Chapter 4: Carbon and its Compounds

Science · CBSE Class 10 · NCERT Solutions

📖 Chapter 4 — Carbon and its Compounds (CBSE Class 10 Science): Detailed notes — covalent bonding, hydrocarbons, functional groups, homologous series, ethanol, ethanoic acid, soaps.

Chapter 4 — Carbon and its Compounds

🔗 1. Bonding in Carbon

Carbon has 4 valence electrons (tetravalency). It shares electrons to form covalent bonds (not ionic — losing/gaining 4 electrons is not favourable). Covalent compounds have low m.p./b.p. and are poor conductors.

Versatile nature: ① Catenation — carbon links with itself to form long chains, branches and rings. ② Tetravalency — bonds with many other atoms (H, O, N, S, halogens). Hence millions of carbon compounds exist.

💎 2. Allotropes of Carbon

Diamond (hardest, each C bonded to 4), Graphite (soft, conducts, each C bonded to 3), Fullerene (C₆₀, football-shaped).

🔬 3. Hydrocarbons

Type	Bond & formula
Alkane (saturated)	single bonds, CₙH₂ₙ₊₂ (CH₄, C₂H₆)
Alkene (unsaturated)	C=C double bond, CₙH₂ₙ (C₂H₄)
Alkyne (unsaturated)	C≡C triple bond, CₙH₂ₙ₋₂ (C₂H₂)

🧪 4. Functional Groups & Homologous Series

−X (halogen) · −OH (alcohol) · −CHO (aldehyde) · >C=O (ketone) · −COOH (carboxylic acid)

Homologous series: a series of compounds with the same functional group, where consecutive members differ by −CH₂. They have similar chemical properties and a gradual change in physical properties.

🔥 5. Chemical Properties

Combustion: burn in air → CO₂ + H₂O + heat/light (why carbon compounds are fuels).
Oxidation: alcohol →(oxidising agent)→ carboxylic acid.
Addition: unsaturated + H₂ →(Ni)→ saturated (hydrogenation — makes vanaspati ghee).
Substitution: alkane + Cl₂ →(sunlight)→ substituted product + HCl.

🍶 6. Ethanol & Ethanoic Acid

Ethanol (C₂H₅OH): liquid; reacts with Na → H₂; on heating with conc. H₂SO₄ → ethene (dehydration). Uses: solvent, medicines. (Drinking alcohol is harmful.)
Ethanoic acid (CH₃COOH): 5–8% solution = vinegar; weak acid, turns blue litmus red; + carbonate → CO₂; + ethanol (acid, heat) → sweet-smelling ester (esterification).

🧼 7. Soaps & Detergents

Soap molecule = ionic hydrophilic head (water-loving) + hydrocarbon hydrophobic tail (oil-loving). In water it forms a micelle — tails trap dirt/oil inside, heads face water — so dirt is washed away. Hard water reacts with soap to form insoluble scum; detergents work even in hard water.

🧠 Remember: Carbon = tetravalency + catenation + covalent. Alkane single, alkene double, alkyne triple. Homologous = differ by −CH₂. Hydrogenation = oil→ghee (Ni). Bromine water decolourised = unsaturated. Soap = micelle.

⭐ Exam Tips: ① Covalent bonding, catenation, tetravalency. ② Hydrocarbon types + formulae. ③ Functional groups + homologous series. ④ Combustion/addition/substitution; hydrogenation. ⑤ Ethanol vs ethanoic acid; esterification. ⑥ Soap micelle + hard water scum.

📘 Textbook Question Answers

Q1-3MCQs: bonds in ethane; butanone functional group; blackened vessel bottom.

Q1: (b) 7 covalent bonds (1 C–C + 6 C–H). Q2: (c) ketone. Q3: (b) the fuel is not burning completely (incomplete combustion gives soot).

Q4Nature of covalent bond in CH₃Cl.

Carbon shares its 4 electrons — one shared pair each with 3 hydrogen atoms and 1 chlorine atom. Each shared pair of electrons is a covalent bond, so CH₃Cl has 4 single covalent bonds (all atoms complete their octet/duplet by sharing).

Q5Electron dot structures: ethanoic acid, H₂S, propanone, F₂.

(a) Ethanoic acid CH₃COOH — CH₃ group + C double-bonded to O and single-bonded to O–H. (b) H₂S — S shares one pair each with two H. (c) Propanone CH₃COCH₃ — central C double-bonded to O, single bonds to two CH₃. (d) F₂ — the two F atoms share one pair of electrons (single bond), each keeping 3 lone pairs.

Q6What is a homologous series? Example.

A series of carbon compounds with the same functional group, where each member differs from the next by a −CH₂ unit; they show similar chemical properties and a gradual change in physical properties. Example: alkanes CH₄, C₂H₆, C₃H₈, C₄H₁₀…

Q7Differentiate ethanol and ethanoic acid.

Ethanoic acid has a sour taste and pungent (vinegar) smell, turns blue litmus red, and reacts with sodium carbonate/bicarbonate giving CO₂ (brisk effervescence). Ethanol is neutral (no effect on litmus) and does NOT react with carbonate.

Q8Why micelle forms; will it form in ethanol?

Soap has a hydrophilic head and a hydrophobic tail. In water the tails avoid water and cluster inward around oil/dirt, while heads face the water, forming a micelle. In ethanol (which dissolves both oil and the tail), no micelle forms.

Q9-11Why carbon compounds are fuels; scum in hard water; soap on litmus.

Q9: Because they burn readily in air, releasing a large amount of heat. Q10: Soap reacts with the calcium/magnesium salts of hard water to form an insoluble precipitate called scum. Q11: Soap is basic — it turns red litmus blue and has no effect on blue litmus.

Q12-14Hydrogenation; which undergo addition; saturated vs unsaturated test.

Q12: Hydrogenation is the addition of H₂ to an unsaturated hydrocarbon/oil using a nickel catalyst, converting it to saturated. Industrial use: making vanaspati ghee from vegetable oils. Q13: Addition reactions (unsaturated): C₃H₆ and C₂H₂. Q14: Add bromine water — unsaturated hydrocarbons decolourise it; saturated ones don't.

Q15Mechanism of cleaning action of soaps.

Dirt is oily and does not dissolve in water. The soap's hydrophobic tails attach to the oil/dirt while the hydrophilic heads face outward into water, forming a micelle that surrounds the dirt. The dirt is thus lifted off and suspended in water, and is washed away with water.

Extra practice (4× the textbook) — MCQ, short answer, long answer & equations.

🎯 Extra MCQ

1The next member after C₂H₆ in the alkane series is —

C₃H₈ (propane).

2Which gives a sweet-smelling substance with ethanol?

Ethanoic acid (forms an ester).

3Denatured alcohol is ethanol made — to drink.

Unfit (poisonous, by adding methanol/copper sulphate).

4Which is a saturated hydrocarbon: C₂H₄, C₂H₂, CH₄?

CH₄ (methane).

📝 Short Answer

5Why does carbon form covalent bonds?

Because losing or gaining 4 electrons needs too much energy; sharing electrons (covalent bonding) lets it complete its octet.

6What is an ester? Give its use.

A sweet-smelling compound formed from an acid and alcohol; used in perfumes and flavourings.

7Why is a solution of soap basic?

Soap is the sodium salt of a weak fatty acid and a strong base (NaOH), so its solution is slightly basic.

8Name two isomers of C₄H₁₀.

n-butane and iso-butane (2-methylpropane).

9Why is detergent preferred over soap for washing?

Detergents lather well and clean even in hard water (they don't form scum).

⚗️ Equations

10Ethanol + sodium →

2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂↑.

11Ethanoic acid + sodium carbonate →

2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂↑.

12Ethanoic acid + ethanol (esterification) →

CH₃COOH + C₂H₅OH →(H₂SO₄)→ CH₃COOC₂H₅ (ester) + H₂O.

📖 Long Answer

13Explain the versatile nature of carbon.

Carbon is versatile because of (1) catenation — it forms long chains, branches and rings by bonding to itself, and (2) tetravalency — it forms four strong covalent bonds with many elements (H, O, N, S, halogens). This gives rise to millions of stable carbon compounds.

14Explain the cleansing action of soap with a diagram idea.

Each soap molecule has a hydrophilic (water-loving) head and a hydrophobic (oil-loving) tail. When soap is added to dirty water, many molecules arrange so that their tails point into the oily dirt and their heads face the water, forming a micelle around the dirt. The micelle lifts the dirt off and keeps it suspended, and it is rinsed away with water.

15Compare saturated and unsaturated hydrocarbons.

Saturated hydrocarbons (alkanes) have only single C–C bonds, burn with a clean blue flame, and undergo substitution reactions. Unsaturated hydrocarbons (alkenes/alkynes) have double/triple bonds, burn with a sooty yellow flame, undergo addition reactions, and decolourise bromine water.