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Class 10 › Science › Chapter 4

Carbon and Its Compounds — Important Questions & Answers

✓ CBSE 2025 Board Ready 14 Questions 3-Mark Format

Core Reactions

1

An organic compound 'X' is a main component of wine. When 'X' is heated with alkaline \(KMnO_{4}\), it changes into compound 'Y' which turns blue litmus red. Identify 'X' and 'Y' and write the chemical equation.

3 marks
+
Identification

Compound 'X' is Ethanol (\(CH_3CH_2OH\)) — the alcohol found in wine.
Compound 'Y' is Ethanoic Acid (\(CH_3COOH\)) — it turns blue litmus red because it is acidic.

Type of Reaction
⚗️This is an oxidation reaction. Alkaline \(KMnO_4\) acts as the oxidising agent, adding oxygen to ethanol and removing hydrogen.
Chemical Equation
\(\text{CH}_3\text{CH}_2\text{OH} \xrightarrow{\text{Alkaline KMnO}_4,\ \Delta} \text{CH}_3\text{COOH} + \text{H}_2\text{O}\)
2

Why does a candle flame look yellow and bright, while a gas stove burns with a clear blue flame? Explain the chemistry behind this difference.

3 marks
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Gas Stove — Blue Flame

LPG (saturated hydrocarbons) + air holes → ample oxygen → complete combustion → carbon fully oxidised → clean, soot-free blue flame.

Candle — Yellow Flame

Wax vapour around the wick cannot mix quickly enough with air → incomplete combustion. Unburnt carbon particles become red-hot and glow yellow, escaping as black soot.

💡Memory tip: Blue = Complete, Yellow = Incomplete
3

Carbon is placed in Group 14. It cannot form \(C^{4+}\) cations or \(C^{4-}\) anions. Explain why it must share electrons to become stable.

3 marks
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Electronic Configuration

Carbon (atomic number 6): configuration (2, 4) → needs 4 more electrons to complete its octet.

Why not C⁴⁻?
⚡Only 6 protons cannot hold 10 electrons — heavy electron-electron repulsion makes it energetically impossible.
Why not C⁴⁺?
🔋Removing 4 electrons successively requires an enormous amount of energy — unavailable under normal conditions.
Conclusion

Carbon shares its 4 valence electrons with other atoms to form stable covalent bonds. This is the basis of the vast organic world.

4

A student added a piece of sodium metal to a test tube containing ethanol. State two main observations and write the balanced chemical equation.

3 marks
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Observation 1

Brisk effervescence (vigorous bubbling) is seen due to rapid evolution of a colourless, odourless gas (hydrogen).

Observation 2

The gas burns with a 'pop' sound when a burning matchstick is brought near, confirming the gas is H₂. Sodium gradually dissolves.

Balanced Chemical Equation
\(2\,CH_3CH_2OH + 2\,Na \rightarrow 2\,CH_3CH_2ONa + H_2\uparrow\)
(Ethanol + Sodium → Sodium Ethoxide + Hydrogen gas)
5

What is a structural isomer? Why is it impossible to have isomers for Methane, Ethane, and Propane?

3 marks
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Definition

Structural isomers are compounds sharing the same molecular formula but with different structural arrangements of the carbon skeleton.

Why No Isomers for C₁–C₃ Alkanes?

Isomerism requires branching. 1, 2, or 3 carbon atoms can only be arranged in a single straight chain — branching is geometrically impossible with so few carbons. Isomers appear only from Butane (C₄H₁₀) onwards.

6

Vegetable oils are healthy, but vanaspati ghee is considered less healthy. Explain the chemical process used to convert oil into ghee with a chemical equation.

3 marks
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The Process — Hydrogenation

Vegetable oils contain unsaturated fatty acids (C=C double bonds). H₂ gas is added in the presence of a Nickel (Ni) catalyst at high temperature, converting double bonds to single bonds → solid saturated fat (vanaspati).

Health Aspect
🩺Unsaturated fats are easier to metabolise. Saturated fats (ghee) can raise LDL (bad) cholesterol, increasing cardiovascular risk.
Chemical Equation
\(\text{R}_2C=CR_2 + H_2 \xrightarrow{Ni,\, \Delta} \text{R}_2CH-CHR_2\)
(R = any alkyl group, e.g., –CH₃)
7

Among \(C_3H_6\), \(C_4H_{10}\), and \(C_5H_{10}\) — identify which belong to the same homologous series and give two reasons.

3 marks
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Identification

\(C_3H_6\) (Propene) and \(C_5H_{10}\) (Pentene) belong to the Alkene series.
(\(C_4H_{10}\) is Butane — an alkane, different series.)

Reason 1 — General Formula

Both fit the alkene general formula \(C_nH_{2n}\): hydrogen atoms = exactly 2× carbon atoms.

Reason 2 — Same Functional Group

Both contain a C=C double bond (unsaturation), giving them identical chemical properties and reaction types (e.g., addition reactions).

Structure & Bonding

8

Draw the electron-dot structure of Ethyne (\(C_2H_2\)). State the total number of single and triple covalent bonds.

3 marks
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Electron Sharing

Each carbon shares 3 pairs of electrons with the other carbon → triple bond. Each carbon also shares 1 pair with a hydrogen atom → single bond.

Structural Representation
H — C ≡ C — H

Each H has 2 electrons around it (duet); each C has 8 electrons (octet).

Bond Count

• 2 single covalent bonds (C–H bonds)
• 1 triple covalent bond (C≡C bond)
• Total shared pairs = 5

Soap, Detergent & Saponification

9

When an ester reacts with NaOH, it breaks into two products. Name this process, state its practical use, and write the balanced equation.

3 marks
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Name of Process

Saponification — the reaction of an ester with a strong base (NaOH) to produce a soap salt and an alcohol.

Practical Use
🧼Used industrially to manufacture soap bars by hydrolyzing fats and oils with alkalis.
Chemical Equation
\(CH_3COOCH_2CH_3 + NaOH \rightarrow CH_3COONa + CH_3CH_2OH\)
(Ethyl Ethanoate + Sodium Hydroxide → Sodium Ethanoate [Soap] + Ethanol)
10

Why does hard water reduce the cleaning capacity of soap, while synthetic detergents work effectively even in hard water?

3 marks
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Soap in Hard Water

Hard water contains dissolved Ca²⁺ and Mg²⁺ ions. Soap molecules react with these ions instantly to form a sticky, insoluble white precipitate called scum — which wastes soap and stains clothes.

Detergents in Hard Water

Detergents are made from ammonium or sulphonate salts. Their charged ends do not react with Ca²⁺ or Mg²⁺ ions, so they remain soluble and produce a rich lather.

✅Result: Detergents clean effectively in hard water; soaps cannot.
Theory Extra — Why does soap clean at all?

Soap molecules are long chains with a hydrophilic (water-loving) head and a hydrophobic (oil-loving) tail. The tails surround grease to form tiny spheres called micelles, which are carried away by water, removing dirt.

Theory Extra — Advantage of detergents over soaps

Detergents work in hard water and even in acidic conditions, while soaps lose effectiveness. However, many synthetic detergents are non-biodegradable, causing environmental pollution — a key disadvantage.

IUPAC Naming & Structures

11

Write the IUPAC names for: (a) \(CH_3{-}CH_2{-}CH(CH_3){-}CH_2{-}OH\)   (b) \(CH_3{-}CH_2{-}C\!\equiv\!C{-}CH_3\)   (c) \(HCHO\)

3 marks
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(a)

2-Methylbutan-1-ol — longest chain = 4C (butanol); –OH at C1; –CH₃ branch at C2.

(b)

Pent-2-yne — 5-carbon chain (pent); triple bond starting at C2 from the end nearest to it.

(c)

Methanal — 1 carbon with aldehyde (–CHO) functional group.

12

Draw structural formulas showing all bonds for: (a) Propanone   (b) Butanoic Acid   (c) Bromopentane

3 marks
+
(a) Propanone (\(CH_3COCH_3\))
H O H | ‖ | H — C — C — C — H | | H H
(b) Butanoic Acid (\(CH_3CH_2CH_2COOH\))
H H H O | | | ‖ H — C — C — C — C | | | \ H H H O — H
(c) 1-Bromopentane (\(CH_3(CH_2)_3CH_2Br\))
H H H H H | | | | | H — C — C — C — C — C — Br | | | | | H H H H H

Ethanol & Ethanoic Acid

13

State two uses of Ethanol and one use of Ethanoic Acid in daily life.

3 marks
+
Uses of Ethanol

1. Industrial solvent — dissolves organic substances; used in paints, varnishes, and lacquers.
2. Medical/antiseptic use — main ingredient in hand sanitisers, cough syrups, and medical rubs.

Use of Ethanoic Acid

Food preservation (vinegar) — a 5–8% aqueous solution of ethanoic acid is vinegar, used to preserve pickles and sauces.

14

A chemist wants to make a sweet-smelling substance used in perfumes. Name the reactants, the process, and write the balanced equation.

3 marks
+
Reactants & Process

Reactants: Ethanoic Acid + Ethanol. Process: Esterification. Catalyst: concentrated \(H_2SO_4\).

Chemical Equation
\(CH_3COOH + C_2H_5OH \underset{\Delta}{\xrightarrow{\text{Conc. }H_2SO_4}} CH_3COOC_2H_5 + H_2O\)
(Ethanoic Acid + Ethanol → Ethyl Ethanoate [sweet-smelling ester] + Water)

Homologous Series, Catenation & Combustion

15

Why does carbon show stronger catenation than silicon, even though both are in Group 14?

3 marks
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Bond Strength — Carbon

Carbon has a very small atomic size, so its nucleus holds shared electron pairs tightly → C–C bond is very strong and stable → long chains and rings form easily.

Silicon's Weakness

Silicon atoms are much larger → shared electrons are farther from the nucleus → Si–Si bonds are weak and break after only a few atoms.

16

Why do saturated hydrocarbons give a clean flame, while unsaturated hydrocarbons produce a smoky yellow flame in air?

3 marks
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Saturated Hydrocarbons (Alkanes)

Lower carbon-to-hydrogen ratio → enough oxygen available to completely oxidise all carbon → complete combustion → clean blue flame.

Unsaturated Hydrocarbons (Alkenes/Alkynes)

Higher carbon percentage → air does not supply enough oxygen → incomplete combustion → unburnt carbon particles glow yellow, escape as black soot (smoke).

🔥This is why LPG (mainly alkanes) gives a clean blue flame on your gas stove.
17

Why can ethene undergo addition reaction with H₂, but ethane cannot? Write reaction conditions.

3 marks
+
Ethene (Unsaturated)

Contains a C=C double bond. One bond can open up to accept extra atoms → addition is possible.

Ethane (Saturated)

Contains only single bonds — all valencies are fully satisfied → no room to add atoms → addition reaction not possible.

Reaction & Conditions
\(CH_2=CH_2 + H_2 \xrightarrow{Ni/Pd,\ \Delta} CH_3-CH_3\)
(Ethene + Hydrogen → Ethane; catalyst: Ni or Pd)
18

What happens when methane reacts with chlorine in the presence of sunlight? Name the reaction type and write the equation.

3 marks
+
Reaction Type

Substitution Reaction — in the presence of sunlight, a highly reactive Cl atom replaces (substitutes) one H atom in methane.

Chemical Equation
\(CH_4 + Cl_2 \xrightarrow{\text{Sunlight}} CH_3Cl + HCl\)
(Methane + Chlorine → Chloromethane + Hydrogen Chloride)
☀️Sunlight provides the energy needed to break the Cl–Cl bond and start this chain reaction.
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