₄ ethene, C₂H₂ ethyne.
RRB Technician Grade III 2024-25 – 29 December 2024 Shift 2 · Ethanol and ethanoic acid

Q126. What product will be formed when ethanol at 443K with excess concentrated sulphuric acid is heated?

  1. (a) Only ethyne
  2. (b) Ethene and water
  3. (c) Ethyne and hydrogen
  4. (d) Ethane and water
Answer & solution

Answer: (b) Ethene and water

Correct Answer: Option 2 — Ethene and water
Heating ethanol (C₂H₅OH) at 443 K (170 °C) with excess concentrated sulphuric acid brings about dehydration — the removal of a molecule of water from the alcohol. The –OH group and one hydrogen from the neighbouring carbon are pulled out together, and the two carbons close the gap by forming a double bond, giving the unsaturated hydrocarbon ethene (C₂H₄).
Here concentrated H₂SO₄ acts as a dehydrating agent (not as an oxidising agent), because it has a very strong affinity for water. It is used in excess and is not consumed, so the reaction is written with H₂SO₄ over the arrow.
⚗️ The Reaction
C₂H₅OH → (conc. H₂SO₄, 443 K) C₂H₄ + H₂O
• The alcohol loses H and OH from adjacent carbons — an elimination (dehydration) reaction.
• Atom count: 2 C, 6 H, 1 O on each side, so the equation is balanced.
• The product ethene decolourises bromine water, proving that it is unsaturated.
❌ Why the other options are incorrect
✗ Only ethyne — ethyne (C₂H₂) would need two molecules of water to be removed; simple dehydration of ethanol stops at the double bond, not the triple bond.
✗ Ethyne and hydrogen — no hydrogen gas is liberated here; the hydrogen leaves as part of the water molecule, and the loss of water gives ethene, not ethyne.
✗ Ethane and water — ethane (C₂H₆) is saturated and would require the addition of hydrogen, whereas this reaction removes atoms; dehydration can never make a saturated product.
📌 SSC & Railway Exam Facts
• Ethanol → (conc. H₂SO₄, 443 K) Ethene + Water — the standard dehydration PYQ.
• Conc. H₂SO₄ = dehydrating agent; it also chars sugar to carbon.
• Ethanol + Na → sodium ethoxide + H₂ gas — the test of the –OH group.
• Ethanol + ethanoic acid → (acid catalyst) ester + water is esterification; the reverse with NaOH is saponification.
• Ethene + H₂ → (Ni catalyst) ethane — hydrogenation, used to make vanaspati ghee from oils.
RRB Technician Grade III 2024-25 – 29 December 2024 Shift 2 · Ethanol and ethanoic acid

Q127. Which of the following alcohols is used in cough syrups?

  1. (a) Methanol
  2. (b) Propanol
  3. (c) Ethanoic acid
  4. (d) Ethanol
Answer & solution

Answer: (d) Ethanol

Correct Answer: Option 4 — Ethanol
Ethanol (C₂H₅OH), commonly called ethyl alcohol or simply "alcohol", is the only member of the alcohol family that is safe in small, controlled doses. It is a colourless liquid with a boiling point of 78 °C, it mixes with water in all proportions, and it is an excellent solvent for many drugs that do not dissolve in water.
For these reasons ethanol is used in cough syrups, tonics, tincture iodine and many other medicinal preparations, where it dissolves the active ingredient and also preserves the mixture.
❌ Why the other options are incorrect
✗ Methanol — CH₃OH (wood spirit) is extremely poisonous. In the body it is oxidised to methanal (formaldehyde), which coagulates protoplasm; even a small quantity causes blindness, and larger amounts cause death. It is added to ethanol to make denatured spirit, deliberately unfit to drink.
✗ Propanol — C₃H₇OH is used as an industrial solvent and in rubbing alcohol/sanitisers, not taken internally in syrups.
✗ Ethanoic acid — CH₃COOH is a carboxylic acid, not an alcohol at all; its 5–8% solution is vinegar, and the pure acid freezes to glacial acetic acid below 16.6 °C.
📌 SSC & Railway Exam Facts
• Ethanol C₂H₅OH — used in cough syrups, tinctures and as a solvent; methanol CH₃OH — poisonous.
• Denatured spirit = ethanol + methanol (+ dye), made unfit for drinking to avoid excise duty.
• Ethanol from sugarcane molasses by fermentation; blended with petrol it makes gasohol.
• Ethanol + Na → sodium ethoxide + H₂; ethanol → (conc. H₂SO₄, 443 K) ethene + water.
• Functional groups: –OH alcohol, –CHO aldehyde, –COOH carboxylic acid, >C=O ketone.
RRB Technician Grade III 2024-25 – 29 December 2024 Shift 3 · Soaps and detergents

Q128. The sodium or potassium salts of which acid are defined as soaps?

  1. (a) Nitric acid
  2. (b) Carboxylic acid
  3. (c) Sulphuric acid
  4. (d) Hydrochloric acid
Answer & solution

Answer: (b) Carboxylic acid

Correct Answer: Option 2 — Carboxylic acid
Soaps are the sodium or potassium salts of long chain carboxylic acids (fatty acids). A carboxylic acid carries the –COOH functional group; when it is neutralised by NaOH or KOH, the acidic hydrogen is replaced by the metal and a –COO⁻Na⁺ salt is formed. Sodium stearate, C₁₇H₃₅COONa, is a typical soap.
This structure is exactly what makes soap clean. The long hydrocarbon chain is hydrophobic and dissolves in grease, while the ionic –COO⁻Na⁺ head is hydrophilic and stays in water. Many such molecules arrange themselves into spherical micelles that trap the dirt and are rinsed away.
Soaps are manufactured by saponification — boiling a vegetable oil or animal fat (an ester of a fatty acid) with sodium hydroxide. Sodium soaps are hard soaps; potassium soaps are soft soaps.
⚗️ The Reaction — Saponification
(C₁₇H₃₅COO)₃C₃H₅ + 3NaOH → 3C₁₇H₃₅COONa + C₃H₅(OH)₃
• The fat (an ester) is hydrolysed by alkali into soap and glycerol.
• C₁₇H₃₅COOH is stearic acid, a long chain carboxylic acid — this is the acid the question is asking about.
• Glycerol is the valuable by-product, used in medicines and cosmetics.
❌ Why the other options are incorrect
✗ Nitric acid — a strong mineral acid (HNO₃) used in making fertilisers and explosives; its sodium salt is sodium nitrate, not a soap.
✗ Sulphuric acid — the "king of chemicals" (H₂SO₄); its long chain sulphonate/sulphate salts give detergents, not soaps.
✗ Hydrochloric acid — a strong mineral acid (HCl) present in gastric juice; its sodium salt is simply common salt, NaCl.
📌 SSC & Railway Exam Facts
• Soap = sodium/potassium salt of a long chain carboxylic acid; detergent = sodium salt of a long chain benzene sulphonic acid or sulphate.
• Functional group of a carboxylic acid is –COOH; its IUPAC suffix is -oic acid.
• Ethanoic acid, CH₃COOH, is the commonest carboxylic acid — vinegar is its 5–8% solution.
• Soap does not lather in hard water; Ca²⁺/Mg²⁺ ions form an insoluble scum.
• The cleansing action of soap depends on micelle formation, which shows the Tyndall effect.
RRB Technician Grade III 2024-25 – 29 December 2024 Shift 3 · Hydrocarbons - alkanes, alkenes, alkynes

Q129. Which of the following is the general formula of alkane?

  1. (a) CₙH₂ₙ₊₂
  2. (b) CₙHₙ
  3. (c) C₂ₙH₂ₙ
  4. (d) CₙH₂ₙ
Answer & solution

Answer: (a) CₙH₂ₙ₊₂

Correct Answer: Option 1 — CₙH₂ₙ₊₂
Alkanes are saturated hydrocarbons — every carbon atom is joined to its neighbours by single covalent bonds (C–C) only, and all the remaining valencies are satisfied by hydrogen. Because each carbon holds the maximum possible number of hydrogen atoms, the general formula works out to CₙH₂ₙ₊₂, where n is the number of carbon atoms.
Putting values of n into it gives the whole homologous series: n = 1 → CH₄ (methane), n = 2 → C₂H₆ (ethane), n = 3 → C₃H₈ (propane), n = 4 → C₄H₁₀ (butane). Each member differs from the next by a –CH₂– unit (14 u), which is the defining property of a homologous series.
Alkanes burn in air with a clean blue flame and are relatively unreactive, which is why they are also called paraffins (Latin *parum affinis* — little affinity).
❌ Why the other options are incorrect
✗ CₙHₙ — this fits nothing in the basic series; it would wrongly suggest one hydrogen per carbon (it is closer to the formula of benzene, C₆H₆, an aromatic compound).
✗ C₂ₙH₂ₙ — on simplifying, the carbon count is doubled for no reason; it is just CₙH₂ₙ written misleadingly and still describes an alkene.
✗ CₙH₂ₙ — this is the general formula of an alkene, an unsaturated hydrocarbon with a C=C double bond (e.g. ethene, C₂H₄).
📌 SSC & Railway Exam Facts
• Alkane CₙH₂ₙ₊₂ (single bond); alkene CₙH₂ₙ (double bond); alkyne CₙH₂ₙ₋₂ (triple bond).
• First members: methane CH₄, ethene C₂H₄, ethyne C₂H₂ (acetylene) — used with oxygen for welding.
• Saturated hydrocarbons burn with a clean blue flame; unsaturated ones give a yellow sooty flame.
• Successive members of a homologous series differ by –CH₂–, i.e. 14 u, and have similar chemical properties.
• Methane is the main constituent of CNG, biogas and natural gas; it is also called marsh gas.
RRB Technician Grade III 2024-25 – 28 December 2024 Shift 2 · Bonding in carbon and catenation

Q130. Carbon has the unique ability to form bonds with other atoms of carbon. This property is known as _______.

  1. (a) Isomerisation
  2. (b) cyclisation
  3. (c) catenation
  4. (d) Crystallisation
Answer & solution

Answer: (c) catenation

Correct Answer: Option 3 — catenation
Catenation is the ability of an element to form bonds with atoms of its own kind, producing long chains, branches and rings. Carbon shows catenation to the maximum extent of all elements, and this single property is the reason why more than 10 million organic compounds are known — far more than the compounds of all the other elements put together.
Carbon can do this because its C—C bond is exceptionally strong and stable (small atomic size, high bond energy) and because carbon is tetravalent — each carbon atom has 4 valence electrons and forms four covalent bonds, so it can hold other carbons and still have bonds left for hydrogen or other atoms.
Carbon chains may be straight (n-butane), branched (isobutane) or closed rings (cyclohexane, benzene), and the bonds may be single, double or triple.
❌ Why the other options are incorrect
✗ Isomerisation — the existence of compounds with the same molecular formula but different structures (e.g. C₄H₁₀ as n-butane and isobutane); it is a *consequence* of catenation, not the bonding property itself.
✗ cyclisation — the formation of a closed ring from an open chain; it is only one special case of catenation.
✗ Crystallisation — a purification technique used to obtain pure solid crystals from a solution; it has nothing to do with carbon–carbon bonding.
📌 SSC & Railway Exam Facts
• Catenation is strongest in carbon; next comes silicon, but its chains are short and unstable.
• Carbon is tetravalent (valency 4), atomic number 6, configuration 2, 4.
• Carbon forms covalent bonds by sharing electrons, so its compounds are poor conductors with low melting and boiling points.
• Allotropes of carbon: diamond (hardest natural substance), graphite (conducts electricity), fullerene (C₆₀, buckminsterfullerene).
• Compounds with the same molecular formula but different structures are isomers; the first alkane to show isomerism is butane (C₄H₁₀).
RRB Technician Grade III 2024-25 – 28 December 2024 Shift 2 · Homologous series

Q131. Which of following pairs belongs to homologous series of alkenes?

  1. (a) C₃H₆ and C₄H₈
  2. (b) CH₄ and C₂H₄
  3. (c) C₂H₆ and C₃H₁₀
  4. (d) CH₄ and C₄H₈
Answer & solution

Answer: (a) C₃H₆ and C₄H₈

Correct Answer: Option 1 — C₃H₆ and C₄H₈
A homologous series is a family of organic compounds with the same general formula and the same functional group, in which each member differs from the next by a —CH₂— unit (14 u). Members of one series show similar chemical properties and a gradual change in physical properties.
Alkenes are unsaturated hydrocarbons containing at least one carbon–carbon double bond (C=C). Their general formula is CₙH₂ₙ, and the series begins with ethene (C₂H₄) because at least two carbon atoms are needed to form a double bond.
Testing the options against CₙH₂ₙ: C₃H₆ (n = 3, 2n = 6 ✓) is propene and C₄H₈ (n = 4, 2n = 8 ✓) is butene. Both are alkenes and they differ by exactly one CH₂, so this pair alone belongs to the alkene homologous series.
❌ Why the other options are incorrect
✗ CH₄ and C₂H₄ — CH₄ (methane) fits CₙH₂ₙ₊₂ and is an alkane; only C₂H₄ (ethene) is an alkene, so the pair is mixed.
✗ C₂H₆ and C₃H₁₀ — C₂H₆ (ethane) is an alkane, and C₃H₁₀ does not exist at all (propane is C₃H₈).
✗ CH₄ and C₄H₈ — again a mixture: CH₄ is an alkane while C₄H₈ (butene) is an alkene.
📌 SSC & Railway Exam Facts
• General formulas: alkane CₙH₂ₙ₊₂, alkene CₙH₂ₙ, alkyne CₙH₂ₙ₋₂.
• Successive members of a homologous series differ by CH₂, i.e. 14 u of molecular mass.
• First members: methane CH₄, ethene C₂H₄, ethyne C₂H₂ (acetylene).
• Saturated hydrocarbons (alkanes) burn with a clean blue flame; unsaturated ones burn with a yellow sooty flame.
• Unsaturation is tested with bromine water, whose brown colour is decolourised by alkenes and alkynes.
RRB Technician Grade III 2024-25 – 28 December 2024 Shift 3 · Chemical properties of carbon compounds (combustion, oxidation, addition, substitution)

Q132. Methane, on burning with oxygen, gives which of the following products?

  1. (a) Carbon monoxide only
  2. (b) Carbonic acid
  3. (c) Carbon monoxide and water
  4. (d) Carbon dioxide and water
Answer & solution

Answer: (d) Carbon dioxide and water

Correct Answer: Option 4 — Carbon dioxide and water
Methane (CH₄) is the first member of the alkane (saturated hydrocarbon) series. Like every hydrocarbon, it burns in a sufficient supply of oxygen to give carbon dioxide and water, releasing a large amount of heat and light. The carbon of the fuel ends up as CO₂ and its hydrogen as H₂O — that is the general rule for the complete combustion of any hydrocarbon.
Methane is the main constituent of natural gas, CNG and biogas (gobar gas). Being a saturated hydrocarbon, it burns with a clean blue flame and leaves no soot, which makes it an excellent domestic fuel.
⚗️ The Reaction
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) + heat + light
• Balance check — C: 1 = 1, H: 4 = 4, O: 4 = 4. The coefficient 2 before O₂ and H₂O is essential.
• This is an oxidation (combustion) reaction and is strongly exothermic.
• In an insufficient supply of oxygen the combustion is incomplete and poisonous carbon monoxide (CO) plus soot are formed instead.
❌ Why the other options are incorrect
✗ Carbon monoxide only — CO appears only in incomplete combustion, and even then water is also formed, since the hydrogen of methane must go somewhere.
✗ Carbonic acid — H₂CO₃ is formed when CO₂ dissolves in water (as in soda water); it is not a product of burning.
✗ Carbon monoxide and water — these are the products of incomplete combustion in a limited supply of oxygen; the question specifies burning with oxygen, so the complete products are CO₂ and H₂O.
📌 SSC & Railway Exam Facts
• Complete combustion of a hydrocarbon → CO₂ + H₂O; incomplete combustion → CO + soot (C) + H₂O.
• Methane CH₄ is the first alkane; the series runs CH₄, C₂H₆ (ethane), C₃H₈ (propane), C₄H₁₀ (butane).
• CNG and biogas are mainly methane; LPG is mainly butane.
• CO is a poisonous gas — it binds to haemoglobin and prevents oxygen transport.
• Methane is a greenhouse gas and is also called marsh gas; the general alkane formula is CₙH₂ₙ₊₂.
RRB Technician Grade III 2024-25 – 28 December 2024 Shift 3 · Chemical properties of carbon compounds (combustion, oxidation, addition, substitution)

Q133. Burning of unsaturated compounds gives which of the following?

  1. (a) Green flame
  2. (b) Red flame
  3. (c) Yellow flame
  4. (d) Clean flame
Answer & solution

Answer: (c) Yellow flame

Correct Answer: Option 3 — Yellow flame
Unsaturated hydrocarbons are those containing a double bond (alkenes, CₙH₂ₙ) or a triple bond (alkynes, CₙH₂ₙ₋₂), such as ethene (C₂H₄) and ethyne (C₂H₂). Compared with saturated compounds they have a much higher percentage of carbon in the molecule.
When they burn in air, the oxygen available is not enough to oxidise all that carbon. The combustion is therefore incomplete, and unburnt carbon particles (soot) are left in the flame. These hot glowing carbon particles make the flame yellow and luminous, and they deposit black soot on any vessel held over it.
Saturated hydrocarbons (alkanes, CₙH₂ₙ₊₂) such as methane have relatively less carbon, burn completely and therefore give a clean blue flame with no soot — which is why LPG and CNG are good domestic fuels.
❌ Why the other options are incorrect
✗ Green flame — a green flame is a flame test colour given by the salts of copper and barium, not by burning hydrocarbons.
✗ Red flame — red or crimson is the flame test colour of strontium and lithium salts; it is unrelated to the saturation of a compound.
✗ Clean flame — a clean blue flame is given by saturated hydrocarbons, which burn completely; unsaturated ones do the opposite and smoke.
📌 SSC & Railway Exam Facts
• Saturated → clean blue flame; Unsaturated → yellow, sooty (luminous) flame.
• Alkane CₙH₂ₙ₊₂ (single bond), Alkene CₙH₂ₙ (double bond), Alkyne CₙH₂ₙ₋₂ (triple bond).
• A sooty flame is the simple test to tell an unsaturated compound from a saturated one; the chemical test is decolourisation of bromine water.
• A blackened cooking vessel means the fuel is burning incompletely owing to an insufficient air supply.
• Ethyne (acetylene, C₂H₂) burnt in pure oxygen gives the very hot oxy-acetylene flame (≈ 3000 °C) used for welding.
RRB Technician Grade III 2024-25 – 28 December 2024 Shift 3 · Soaps and detergents

Q134. Which of the following has two ends: one which is hydrophilic and the other which is hydrophobic?

  1. (a) Water
  2. (b) Cream
  3. (c) Perfume
  4. (d) Soap
Answer & solution

Answer: (d) Soap

Correct Answer: Option 4 — Soap
A soap is the sodium or potassium salt of a long-chain fatty acid, for example sodium stearate, C₁₇H₃₅COONa. Its molecule has two very different ends:
(i) a long hydrocarbon tail which is non-polar, water-repelling (hydrophobic) but oil-loving (oleophilic); and (ii) a short ionic head, −COO⁻Na⁺, which is polar and water-loving (hydrophilic). A molecule with both a water-loving and a water-hating end is described as amphipathic.
When soap is put in water, the molecules arrange themselves into clusters called micelles: the hydrophobic tails point inwards and trap the oil or grease of the dirt, while the hydrophilic heads face outwards into the water. The grease is thus held in the middle of the micelle and is carried away with the rinsing water — this is how soap cleans.
❌ Why the other options are incorrect
✗ Water — H₂O is a single polar molecule; it is hydrophilic all over and has no water-hating end, which is exactly why it alone cannot wash away grease.
✗ Cream — cream is a colloid (an emulsion) of fat in water, a mixture; it is not a single molecule with two distinct ends.
✗ Perfume — a solution of fragrant oils in alcohol; it evaporates quickly but its molecules have no hydrophilic–hydrophobic double nature.
📌 SSC & Railway Exam Facts
• Soap = sodium/potassium salt of a fatty acid; the cleaning cluster it forms is called a micelle.
• Saponification = heating a fat or oil with NaOH to give soap + glycerol.
• Soap does not lather well in hard water (which has Ca²⁺ and Mg²⁺ ions) and forms an insoluble scum; detergents work even in hard water.
• Hydrophilic = water-loving head; Hydrophobic = water-hating hydrocarbon tail.
• Detergents are ammonium or sulphonate salts of long-chain hydrocarbons and are used in shampoos and washing powders.
RRB Technician Grade III 2024-25 – 27 December 2024 Shift 1 · Hydrocarbons - alkanes, alkenes, alkynes

Q135. Which of the following is the correct formula for pentane?

  1. (a) C₅H₁₂
  2. (b) C₅H₉
  3. (c) C₅H₁₄
  4. (d) C₅H₁₀
Answer & solution

Answer: (a) C₅H₁₂

Correct Answer: Option 1 — C₅H₁₂
Pentane ends in -ane, so it belongs to the alkane family — the saturated hydrocarbons, in which every carbon–carbon bond is a single bond. The root pent- tells us there are 5 carbon atoms.
🔢 Calculation
• General formula of an alkane: CₙH₂ₙ₊₂
• Here n = 5
• Hydrogen atoms = (2 × 5) + 2 = 12
• Formula of pentane = C₅H₁₂
Pentane is a colourless, volatile liquid and is one of the components of petrol. It has three structural isomers — n-pentane, iso-pentane and neo-pentane — the first member of the alkane series to show branched-chain isomerism along with butane.
❌ Why the other options are incorrect
✗ C₅H₉ — this does not fit any of the three general formulae; CₙH₂ₙ₊₂ = C₅H₁₂, CₙH₂ₙ = C₅H₁₀, CₙH₂ₙ₋₂ = C₅H₈. C₅H₉ would be a free radical, not a stable hydrocarbon.
✗ C₅H₁₄ — carbon's valency is 4, and a 5-carbon chain cannot hold 14 hydrogen atoms; the maximum possible is 12.
✗ C₅H₁₀ — this is CₙH₂ₙ, which is pentene, an unsaturated alkene with one double bond, not pentane.
📌 SSC & Railway Exam Facts
• Alkanes: CH₄ methane, C₂H₆ ethane, C₃H₈ propane, C₄H₁₀ butane, C₅H₁₂ pentane, C₆H₁₄ hexane.
• Alkanes CₙH₂ₙ₊₂, Alkenes CₙH₂ₙ, Alkynes CₙH₂ₙ₋₂.
• Successive members of a homologous series differ by −CH₂− and by 14 u in molecular mass.
• Butane (C₄H₁₀) is the first alkane to show chain isomerism; pentane has 3 isomers.
• LPG is mainly butane, CNG is mainly methane and petrol is a mixture of C₅–C₁₂ hydrocarbons.
RRB Technician Grade III 2024-25 – 27 December 2024 Shift 2 · Hydrocarbons - alkanes, alkenes, alkynes

Q136. Which of the following is a saturated carbon compound?

  1. (a) Alkane
  2. (b) Alkyne
  3. (c) Alkene
  4. (d) Ethylene
Answer & solution

Answer: (a) Alkane

Correct Answer: Option 1 — Alkane
A carbon compound is called saturated when every carbon–carbon linkage in it is a single covalent bond (C–C), so each carbon holds the maximum possible number of hydrogen atoms and the molecule cannot take up any more. Such hydrocarbons are the alkanes, general formula CₙH₂ₙ₊₂ — CH₄ (methane), C₂H₆ (ethane), C₃H₈ (propane), C₄H₁₀ (butane).
Compounds with a double bond (C=C) or a triple bond (C≡C) are unsaturated, because two carbons are sharing extra electron pairs instead of bonding to hydrogen; such molecules can add hydrogen and become saturated.
Saturated and unsaturated compounds are told apart in the laboratory by their flame: alkanes burn with a clean blue flame, while unsaturated hydrocarbons burn with a yellow, sooty flame because of their higher percentage of carbon. Unsaturated compounds also decolourise bromine water, whereas alkanes do not — the standard test.
❌ Why the other options are incorrect
✗ Alkyne — an unsaturated hydrocarbon containing a carbon–carbon triple bond (C≡C), general formula CₙH₂ₙ₋₂, e.g. ethyne, C₂H₂.
✗ Alkene — an unsaturated hydrocarbon containing a carbon–carbon double bond (C=C), general formula CₙH₂ₙ, e.g. ethene, C₂H₄.
✗ Ethylene — this is simply the common name of ethene (C₂H₄), an alkene; it carries a double bond and so is unsaturated, not saturated.
📌 SSC & Railway Exam Facts
• General formulae: alkane CₙH₂ₙ₊₂, alkene CₙH₂ₙ, alkyne CₙH₂ₙ₋₂.
• Alkanes are also called paraffins; their typical reaction is substitution (with Cl₂ in sunlight).
• Unsaturated hydrocarbons give addition reactions and decolourise bromine water — alkanes do not.
• Hydrogenation of oil (vegetable oil + H₂ over nickel catalyst) converts unsaturated oil into saturated vanaspati ghee.
• Ethylene (ethene) is the gas used for the artificial ripening of fruits.
RRB Technician Grade III 2024-25 – 27 December 2024 Shift 2 · Chemical properties of carbon compounds (combustion, oxidation, addition, substitution)

Q137. Which of the following is used in the oxidation of alcohol into carboxylic acid?

  1. (a) Zinc
  2. (b) Alkaline potassium permanganate
  3. (c) Palladium
  4. (d) Nickel
Answer & solution

Answer: (b) Alkaline potassium permanganate

Correct Answer: Option 2 — Alkaline potassium permanganate
An alcohol can be converted into the corresponding carboxylic acid by oxidation, which means adding oxygen to the –OH group. The reagents that supply this oxygen are called oxidising agents, and the two used in the NCERT course are alkaline potassium permanganate (alkaline KMnO₄) and acidified potassium dichromate (acidified K₂Cr₂O₇).
When ethanol is warmed with alkaline KMnO₄, the purple colour of the permanganate disappears as it is consumed, and ethanoic acid (acetic acid) is formed. Because these reagents add oxygen to another substance, they are also called oxidising agents in a general sense.
⚗️ The Reaction
CH₃CH₂OH → (alkaline KMnO₄ + heat) CH₃COOH
• Ethanol (CH₃CH₂OH) is oxidised to ethanoic acid (CH₃COOH) — the –CH₂OH group becomes –COOH.
• The same change is brought about by acidified K₂Cr₂O₇, where the orange dichromate turns green.
• A 5–8% solution of ethanoic acid in water is vinegar, and pure ethanoic acid freezes in cold weather to give glacial acetic acid.
❌ Why the other options are incorrect
✗ Zinc — zinc is a reducing agent, not an oxidising agent; with acids it liberates H₂ gas (Zn + 2HCl → ZnCl₂ + H₂).
✗ Palladium — Pd is a hydrogenation catalyst, used to add hydrogen across double bonds — the opposite of oxidation.
✗ Nickel — Ni is the catalyst used in the hydrogenation of vegetable oils into vanaspati ghee; it reduces, it does not oxidise.
📌 SSC & Railway Exam Facts
• Oxidising agents for alcohols: alkaline KMnO₄ and acidified K₂Cr₂O₇.
• KMnO₄ is purple and gets decolourised; K₂Cr₂O₇ is orange and turns green.
• Ethanol → ethanoic acid on oxidation; hence alcohols are said to undergo oxidation to acids.
• Vinegar = 5–8% ethanoic acid; glacial acetic acid freezes at about 17 °C.
• Catalysts for addition (hydrogenation): Ni, Pd, Pt.
RRB Technician Grade III 2024-25 – 27 December 2024 Shift 2 · Chemical properties of carbon compounds (combustion, oxidation, addition, substitution)

Q138. Which of the following pairs of the name reaction and its example is/are correctly matched?
(a) Saponification: The reaction of ethanol with conc. H₂SO₄ to form ethene.
(b) Esterification: The reaction of ethanoic acid with ethanol in the presence of acid catalyst to form ester.
(c) Dehydration: The reaction of esters with sodium hydroxide to form sodium salt of carboxylic acid.

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

Answer: (d) Only (b) is correct.

Correct Answer: Option 4 — Only (b) is correct.
The paper has swapped the names of (a) and (c). Statement (b) alone names its reaction correctly.
(b) Esterification is correct. A carboxylic acid heated with an alcohol in the presence of a few drops of conc. H₂SO₄ (acid catalyst) gives a sweet-smelling ester. Esters give fruits their fragrance and are used in perfumes and flavouring agents.
(a) is wrongly named. Heating ethanol with conc. H₂SO₄ at 443 K removes a molecule of water and gives ethene — that is a dehydration reaction, and conc. H₂SO₄ acts as a dehydrating agent, not saponification.
(c) is wrongly named. Heating an ester with sodium hydroxide splits it back into the sodium salt of the carboxylic acid and an alcohol — that is saponification, the very reaction by which soap is made, not dehydration.
⚗️ The Reactions
CH₃COOH + C₂H₅OH → (conc. H₂SO₄) CH₃COOC₂H₅ + H₂O (esterification)
CH₃CH₂OH → (conc. H₂SO₄, 443 K) CH₂=CH₂ + H₂O (dehydration)
CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH (saponification)
• Esterification is reversible; saponification is its alkaline hydrolysis, the reverse step.
• In soap making, the ester is a fat or oil and the product sodium salt of a fatty acid is the soap.
❌ Why the other options are incorrect
✗ Only (a) is correct — (a) actually describes dehydration, so it is mismatched; and it wrongly leaves out the correct pair (b).
✗ (a), (b) and (c) are correct — only (b) is correctly matched; the names in (a) and (c) have been interchanged.
✗ Only (c) is correct — (c) actually describes saponification, not dehydration, so it is mismatched.
📌 SSC & Railway Exam Facts
• Ester + NaOH → soap + alcohol is called saponification.
• Acid + alcohol → ester + water (conc. H₂SO₄ catalyst) is esterification; esters smell sweet.
• Conc. H₂SO₄ is a dehydrating agent; ethanol at 443 K gives ethene.
• Soap = sodium/potassium salt of a long-chain fatty acid; it cleans by forming micelles.
• Ethanol at 373 K with conc. H₂SO₄ gives ether, at 443 K it gives ethene — a common trap.
RRB Technician Grade III 2024-25 – 27 December 2024 Shift 2 · Functional groups

Q139. Identify the nature of the functional group present in butanal.

  1. (a) Ketone
  2. (b) Alcohol
  3. (c) Aldehydes
  4. (d) Alkene
Answer & solution

Answer: (c) Aldehydes

Correct Answer: Option 3 — Aldehydes
In the IUPAC system the suffix of the name tells you the functional group. The suffix “-al” always stands for an aldehyde, whose functional group is –CHO (the carbonyl group attached to a hydrogen atom).
Butanal therefore has a four-carbon chain (but-) that is saturated (-an-) and ends in an aldehyde group (-al). Its formula is CH₃CH₂CH₂CHO (C₄H₈O). Note that the –CHO group is always at the end of the chain, so it is automatically carbon number 1.
The other common suffixes are worth memorising together: -ol → alcohol (–OH), -one → ketone (>C=O), -oic acid → carboxylic acid (–COOH), -ene → alkene (C=C) and -yne → alkyne (C≡C).
❌ Why the other options are incorrect
✗ Ketone — a ketone ends in “-one” (e.g. propanone, CH₃COCH₃) and its >C=O group sits between two carbon atoms, never at the chain end.
✗ Alcohol — an alcohol ends in “-ol” (e.g. butanol, C₄H₉OH) and contains the hydroxyl group –OH, not –CHO.
✗ Alkene — an alkene ends in “-ene” (e.g. butene, C₄H₈) and is defined by a carbon–carbon double bond, which is a bond type, not an oxygen-containing group.
📌 SSC & Railway Exam Facts
• Suffix key: -ol = alcohol, -al = aldehyde, -one = ketone, -oic acid = carboxylic acid.
• Functional groups: alcohol –OH, aldehyde –CHO, ketone >C=O, acid –COOH.
• The smallest aldehyde is methanal (HCHO, formaldehyde); its 40% solution is formalin, used to preserve specimens.
• Butanal = CH₃CH₂CH₂CHO; the –CHO group is always at carbon 1 of the chain.
• Aldehydes and ketones both contain the carbonyl (>C=O) group — the difference is its position in the chain.
RRB Technician Grade III 2024-25 – 27 December 2024 Shift 2 · Isomerism

Q140. Which of the following is a property of isomers in organic chemistry?

  1. (a) They have different molecular formulas but the same structural arrangement.
  2. (b) They have the same molecular formula but different structural arrangements.
  3. (c) They have the same molecular formula and the same structural arrangement.
  4. (d) They have no carbon atoms in their structure.
Answer & solution

Answer: (b) They have the same molecular formula but different structural arrangements.

Correct Answer: Option 2 — They have the same molecular formula but different structural arrangements.
Isomers are compounds that have the same molecular formula — the same number of atoms of each element — but a different arrangement of those atoms, and therefore different physical and chemical properties. The phenomenon itself is called isomerism.
The classic school example is butane, C₄H₁₀, which exists in two forms: n-butane, a straight chain of four carbons (CH₃–CH₂–CH₂–CH₃), and isobutane (2-methylpropane), a branched chain (CH₃–CH(CH₃)–CH₃). Both contain 4 carbon and 10 hydrogen atoms, yet n-butane boils at about −0.5 °C and isobutane at about −12 °C.
Isomerism is a direct consequence of carbon's tetravalency and its power of catenation — its ability to link with other carbon atoms in straight chains, branched chains and rings. This is a major reason why there are millions of carbon compounds. The first three alkanes — methane, ethane and propane — have no isomers; isomerism starts from butane (C₄H₁₀).
❌ Why the other options are incorrect
✗ They have different molecular formulas but the same structural arrangement — this reverses the definition; a different molecular formula makes them entirely different compounds, not isomers.
✗ They have the same molecular formula and the same structural arrangement — identical formula and identical structure means it is the same compound, not a pair of isomers.
✗ They have no carbon atoms in their structure — isomerism is discussed here for organic (carbon) compounds; the option contradicts the very subject of the question.
📌 SSC & Railway Exam Facts
• Isomers = same molecular formula, different structure (and hence different properties).
• C₄H₁₀ has 2 isomers, C₅H₁₂ has 3 isomers (n-, iso- and neo-pentane).
• Methane, ethane and propane have no isomers; isomerism begins at butane.
• Catenation (self-linking) and tetravalency of carbon explain the huge number of organic compounds.
• Isotopes, isobars, isotones belong to atomic structure; isomers belong to organic chemistry — do not mix them up.
RRB Technician Grade III 2024-25 – 27 December 2024 Shift 2 · Homologous series

Q141. Among the following which is NOT a homologous compound as compared to others?

  1. (a) C₆H₅OH
  2. (b) C₂H₅OH
  3. (c) CH₃OH
  4. (d) C₃H₇OH
Answer & solution

Answer: (a) C₆H₅OH

Correct Answer: Option 1 — C₆H₅OH
A homologous series is a family of organic compounds that have the same functional group, fit a single general formula, and in which each member differs from the next by one –CH₂– unit (14 u of molecular mass). Members show a gradual gradation in physical properties but similar chemical properties.
The alcohol series has the general formula CₙH₂ₙ₊₁OH: CH₃OH (methanol, n = 1), C₂H₅OH (ethanol, n = 2) and C₃H₇OH (propanol, n = 3) all fit it perfectly and differ from one another by exactly –CH₂–, so these three are true homologues.
C₆H₅OH is phenol, in which the –OH group is attached directly to a benzene ring (an aromatic ring), not to an open-chain alkyl group. It does not obey CₙH₂ₙ₊₁OH (for n = 6 the alcohol would be C₆H₁₃OH), so it belongs to a different series and is the odd one out.
❌ Why the other options are incorrect
✗ C₂H₅OH — ethanol, the second member of the alcohol series; it differs from CH₃OH by exactly one –CH₂–, so it is a homologue.
✗ CH₃OH — methanol, the first member of the same series; it fits CₙH₂ₙ₊₁OH with n = 1.
✗ C₃H₇OH — propanol, the third member; it differs from ethanol by one –CH₂– and fits the general formula with n = 3.
📌 SSC & Railway Exam Facts
• In a homologous series, successive members differ by –CH₂–, i.e. by 14 u of molecular mass.
• General formulae: alkane CₙH₂ₙ₊₂, alkene CₙH₂ₙ, alkyne CₙH₂ₙ₋₂, alcohol CₙH₂ₙ₊₁OH.
• CH₃OH = methanol (wood spirit) — highly poisonous, causes blindness; C₂H₅OH = ethanol, present in alcoholic drinks.
• C₆H₅OH = phenol (carbolic acid), an aromatic compound used as a disinfectant.
• Members of a homologous series have similar chemical properties but gradually changing physical properties.
RRB Technician Grade III 2024-25 – 27 December 2024 Shift 3 · Homologous series

Q142. A series of compounds in which the same functional group substitutes for hydrogen in a carbon chain is called __________.

  1. (a) isobars
  2. (b) isochores
  3. (c) isomers
  4. (d) homologous series
Answer & solution

Answer: (d) homologous series

Correct Answer: Option 4 — homologous series
A homologous series is a family of organic compounds in which the same functional group replaces hydrogen in a carbon chain, so that all the members share one general formula and similar chemical properties. Successive members differ from one another by a single –CH₂– unit, that is, by 14 u of molecular mass.
For example, replacing one hydrogen of an alkane by the –OH group builds the alcohol series, CₙH₂ₙ₊₁OH: CH₃OH (methanol) → C₂H₅OH (ethanol) → C₃H₇OH (propanol). In the same way the –COOH group builds the carboxylic acid series, and –CHO the aldehyde series.
Within a series, the chemical properties stay similar because the reacting part — the functional group — is unchanged, while the physical properties (melting point, boiling point, density) change gradually as the chain lengthens and the molecular mass rises.
❌ Why the other options are incorrect
✗ isobars — a term from atomic structure, not organic chemistry: atoms of different elements with different atomic numbers but the same mass number, e.g. ₂₀Ca⁴⁰ and ₁₈Ar⁴⁰.
✗ isochores — a term from physics/thermodynamics for a process at constant volume; it has nothing to do with carbon chains.
✗ isomers — compounds with the same molecular formula but different structures, e.g. n-butane and isobutane (C₄H₁₀); they are a single pair of compounds, not a series built by one functional group.
📌 SSC & Railway Exam Facts
• Successive homologues differ by –CH₂–, i.e. by 14 u of molecular mass.
• General formulae: alkane CₙH₂ₙ₊₂, alkene CₙH₂ₙ, alkyne CₙH₂ₙ₋₂, alcohol CₙH₂ₙ₊₁OH.
• In a homologous series chemical properties are similar, physical properties change gradually.
• Functional groups: –OH alcohol, –CHO aldehyde, >C=O ketone, –COOH carboxylic acid.
• Isomers = same formula, different structure; homologues = different formula, same functional group — a classic PYQ contrast.
RRB Technician Grade III 2024-25 – 27 December 2024 Shift 3 · Hydrocarbons - alkanes, alkenes, alkynes

Q143. Which of the following statements regarding saturated and unsaturated carbon compounds is correct?
Statement I: Saturated carbon compounds are hydrocarbons where carbon atoms are connected by single covalent bonds.
Statement II: Unsaturated carbon compounds are hydrocarbons where carbon atoms are connected by single covalent bonds.

  1. (a) Neither statement I nor II is correct.
  2. (b) Both statements I and II are correct.
  3. (c) Only statement II is correct.
  4. (d) Only statement I is correct.
Answer & solution

Answer: (d) Only statement I is correct.

Correct Answer: Option 4 — Only statement I is correct.
Statement I is correct. A saturated carbon compound is one in which the carbon atoms are joined only by single covalent bonds (C–C), so every carbon carries the maximum number of hydrogen atoms it can. These are the alkanes, CₙH₂ₙ₊₂ — CH₄, C₂H₆, C₃H₈ and so on.
Statement II is incorrect. An unsaturated compound is defined by exactly the opposite feature — it contains at least one carbon–carbon double bond (C=C) or triple bond (C≡C). These are the alkenes, CₙH₂ₙ (e.g. ethene, C₂H₄) and the alkynes, CₙH₂ₙ₋₂ (e.g. ethyne, C₂H₂). The statement has simply repeated the definition of a saturated compound.
The two families are told apart experimentally: unsaturated compounds decolourise bromine water and give addition reactions with hydrogen over Ni/Pd/Pt, whereas saturated compounds do neither and instead give substitution reactions. Saturated hydrocarbons also burn with a clean blue flame, while unsaturated ones burn with a yellow, sooty flame.
❌ Why the other options are incorrect
✗ Neither statement I nor II is correct — statement I is a textbook-correct definition of a saturated compound, so this cannot be right.
✗ Both statements I and II are correct — they cannot both be true, since they give the same definition to two opposite terms.
✗ Only statement II is correct — statement II is precisely the one that is wrong; unsaturated compounds need a double or triple bond.
📌 SSC & Railway Exam Facts
• Saturated = single bonds only (alkanes); unsaturated = double or triple bond (alkenes, alkynes).
• General formulae: alkane CₙH₂ₙ₊₂, alkene CₙH₂ₙ, alkyne CₙH₂ₙ₋₂.
• Bromine water test: unsaturated compounds decolourise the brown-orange colour; alkanes do not.
• Alkanes burn with a blue flame; unsaturated hydrocarbons give a yellow, sooty flame.
• Alkanes react by substitution; alkenes and alkynes react by addition (hydrogenation over Ni).
RRB Technician Grade III 2024-25 – 26 December 2024 Shift 1 · Homologous series

Q144. CH₃OH, C₂H₅OH, C₃H₇OH and C₄H₉OH compounds exhibit almost similar chemical properties because they belong to:

  1. (a) a heterologous series
  2. (b) an electrochemical series
  3. (c) a homologous series
  4. (d) a periodic series
Answer & solution

Answer: (c) a homologous series

Correct Answer: Option 3 — a homologous series
A homologous series is a group of organic compounds having the same functional group, the same general formula, and in which each successive member differs from the previous one by a −CH₂− unit (14 u in mass).
All four compounds given — CH₃OH, C₂H₅OH, C₃H₇OH, C₄H₉OH — contain the alcohol group −OH and fit the general formula CₙH₂ₙ₊₁OH:
• CH₃OH (methanol), C₂H₅OH (ethanol), C₃H₇OH (propanol), C₄H₉OH (butanol).
• Consecutive members differ by CH₂, i.e. by 14 u in molecular mass.
Because chemical properties are decided by the functional group, every member of the series shows almost identical chemical behaviour. Their physical properties, however, such as boiling point and density, change gradually as the chain grows longer.
❌ Why the other options are incorrect
✗ a heterologous series — not a recognised term in organic chemistry; it is coined only to look like the right answer.
✗ an electrochemical series — this is the arrangement of metals/electrodes by their standard electrode potential, used in electrochemistry, not for organic compounds.
✗ a periodic series — the periodic table arranges elements by atomic number; it does not classify compounds such as alcohols.
📌 SSC & Railway Exam Facts
• Alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ, alkynes CₙH₂ₙ₋₂, alcohols CₙH₂ₙ₊₁OH.
• Successive homologues differ by −CH₂−, i.e. 14 u.
• Functional groups: alcohol −OH, aldehyde −CHO, ketone >C=O, carboxylic acid −COOH.
• CH₃OH = methanol (wood spirit, poisonous); C₂H₅OH = ethanol (drinking alcohol).
• Members of a homologous series have similar chemical but gradually changing physical properties.
RRB Technician Grade III 2024-25 – 26 December 2024 Shift 2 · Hydrocarbons - alkanes, alkenes, alkynes

Q145. Compounds of carbon, which are linked by only single bonds between the carbon atoms are called ___________.

  1. (a) Ionic compounds
  2. (b) Both ionic and saturated compounds
  3. (c) Unsaturated compounds
  4. (d) Saturated compounds
Answer & solution

Answer: (d) Saturated compounds

Correct Answer: Option 4 — Saturated compounds
Carbon compounds in which all the carbon–carbon bonds are single covalent bonds are called saturated compounds. The word "saturated" means each carbon has used all four of its valencies to hold the maximum possible number of hydrogen atoms, so no more atoms can be added to the chain.
The saturated hydrocarbons form the alkane family, whose general formula is CₙH₂ₙ₊₂ — for example CH₄ (methane), C₂H₆ (ethane), C₃H₈ (propane) and C₄H₁₀ (butane).
Saturated hydrocarbons are chemically less reactive, they undergo substitution reactions (with chlorine in sunlight), and they burn with a clean blue flame because combustion is complete.
❌ Why the other options are incorrect
✗ Ionic compounds — these are formed by the transfer of electrons between a metal and a non-metal, e.g. NaCl. Carbon compounds are covalent, formed by the sharing of electrons.
✗ Both ionic and saturated compounds — half of the statement is wrong, since carbon does not normally form ionic bonds; carbon has 4 valence electrons and gaining or losing four would need too much energy.
✗ Unsaturated compounds — these contain at least one double bond (alkenes, CₙH₂ₙ) or triple bond (alkynes, CₙH₂ₙ₋₂), such as C₂H₄ (ethene) and C₂H₂ (ethyne); they are more reactive and give addition reactions.
📌 SSC & Railway Exam Facts
• Alkanes CₙH₂ₙ₊₂ (saturated), Alkenes CₙH₂ₙ, Alkynes CₙH₂ₙ₋₂ (both unsaturated).
• Saturated hydrocarbons burn with a blue flame; unsaturated ones burn with a yellow sooty flame.
• Unsaturation is tested by decolourising bromine water or alkaline KMnO₄.
• Catenation — carbon's ability to link with itself in long chains, rings and branches — gives rise to millions of organic compounds.
• The first four alkanes are methane, ethane, propane, butane; CNG and LPG are mainly saturated hydrocarbons.
RRB Technician Grade III 2024-25 – 26 December 2024 Shift 3 · IUPAC nomenclature

Q146. What would be the name of hydrocarbon having three carbon chain with a double bond?

  1. (a) Propyne
  2. (b) Propane
  3. (c) Propanone
  4. (d) Propene
Answer & solution

Answer: (d) Propene

Correct Answer: Option 4 — Propene
In the IUPAC system the name of a hydrocarbon is built from two parts — a root word that tells the number of carbon atoms, and a suffix that tells the kind of bond present.
• Three carbon atoms → root word prop-
• Only single bonds → alkane, suffix -ane
• One double bond → alkene, suffix -ene
• One triple bond → alkyne, suffix -yne
A chain of three carbons carrying one double bond is therefore prop + ene = propene, whose formula is C₃H₆. It belongs to the alkene family with general formula CₙH₂ₙ, so for n = 3 we get C₃H₆, and it is an unsaturated hydrocarbon.
❌ Why the other options are incorrect
✗ Propyne — the suffix -yne means a triple bond, so propyne is C₃H₄, an alkyne, not an alkene.
✗ Propane — the suffix -ane means all single bonds; propane is the saturated hydrocarbon C₃H₈, the common LPG gas.
✗ Propanone — the ending -one is the ketone functional group; propanone is CH₃COCH₃ (acetone), not a plain hydrocarbon.
📌 SSC & Railway Exam Facts
• Root words: meth- (1), eth- (2), prop- (3), but- (4), pent- (5), hex- (6).
• Alkanes CₙH₂ₙ₊₂, Alkenes CₙH₂ₙ, Alkynes CₙH₂ₙ₋₂.
• Ethene (C₂H₄) is used for the artificial ripening of fruits; ethyne (C₂H₂) is the oxy-acetylene welding gas.
• Functional group suffixes: -ol alcohol, -al aldehyde, -one ketone, -oic acid carboxylic acid.
• Unsaturated hydrocarbons decolourise bromine water; saturated ones do not.
RRB Technician Grade III 2024-25 – 26 December 2024 Shift 3 · Ethanol and ethanoic acid

Q147. Which of the following compounds is used in tincture iodine?

  1. (a) Ethanoic acid
  2. (b) Propanol
  3. (c) Ethanol
  4. (d) Methanol
Answer & solution

Answer: (c) Ethanol

Correct Answer: Option 3 — Ethanol
Tincture iodine is a solution of iodine in alcohol, specifically a mixture of iodine dissolved in ethanol and water. It is a well-known antiseptic, applied to cuts and wounds to kill germs.
Ethanol (C₂H₅OH) is chosen as the solvent because iodine is only slightly soluble in water but dissolves readily in ethanol, and ethanol itself is a good antiseptic and is not poisonous in the small amounts involved. A "tincture" in pharmacy always means a medicinal solution prepared in alcohol.
Ethanol is a colourless liquid with a boiling point of 78 °C, is the alcohol present in all alcoholic drinks, and belongs to the alcohol family with the −OH functional group.
❌ Why the other options are incorrect
✗ Ethanoic acid — this is CH₃COOH, acetic acid; its 5–8% solution in water is vinegar, used to preserve food, not as the solvent of tincture iodine.
✗ Propanol — C₃H₇OH is a laboratory and industrial solvent, but tincture iodine is standardised with ethanol, not propanol.
✗ Methanol — CH₃OH, wood spirit, is highly poisonous; even small amounts cause blindness and death, so it can never be used on skin or in medicine.
📌 SSC & Railway Exam Facts
• Ethanol C₂H₅OH — b.p. 78 °C; Methanol CH₃OH — b.p. 65 °C, extremely toxic.
• Denatured alcohol is ethanol made unfit for drinking by adding methanol and a blue dye.
• Ethanol + conc. H₂SO₄ at 170 °C → ethene, a dehydration reaction.
• Ethanol + Na → sodium ethoxide + H₂; it is the test that shows the −OH group.
• Ethanoic acid has a melting point of 17 °C, so it freezes in cold weather and is called glacial acetic acid.
RRB Technician Grade III 2024-25 – 26 December 2024 Shift 3 · Soaps and detergents

Q148. Which of the following statements is correct about soaps and detergents?

  1. (a) Soaps molecules have two ends – one is hydrophilic and the other end is hydrophobic.
  2. (b) Hydrophobic interacts with water molecules.
  3. (c) Hydrophilic ends interact with hydrocarbons.
  4. (d) Soap molecules have no ends.
Answer & solution

Answer: (a) Soaps molecules have two ends – one is hydrophilic and the other end is hydrophobic.

Correct Answer: Option 1 — Soaps molecules have two ends – one is hydrophilic and the other end is hydrophobic.
A soap molecule is the sodium or potassium salt of a long-chain fatty acid, for example C₁₇H₃₅COONa (sodium stearate). Every such molecule has two distinctly different ends:
• A long hydrocarbon tail which is hydrophobic (water-repelling) but oil-loving — it dissolves in grease and dirt.
• An ionic head, −COO⁻Na⁺, which is hydrophilic (water-loving) — it dissolves in water.
This dual nature is precisely what makes cleaning possible. When soap is added to dirty cloth, the hydrophobic tails bury themselves in the oily dirt while the hydrophilic heads stay in the water. The soap molecules arrange themselves into a spherical cluster called a micelle, with the dirt trapped inside. Agitation then lifts the micelles away and the dirt is rinsed off.
❌ Why the other options are incorrect
✗ Hydrophobic interacts with water molecules — the opposite is true; "hydrophobic" literally means water-fearing, and this end interacts with oil and grease.
✗ Hydrophilic ends interact with hydrocarbons — the hydrophilic ionic head interacts with water; it is the hydrocarbon tail that mixes with hydrocarbons.
✗ Soap molecules have no ends — if a soap molecule had no two distinct ends, no micelle could form and soap could not clean at all.
📌 SSC & Railway Exam Facts
• Soap = sodium/potassium salt of a long-chain carboxylic (fatty) acid; the process of making it is saponification.
• Soaps do not lather in hard water because Ca²⁺ and Mg²⁺ form an insoluble scum; detergents work even in hard water.
• Detergents are ammonium or sulphonate salts of long-chain hydrocarbons.
• The micelle forms in water but not in ethanol, since the tails would dissolve in ethanol.
• Detergents with branched chains are non-biodegradable and cause water pollution and foaming in rivers.
RRB Technician Grade III 2024-25 – 24 December 2024 Shift 1 · Chemical properties of carbon compounds (combustion, oxidation, addition, substitution)

Q149. The chemical reaction that carries hydrogenation of vegetable oils in the presence of Ni catalyst is called:

  1. (a) addition reaction
  2. (b) elimination reaction
  3. (c) rearrangement reaction
  4. (d) combustion reaction
Answer & solution

Answer: (a) addition reaction

Correct Answer: Option 1 — addition reaction
Vegetable oils are unsaturated — their carbon chains contain double bonds (C=C). When such an unsaturated compound reacts with hydrogen in the presence of a catalyst, the hydrogen atoms simply add across the double bond and nothing is removed. A reaction in which two reactants combine and no atom leaves the molecule is called an addition reaction.
⚗️ The Reaction
Vegetable oil (unsaturated) + H₂ → (Ni catalyst) Vanaspati ghee (saturated)
• The double bonds are converted into single bonds, so the liquid oil becomes a semi-solid fat.
• Nickel (Ni) acts as the catalyst — it speeds up the reaction without being consumed. Palladium (Pd) and platinum (Pt) can also be used.
• This is exactly how vanaspati ghee and margarine are manufactured from oils.
❌ Why the other options are incorrect
✗ elimination reaction — here a small molecule is removed from a compound, the opposite of what happens in hydrogenation.
✗ rearrangement reaction — atoms of the same molecule reorganise to form an isomer; no hydrogen is added at all.
✗ combustion reaction — burning in oxygen to give CO₂, H₂O, heat and light; no oxygen is involved in hydrogenation.
📌 SSC & Railway Exam Facts
• Unsaturated compounds give addition reactions; saturated ones give substitution reactions.
• Hydrogenation of oils uses a nickel (Ni) catalyst; the product is vanaspati ghee.
• Doctors advise unsaturated (vegetable) oils over saturated fats for a healthy heart.
• Bromine water test: unsaturated compounds decolourise brown bromine water by addition.
• A catalyst changes the rate of a reaction without itself being consumed.
RRB Technician Grade III 2024-25 – 24 December 2024 Shift 2 · Chemical properties of carbon compounds (combustion, oxidation, addition, substitution)

Q150. Carbon monoxide reacts with hydrogen molecule to yield which of the following compounds?

  1. (a) CH₄(g) + O₂(g)
  2. (b) H₂O(l)
  3. (c) CH₃OH(l)
  4. (d) C(s) + H₂O(l)
Answer & solution

Answer: (c) CH₃OH(l)

Correct Answer: Option 3 — CH₃OH(l)
When carbon monoxide (CO) reacts with hydrogen (H₂) under pressure in the presence of a catalyst, the product is methanol (methyl alcohol, CH₃OH). The 1 : 2 mixture of CO and H₂ used for this is called synthesis gas; the industrial mixture of CO + H₂ is also known as water gas.
⚗️ The Reaction
CO(g) + 2H₂(g) → (ZnO–Cr₂O₃, 300 °C, 200 atm) CH₃OH(l)
• The reaction is a combination (addition) of hydrogen to carbon monoxide and needs a zinc oxide–chromium oxide catalyst at high temperature and pressure.
• Check the balance: 1 C, 1 O and 4 H on each side — the equation is balanced.
• Methanol belongs to the alcohol family, whose functional group is −OH.
❌ Why the other options are incorrect
✗ CH₄(g) + O₂(g) — these are the reactants for the combustion of methane, not products of this reaction; no free oxygen is produced when CO adds hydrogen.
✗ H₂O(l) — water alone would need the carbon of CO to be left over as free carbon; hydrogen adds to CO instead of merely stripping its oxygen.
✗ C(s) + H₂O(l) — this would mean CO is completely reduced to carbon; industrially, hydrogen builds an organic molecule with CO rather than knocking the carbon out.
📌 SSC & Railway Exam Facts
• Water gas = CO + H₂; producer gas = CO + N₂; synthesis gas is the CO + H₂ feed for methanol.
• Methanol (CH₃OH) is called wood spirit; it is poisonous and can cause blindness or death.
• Ethanol (C₂H₅OH) is the alcohol of drinks; it is made unfit to drink by adding methanol — denatured alcohol.
• Carbon monoxide is a colourless, odourless, poisonous gas that binds haemoglobin far more strongly than oxygen.
• Alcohols end in −ol and contain the functional group −OH.
RRB Technician Grade III 2024-25 – 24 December 2024 Shift 2 · Soaps and detergents

Q151. Which of the following is a common drawback of using soap in hard water?

  1. (a) Soap dissolves too quickly in hard water.
  2. (b) Soap creates too much lather.
  3. (c) Formation of soap scum due to the reaction with calcium and magnesium ions.
  4. (d) Soap loses its colour in hard water.
Answer & solution

Answer: (c) Formation of soap scum due to the reaction with calcium and magnesium ions.

Correct Answer: Option 3 — Formation of soap scum due to the reaction with calcium and magnesium ions.
Hard water contains dissolved salts of calcium (Ca²⁺) and magnesium (Mg²⁺). A soap is the sodium or potassium salt of a long-chain fatty acid, e.g. sodium stearate, C₁₇H₃₅COONa. In hard water the Na⁺ of the soap is replaced by Ca²⁺ or Mg²⁺, and the calcium/magnesium salt formed is insoluble. It separates as a sticky, curdy white precipitate called scum.
⚗️ The Reaction
2C₁₇H₃₅COONa(aq) + Ca²⁺(aq) → (C₁₇H₃₅COO)₂Ca(s) + 2Na⁺(aq)
• The (s) shows the calcium salt is an insoluble precipitate — this is the scum.
• Soap keeps getting used up in removing the hardness, so very little lather forms and cleaning is poor; the scum also sticks to clothes.
❌ Why the other options are incorrect
✗ Soap dissolves too quickly in hard water. — the opposite is true; soap is precipitated out and becomes less available in solution.
✗ Soap creates too much lather. — in hard water soap gives very poor lather until all the Ca²⁺ and Mg²⁺ ions have been removed.
✗ Soap loses its colour in hard water. — colour has nothing to do with hardness; the real problem is a chemical reaction producing scum.
📌 SSC & Railway Exam Facts
• Detergents lather well in hard water because their calcium and magnesium salts are soluble.
• Soap is made by saponification — heating oil or fat with NaOH.
• A soap molecule has a hydrophilic (−COONa) head and a hydrophobic hydrocarbon tail; in water it forms micelles.
• Temporary hardness (bicarbonates) is removed by boiling; permanent hardness (chlorides, sulphates) by washing soda, Na₂CO₃·10H₂O.
• Soap = soft soap (KOH) and hard soap (NaOH); detergents are sodium salts of sulphonic acids.
RRB Technician Grade III 2024-25 – 23 December 2024 Shift 1 · IUPAC nomenclature

Q152. Select the correct IUPAC names for CH₃COCH₃ and C₂H₅OH compounds, respectively.

  1. (a) Propanone and methanol
  2. (b) Propanone and ethanol
  3. (c) Ethanol and propanone
  4. (d) Propanoic acid and ethanol
Answer & solution

Answer: (b) Propanone and ethanol

Correct Answer: Option 2 — Propanone and ethanol
An IUPAC name is built as root word + suffix, where the root tells the number of carbon atoms and the suffix tells the functional group.
CH₃COCH₃ has 3 carbon atoms (root prop-) and a ketone group (>C=O) sitting in the middle of the chain, whose suffix is -one. Its IUPAC name is therefore propanone; its common name is acetone, the familiar nail-polish remover and laboratory solvent.
C₂H₅OH has 2 carbon atoms (root eth-) and an alcohol group (–OH), whose suffix is -ol. Its IUPAC name is therefore ethanol; its common name is ethyl alcohol, the alcohol present in all alcoholic drinks.
❌ Why the other options are incorrect
✗ Propanone and methanol — the ketone is right, but methanol (CH₃OH) has only one carbon atom, while C₂H₅OH has two, so it must be ethanol. Methanol is highly poisonous and can cause blindness.
✗ Ethanol and propanone — both names are correct in themselves, but their order is reversed; the question asks for the two names "respectively".
✗ Propanoic acid and ethanol — propanoic acid is CH₃CH₂COOH, which carries a carboxylic acid group (–COOH), not a ketone group, so the first name is wrong.
📌 SSC & Railway Exam Facts
• Root words: meth- (1), eth- (2), prop- (3), but- (4), pent- (5), hex- (6) carbon atoms.
• Suffixes: -ol (alcohol), -al (aldehyde), -one (ketone), -oic acid (carboxylic acid); -ane/-ene/-yne for single/double/triple bonds.
• CH₃COCH₃ = propanone (acetone); C₂H₅OH = ethanol; CH₃COOH = ethanoic acid (acetic acid).
• Vinegar is a 5–8% solution of ethanoic acid in water; pure ethanoic acid freezes in winter and is called glacial acetic acid.
• Denatured alcohol is ethanol made unfit for drinking by adding methanol and a dye.
RRB Technician Grade III 2024-25 – 23 December 2024 Shift 1 · Chemical properties of carbon compounds (combustion, oxidation, addition, substitution)

Q153. Chlorination of alkane is an example of:

  1. (a) rearrangement reaction
  2. (b) both addition and substitution chain reaction
  3. (c) addition reaction
  4. (d) substitution reaction
Answer & solution

Answer: (d) substitution reaction

Correct Answer: Option 4 — substitution reaction
Alkanes (CnH₂n₊₂) are saturated hydrocarbons — every carbon atom is joined to its neighbours by single bonds only, so the molecule is already "full" and has no room to take on extra atoms. The only way a new atom can enter is by replacing an atom already present, and a reaction of that kind is called a substitution reaction.
In the presence of sunlight (ultraviolet light), chlorine replaces the hydrogen atoms of an alkane one at a time.
⚗️ The Reaction
CH₄ + Cl₂ → (sunlight) CH₃Cl + HCl
• One hydrogen atom of methane is replaced by a chlorine atom — that is substitution.
• The reaction needs sunlight/UV and goes on stepwise to CH₂Cl₂, CHCl₃ (chloroform) and finally CCl₄.
• Because alkanes are saturated, they are generally unreactive and give substitution, never addition.
❌ Why the other options are incorrect
✗ rearrangement reaction — here the atoms of a molecule simply reorganise into an isomer; no new atom enters the molecule, which is not what chlorination does.
✗ both addition and substitution chain reaction — chlorination of an alkane is a substitution chain reaction only; there is no addition step, because a saturated alkane has no double or triple bond to add across.
✗ addition reaction — addition takes place across a double or triple bond, that is, in unsaturated compounds (alkenes and alkynes), e.g. ethene + H₂ → ethane in the presence of a nickel catalyst.
📌 SSC & Railway Exam Facts
• Saturated hydrocarbons (alkanes) give substitution reactions; unsaturated ones (alkenes, alkynes) give addition reactions.
• CH₄ + Cl₂ → (sunlight) CH₃Cl + HCl is the classic substitution reaction of the syllabus.
• Hydrogenation of vegetable oil with a nickel catalyst gives vanaspati ghee — an addition reaction.
• General formulas: alkane CnH₂n₊₂, alkene CnH₂n, alkyne CnH₂n₋₂.
• Saturated hydrocarbons burn with a clean blue flame; unsaturated ones burn with a yellow sooty flame.
RRB Technician Grade III 2024-25 – 23 December 2024 Shift 1 · Soaps and detergents

Q154. Hydrophobic end and hydrophilic end of soap molecule:

  1. (a) interacts with hydrocarbons and interacts with water, respectively
  2. (b) interacts with water and interacts with acid, respectively
  3. (c) interacts with water and interacts with hydrocarbons, respectively
  4. (d) interacts with hydrocarbons and interacts with base, respectively
Answer & solution

Answer: (a) interacts with hydrocarbons and interacts with water, respectively

Correct Answer: Option 1 — interacts with hydrocarbons and interacts with water, respectively
Every soap molecule — for example sodium stearate, C₁₇H₃₅COO⁻Na⁺ — has two ends of opposite nature, and this dual nature is the whole secret of cleaning:
• The long hydrocarbon tail is hydrophobic (water-hating). Being non-polar, it interacts with oil, grease and hydrocarbons and dissolves in them.
• The ionic –COO⁻Na⁺ head is hydrophilic (water-loving). Being polar and charged, it interacts with water and dissolves in it.
The question names them in the order hydrophobic end first, hydrophilic end second, so the correct pairing is hydrocarbons, then water.
When soap is put into water carrying greasy dirt, the tails bury themselves in the oil droplet while the heads face outwards into the water, forming a sphere called a micelle. The dirt stays trapped inside the micelle and is carried away with the rinse water.
❌ Why the other options are incorrect
✗ interacts with water and interacts with acid, respectively — the order is reversed and "acid" is wrong; the hydrophilic end interacts with water, not with an acid.
✗ interacts with water and interacts with hydrocarbons, respectively — the two ends have simply been swapped; this is the commonest trap set in this question.
✗ interacts with hydrocarbons and interacts with base, respectively — the first half is right but the second is wrong; the hydrophilic head interacts with water, not with a base.
📌 SSC & Railway Exam Facts
• Hydrophobic = water-hating hydrocarbon tail; hydrophilic = water-loving ionic head.
• The spherical cluster of soap molecules that traps the dirt is called a micelle.
• Soap = sodium/potassium salt of a higher fatty acid, made by saponification: fat/oil + NaOH → soap + glycerol.
• Soap does not lather in hard water — Ca²⁺ and Mg²⁺ ions form an insoluble scum; detergents work even in hard water.
• A soap solution shows the Tyndall effect, because a solution of micelles is a colloid.
RRB Technician Grade III 2024-25 – 23 December 2024 Shift 2 · Hydrocarbons - alkanes, alkenes, alkynes

Q155. C₂H₂, C₂H₄, and C₂H₆ belong to which group of hydrocarbons?

  1. (a) Alkane, alkyne, and alkene hydrocarbons, respectively
  2. (b) Alkane, alkene, and alkyne hydrocarbons, respectively
  3. (c) Alkyne, alkene, and alkane hydrocarbons, respectively
  4. (d) Alkene, alkyne, and alkane hydrocarbons, respectively
Answer & solution

Answer: (c) Alkyne, alkene, and alkane hydrocarbons, respectively

Correct Answer: Option 3 — Alkyne, alkene, and alkane hydrocarbons, respectively
Hydrocarbons contain only carbon and hydrogen. They are classified by the type of bond between the carbon atoms, and each family has its own general formula:
• Alkanes — CₙH₂ₙ₊₂ — only single C–C bonds, saturated.
• Alkenes — CₙH₂ₙ — at least one double bond, unsaturated.
• Alkynes — CₙH₂ₙ₋₂ — at least one triple bond, unsaturated.
Putting n = 2 in each formula: C₂H₆ (2n+2 = 6) is ethane, an alkane; C₂H₄ (2n = 4) is ethene (ethylene), an alkene; C₂H₂ (2n−2 = 2) is ethyne (acetylene), an alkyne. Reading the question's order C₂H₂, C₂H₄, C₂H₆ therefore gives alkyne, alkene, alkane.
❌ Why the other options are incorrect
✗ Alkane, alkyne, and alkene, respectively — it treats C₂H₂ as an alkane; an alkane with 2 carbons must be C₂H₆.
✗ Alkane, alkene, and alkyne, respectively — this is the answer to the reverse order (C₂H₆, C₂H₄, C₂H₂); the sequence has been read backwards.
✗ Alkene, alkyne, and alkane, respectively — it mixes up C₂H₂ and C₂H₄; the one with fewer hydrogens is the more unsaturated alkyne.
📌 SSC & Railway Exam Facts
• CₙH₂ₙ₊₂ = alkane, CₙH₂ₙ = alkene, CₙH₂ₙ₋₂ = alkyne.
• Ethyne (acetylene, C₂H₂) burns with oxygen to give the oxy-acetylene flame (about 3000 °C) used for welding.
• Ethene (C₂H₄) is used to artificially ripen fruits.
• Saturated hydrocarbons burn with a clean blue flame; unsaturated ones with a yellow sooty flame.
• Two successive members of a homologous series differ by –CH₂–, i.e. 14 u.
RRB Technician Grade III 2024-25 – 23 December 2024 Shift 3 · Hydrocarbons - alkanes, alkenes, alkynes

Q156. Which statement is correct for unsaturated hydrocarbons?

  1. (a) Methane, ethane and butane are unsaturated hydrocarbons.
  2. (b) Alkane, alkenes and alkynes are unsaturated hydrocarbons.
  3. (c) Cyclo pentane and hexane are unsaturated hydrocarbons.
  4. (d) Alkenes and alkynes are unsaturated hydrocarbons.
Answer & solution

Answer: (d) Alkenes and alkynes are unsaturated hydrocarbons.

Correct Answer: Option 4 — Alkenes and alkynes are unsaturated hydrocarbons
A hydrocarbon is saturated when every carbon–carbon linkage is a single bond, so each carbon holds the maximum possible number of hydrogen atoms. It is unsaturated when it contains at least one double or triple bond between carbon atoms, i.e. fewer hydrogens than the maximum.
• Alkanes — CₙH₂ₙ₊₂ — only single bonds → saturated (methane CH₄, ethane C₂H₆, butane C₄H₁₀).
• Alkenes — CₙH₂ₙ — one C=C double bond → unsaturated (ethene C₂H₄).
• Alkynes — CₙH₂ₙ₋₂ — one C≡C triple bond → unsaturated (ethyne C₂H₂).
Unsaturated hydrocarbons are chemically more reactive: they undergo addition reactions, for example adding H₂ over a nickel catalyst to become saturated — the reaction used to make vanaspati ghee from vegetable oils.
❌ Why the other options are incorrect
✗ Methane, ethane and butane are unsaturated hydrocarbons — all three are alkanes with only single bonds, hence saturated.
✗ Alkane, alkenes and alkynes are unsaturated hydrocarbons — it wrongly includes alkanes, the very definition of a saturated family.
✗ Cyclo pentane and hexane are unsaturated hydrocarbons — both contain only single bonds; a ring does not make a compound unsaturated.
📌 SSC & Railway Exam Facts
• Saturated → addition is not possible, only substitution (with Cl₂ in sunlight).
• Unsaturated → addition reactions; they decolourise bromine water, the standard test.
• Saturated hydrocarbons burn with a clean blue flame; unsaturated ones with a yellow sooty flame.
• Hydrogenation of oils uses a nickel (Ni) catalyst and converts unsaturated oils to saturated fats.
• Ethene (C₂H₄) ripens fruit; ethyne (C₂H₂) gives the oxy-acetylene welding flame.
RRB Technician Grade III 2024-25 – 23 December 2024 Shift 3 · Hydrocarbons - alkanes, alkenes, alkynes

Q157. Which of the following is the correct formula for ethyne?

  1. (a) C₂H₆
  2. (b) C₂H₂
  3. (c) CH₄
  4. (d) C₂H₄
Answer & solution

Answer: (b) C₂H₂

Correct Answer: Option 2 — C₂H₂
In the IUPAC system, the number of carbon atoms fixes the prefix (meth-, eth-, prop-, but-…) and the type of carbon–carbon bond fixes the suffix:
• -ane → all single bonds (alkane, CₙH₂ₙ₊₂)
• -ene → a double bond (alkene, CₙH₂ₙ)
• -yne → a triple bond (alkyne, CₙH₂ₙ₋₂)
Ethyne = eth (2 carbon atoms) + yne (triple bond) → an alkyne with n = 2, so CₙH₂ₙ₋₂ = C₂H₂. Its structure is H–C≡C–H, the simplest alkyne. Its common name is acetylene; burnt with oxygen it gives the oxy-acetylene flame at about 3000 °C, used for welding and cutting metals.
❌ Why the other options are incorrect
✗ C₂H₆ — this is ethane, an alkane (suffix -ane) with only a single C–C bond.
✗ CH₄ — this is methane, the first alkane; it has only one carbon, so it cannot be an "eth-" compound.
✗ C₂H₄ — this is ethene (ethylene), an alkene with a double bond; confusing -ene with -yne is the usual error.
📌 SSC & Railway Exam Facts
• Ethyne = acetylene = C₂H₂, H–C≡C–H, the first member of the alkyne series.
• CH₄ methane, C₂H₆ ethane, C₃H₈ propane, C₄H₁₀ butane — the first four alkanes.
• Oxy-acetylene flame ≈ 3000 °C; acetylene alone burns with a sooty flame due to incomplete combustion.
• Suffix guide: -ane (single), -ene (double), -yne (triple).
• Carbon shows catenation and tetravalency, which is why it forms millions of compounds.
RRB Technician Grade III 2024-25 – 20 December 2024 Shift 2 · Soaps and detergents

Q158. With which of the following does the hydrophilic end of soaps react?

  1. (a) Hydrocarbon
  2. (b) Both water and oil
  3. (c) Water
  4. (d) Oil
Answer & solution

Answer: (c) Water

Correct Answer: Option 3 — Water
A soap is the sodium or potassium salt of a long-chain fatty acid, for example sodium stearate, C₁₇H₃₅COO⁻Na⁺. Every soap molecule has two completely different ends:
• A long hydrocarbon tail that is hydrophobic (water-hating) and oil-loving — it dissolves in grease and oil.
• A charged –COO⁻Na⁺ head that is hydrophilic (water-loving) — it is attracted to and dissolves in water.
So the hydrophilic end interacts with water. When soap is added to water carrying greasy dirt, the tails bury themselves inside the oil droplet while the heads point outwards into the water, forming a spherical cluster called a micelle. The dirt is trapped inside the micelle and is carried away with the rinse water.
❌ Why the other options are incorrect
✗ Hydrocarbon — the hydrocarbon (oil/grease) is attacked by the hydrophobic tail, not by the hydrophilic head.
✗ Both water and oil — the whole soap molecule acts on both, but the hydrophilic end alone interacts only with water; that is what the question asks.
✗ Oil — oil is dissolved by the non-polar hydrophobic tail; the charged hydrophilic head is repelled by oil.
📌 SSC & Railway Exam Facts
• Soap = sodium/potassium salt of a higher fatty acid; it is made by saponification: fat/oil + NaOH → soap + glycerol.
• The spherical cluster formed by soap molecules in water is called a micelle.
• Soap does not lather in hard water — Ca²⁺ and Mg²⁺ ions form an insoluble scum; detergents work even in hard water.
• Detergents are ammonium or sulphonate salts of long-chain hydrocarbons and are not readily biodegradable.
• Hydrophilic = water-loving head; hydrophobic = water-hating hydrocarbon tail.
RRB JE CBT-1 2024-25 – 18 December 2024 Shift 2 · Esters and esterification

Q159. The product of the reaction between ethanol and acetic acid is:

  1. (a) ethanal
  2. (b) acetate
  3. (c) ester
  4. (d) methanol
Answer & solution

Answer: (c) ester

Answer: Option 3 — ester
Ethanol (an alcohol) reacts with acetic acid (ethanoic acid) in the presence of an acid to form a sweet-smelling ester and water.
This reaction is called esterification.
Revision note — Ethanol, ethanoic acid and esters
• Esterification: CH_(3)COOH + C_(2)H_(5)OH → CH_(3)COOC_(2)H_(5) + H_(2)O, catalysed by conc. H_(2)SO_(4).
• The product ethyl ethanoate (ethyl acetate) is an ester with a fruity, sweet smell — used in perfumes and flavouring agents.
• Saponification: an ester + NaOH gives back the alcohol and the sodium salt of the acid — the reaction used to make soap.
• Ethanoic acid is commonly called acetic acid; a 5–8% solution in water is vinegar.
• Pure ethanoic acid freezes at 290 K in cold climates, so it is called glacial acetic acid.
• Ethanol + sodium gives hydrogen gas and sodium ethoxide.
Do not mix these up
• Ethanal vs ethanoate — ethanal (CH_(3)CHO) is the aldehyde formed by oxidising ethanol; an ethanoate is the ester or salt of ethanoic acid.
Also asked from this topic: the type of reaction between ethanoic acid and ethanol with an acid catalyst (esterification).
"Acetate" alone is only a part of the name — the class of compound formed is an ester.
RRB JE CBT-1 2024-25 – 17 December 2024 Shift 1 · Bonding in carbon and catenation

Q160. Which of the following statements is NOT a reason for the versatile nature of carbon?

  1. (a) Carbon has tetravalency
  2. (b) Carbon can form single, double, and triple bonds
  3. (c) Carbon exhibits catenation
  4. (d) Carbon can form ionic bonds with itself
Answer & solution

Answer: (d) Carbon can form ionic bonds with itself

Answer: Option 4 — Carbon can form ionic bonds with itself
Two carbon atoms have the same electronegativity, so neither can pull an electron away from the other.
They share electrons — every C–C bond is covalent, never ionic.
Revision note — Why carbon forms so many compounds
• Tetravalency: carbon has 4 valence electrons (2, 4) and forms 4 covalent bonds.
• Catenation: carbon links with other carbon atoms into long chains, branches and rings; the C–C bond is strong and stable.
• Carbon forms single (C–C), double (C=C) and triple (C≡C) bonds.
• Carbon cannot easily gain 4 (C⁴⁻) or lose 4 (C⁴⁺) electrons — so it shares them.
• Its small size lets the nucleus hold the shared electrons tightly, giving strong bonds.
• Silicon also shows catenation, but its chains (up to about 7–8 atoms) are reactive and short.
Also asked from this topic: the properties that let carbon form so many compounds (catenation and tetravalency); why carbon forms a large number of compounds (the C–C bond is very strong and stable).
RRB JE CBT-1 2024-25 – 17 December 2024 Shift 1 · Homologous series

Q161. Which of the following statements is true about the physical properties of compounds in a homologous series?

  1. (a) Their boiling points decrease as the number of carbon atoms increase.
  2. (b) They have a regular change in melting and boiling points.
  3. (c) Their physical properties do not change.
  4. (d) They have identical melting and boiling points.
Answer & solution

Answer: (b) They have a regular change in melting and boiling points.

Answer: Option 2 — They have a regular change in melting and boiling points.
Each member differs from the next by –CH_(2)– (14 u), so molecular mass rises step by step.
Physical properties such as melting and boiling points therefore change gradually (gradation).
Revision note — Homologous series
• A homologous series has the same functional group and the same general formula.
• Successive members differ by –CH_(2)–, i.e. 14 u in molecular mass.
• Chemical properties are similar (same functional group); physical properties show gradation.
• Boiling point rises with the number of carbon atoms: methane < ethane < propane.
• Alkanes C_(n)H_(2n+2) (CH_(4), C_(2)H_(6), C_(3)H_(8)); alkenes C_(n)H_(2n); alkynes C_(n)H_(2n−2).
Do not mix these up
• Chemical vs physical properties — chemical properties stay similar; physical properties change regularly.
Also asked from this topic: compounds differing by –CH_(2)– (homologous series); what stays similar in the series (chemical properties); what rises as an alkane chain grows (boiling point).
RRB JE CBT-1 2024-25 – 17 December 2024 Shift 3 · Homologous series

Q162. A homologous series is a group of organic compounds that have:

  1. (a) the same functional group but differ by a CH₂ group
  2. (b) the same molecular formula
  3. (c) the same structural formula
  4. (d) no relation to each other
Answer & solution

Answer: (a) the same functional group but differ by a CH₂ group

Answer: Option 1 — the same functional group but differ by a CH₂ group
A homologous series is a family of carbon compounds with the same functional group, in which successive members differ by a −CH_(2)− unit (14 u).
Revision note — Homologous series (Class 10)
• Members share one general formula — alkanes C_(n)H_(2n+2), alkenes C_(n)H_(2n), alkynes C_(n)H_(2n−2).
• Successive members differ by CH_(2) in formula and by 14 u in molecular mass.
• Chemical properties are similar (decided by the functional group).
• Physical properties (melting point, boiling point, solubility) change gradually with molecular mass.
• Example: methane CH_(4), ethane C_(2)H_(6), propane C_(3)H_(8); alcohols CH_(3)OH, C_(2)H_(5)OH.
Do not mix these up
• Homologous series vs isomers — isomers have the same molecular formula but different structures; homologues differ by CH_(2).
Also asked from this topic: what compounds differing by a −CH_(2)− unit form (a homologous series); what rises steadily as the chain lengthens (boiling point).
RRB JE CBT-1 2024-25 – 16 December 2024 Shift 1 · Chemical properties of carbon compounds (combustion, oxidation, addition, substitution)

Q163. Which of the following reactions represents the combustion of methane?

  1. (a) CH₄ + Cl₂ → CH₃Cl + HCl
  2. (b) CH₄ + O₂ → CO₂ + 2H₂O
  3. (c) CH₄ + O₂ → CO + H₂O
  4. (d) CH₄ + H₂O → CO + H₂
Answer & solution

Answer: (b) CH₄ + O₂ → CO₂ + 2H₂O

Answer: Option 2 — CH₄ + O₂ → CO₂ + 2H₂O
Combustion means burning in oxygen. Methane burns in air to give carbon dioxide, water, heat and light.
The balanced equation is CH_(4) + 2O_(2) → CO_(2) + 2H_(2)O (the option shows the right products; its O_(2) is left unbalanced).
Revision note — Combustion of carbon compounds
• Carbon and its compounds burn in oxygen to give CO_(2) + H_(2)O + heat + light.
• Complete combustion (enough air) gives a blue, non-luminous flame with no soot — as in an LPG or gas stove.
• Incomplete combustion (limited air) gives CO and soot — a yellow, sooty flame that blackens vessels.
• Saturated hydrocarbons (methane, ethane) burn with a clean blue flame; unsaturated ones give a yellow sooty flame.
• Methane (CH_(4)) is the main component of CNG and biogas.
• CH_(4) + Cl_(2) → CH_(3)Cl + HCl (in sunlight) is a substitution reaction, not combustion.
Do not mix these up
• Combustion vs substitution — combustion needs O_(2) and gives CO_(2) + H_(2)O; substitution replaces H with Cl in sunlight.
Also asked from this topic: the sign of complete combustion of methane in a gas burner (a blue, non-luminous flame with no residue); the type of reaction when methane reacts with chlorine in sunlight (substitution).
RRB JE CBT-1 2024-25 – 16 December 2024 Shift 2 · Homologous series

Q164. Which of the following is the correct definition of a homologous series?

  1. (a) A series of compounds having identical physical properties
  2. (b) A series of compounds having different functional groups
  3. (c) A series of compounds with the same molecular mass
  4. (d) A series of compounds having the same functional group and similar chemical properties
Answer & solution

Answer: (d) A series of compounds having the same functional group and similar chemical properties

Answer: Option 4 — A series of compounds having the same functional group and similar chemical properties
Members of a homologous series carry the same functional group, so they react alike — similar chemical properties.
Revision note — Homologous series
• Successive members differ by a –CH_(2)– unit, i.e. by 14 u in molecular mass.
• All members fit one general formula — alkanes C_(n)H_(2n+2), alkenes C_(n)H_(2n), alkynes C_(n)H_(2n−2).
• Chemical properties stay similar (set by the functional group).
• Physical properties (melting point, boiling point, solubility) change gradually as molecular mass rises.
• Example: CH_(3)OH, C_(2)H_(5)OH, C_(3)H_(7)OH — the alcohol series (–OH group).
• Example: CH_(4), C_(2)H_(6), C_(3)H_(8) — methane, ethane, propane (alkanes).
Do not mix these up
• Homologous series vs isomers — homologues have different formulae (differ by CH_(2)); isomers have the same formula but different structures.
Also asked from this topic: the grouping of compounds that differ by a –CH_(2)– unit (homologous series); what methane, ethane and propane represent (a homologous series of alkanes).
RRB JE CBT-1 2024-25 – 16 December 2024 Shift 2 · Bonding in carbon and catenation

Q165. Which of the following options represents the correct Lewis structure for methane (CH₄)?

  1. (a) A carbon atom bonded to four hydrogen atoms with triple bonds
  2. (b) A carbon atom bonded to two hydrogen atoms with double bonds
  3. (c) A carbon atom bonded to one hydrogen atom with a double bond
  4. (d) A carbon atom bonded to four hydrogen atoms with single bonds
Answer & solution

Answer: (d) A carbon atom bonded to four hydrogen atoms with single bonds

Answer: Option 4 — A carbon atom bonded to four hydrogen atoms with single bonds
Carbon (2, 4) has 4 valence electrons; it shares one with each of 4 hydrogen atoms, forming 4 single covalent bonds.
Carbon completes its octet and each H its duplet.
Revision note — Covalent bonding in carbon
• Carbon is tetravalent — it forms 4 covalent bonds by sharing electrons, not by losing or gaining 4.
• Methane (CH_(4)): 4 C–H single bonds; tetrahedral shape, bond angle 109.5°.
• Single bond = 1 shared pair; double = 2 pairs (ethene, C_(2)H_(4)); triple = 3 pairs (ethyne, C_(2)H_(2)).
• O_(2) has a double bond; N_(2) has a triple bond.
• Catenation plus tetravalency let carbon form millions of compounds.
• Covalent compounds have low melting and boiling points and are poor conductors of electricity.
Also asked from this topic: the properties that let carbon form millions of compounds (strong covalent bonding and tetravalency); why covalent compounds have low melting points (weak intermolecular forces).
RRB JE CBT-1 2024-25 – 16 December 2024 Shift 3 · Bonding in carbon and catenation

Q166. Covalent molecules have low melting and boiling point because:

  1. (a) they have weak intermolecular forces
  2. (b) they are generally gaseous in nature
  3. (c) they have strong bonds within the molecule
  4. (d) they have strong electrostatic intermolecular forces
Answer & solution

Answer: (a) they have weak intermolecular forces

Answer: Option 1 — they have weak intermolecular forces
Melting or boiling separates molecules from one another, not atoms inside a molecule.
In covalent compounds the forces between molecules are weak, so little heat is needed.
Revision note — Covalent compounds
• A covalent bond forms by sharing of electron pairs between two atoms (single, double or triple bond).
• The bonds within a molecule are strong; the forces between molecules are weak.
• Hence covalent compounds have low melting and boiling points and many are liquids or gases at room temperature.
• They are generally poor conductors of electricity — no ions are formed.
• Examples: CH_(4), H_(2)O, CO_(2), NH_(3), Cl_(2); carbon forms covalent bonds because it can neither gain nor lose 4 electrons easily.
• Ionic compounds (NaCl, MgCl_(2)) have high melting and boiling points due to strong electrostatic attraction between ions.
Do not mix these up
• Intramolecular vs intermolecular — the covalent bond inside the molecule is strong; the low melting point comes from the weak pull between molecules.
Also asked from this topic: why covalent compounds have low melting points (weak intermolecular forces); the property NOT typical of covalent carbon compounds (good conductors of electricity).
RRB JE CBT-1 2024-25 – 16 December 2024 Shift 3 · Hydrocarbons - alkanes, alkenes, alkynes

Q167. Which of the following is the correct formula for the first member of the alkene series?

  1. (a) C₄H₈
  2. (b) C₂H₄
  3. (c) C₃H₆
  4. (d) C₅H₁₀
Answer & solution

Answer: (b) C₂H₄

Answer: Option 2 — C₂H₄
Alkenes have general formula C_(n)H_(2n) and need a C=C double bond, so the smallest has n = 2.
n = 2 ⇒ C_(2)H_(4), ethene (CH_(2)=CH_(2)).
Revision note — Alkanes, alkenes and alkynes
• Alkanes — C_(n)H_(2n+2), only single bonds, saturated; first member methane CH_(4).
• Alkenes — C_(n)H_(2n), one C=C double bond, unsaturated; ethene C_(2)H_(4), propene C_(3)H_(6), butene C_(4)H_(8).
• Alkynes — C_(n)H_(2n−2), one C≡C triple bond; first member ethyne (acetylene) C_(2)H_(2).
• Successive members of a homologous series differ by –CH_(2)– (14 u).
• Unsaturated hydrocarbons give addition reactions — e.g. hydrogenation of vegetable oils with a nickel catalyst.
• Ethene decolourises bromine water — the test for unsaturation.
Do not mix these up
• C_(3)H_(6) — it fits C_(n)H_(2n) but it is propene (second alkene); as a ring it is cyclopropane, a saturated compound.
Also asked from this topic: the general formula for alkanes (C_(n)H_(2n+2)); the hydrocarbon that most readily gives addition reactions (ethene).
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