Class 10 Science Chapter 4: Carbon and its Compounds

Carbon and its Compounds is Chapter 4 of NCERT Class 10 Science. It explains why carbon forms covalent bonds, why it makes so many compounds, how these compounds are named, and the main reactions of hydrocarbons, ethanol and ethanoic acid, along with soaps and detergents. Watch the six video lessons below, then use these notes and solved examples to revise.

What you will learn

Covalent bonding in carbon and the single, double and triple bonds it forms
Allotropes of carbon and the versatile nature of carbon (catenation and tetravalency)
Saturated and unsaturated hydrocarbons, isomers, functional groups and homologous series
IUPAC nomenclature of simple carbon compounds
Combustion, oxidation, addition and substitution reactions, plus properties of ethanol, ethanoic acid and soaps

Video Lessons

Carbon and its Compounds - Part 1

Carbon and its Compounds - Part 2

Carbon and its Compounds - Part 3

Carbon and its Compounds - Part 4

Carbon and its Compounds - Part 5

Carbon and its Compounds - Part 6

Key Concepts and Formulas

Covalent Bonding in Carbon

Carbon (Z = 6) has the configuration 2, 4. Gaining 4 electrons (C⁴⁻) would be hard because 6 protons cannot hold 10 electrons, and losing 4 electrons (C⁴⁺) needs too much energy. So carbon shares electrons.

Single bond (1 shared pair):  H−H
Double bond (2 shared pairs): O=O
Triple bond (3 shared pairs): N≡N
Methane CH₄: 4 single C−H bonds

Covalent bond: a bond formed by sharing a pair of electrons between two atoms
Covalent compounds have low melting and boiling points and are poor conductors of electricity because they do not form ions

Allotropes of Carbon

Diamond: each carbon is bonded to four others in a rigid 3D network; hardest natural substance; does not conduct electricity
Graphite: each carbon is bonded to three others in hexagonal layers; layers slide easily; good conductor of electricity
Fullerene: C₆₀ molecule shaped like a football, called buckminsterfullerene

Versatile Nature of Carbon

Catenation: the ability of carbon to bond with other carbon atoms to form long chains, branched chains and rings
Tetravalency: a valency of 4 lets carbon bond with four other atoms such as H, O, N, S and halogens
Carbon's small size makes its bonds strong and stable

Saturated and Unsaturated Hydrocarbons

Compounds of only carbon and hydrogen are called hydrocarbons. Alkanes are saturated (single bonds only), while alkenes and alkynes are unsaturated.

Alkane: CₙH₂ₙ₊₂  (methane CH₄, ethane C₂H₆)
Alkene: CₙH₂ₙ    (ethene C₂H₄)
Alkyne: CₙH₂ₙ₋₂  (ethyne C₂H₂)

Structural isomers: same molecular formula, different structures; butane and isobutane are both C₄H₁₀
Carbon also forms rings, such as cyclohexane (C₆H₁₂) and benzene (C₆H₆)

Functional Groups and Homologous Series

An atom or group that replaces hydrogen and decides the properties of a compound is a functional group.

Halo: −Cl, −Br
Alcohol: −OH
Aldehyde: −CHO
Ketone: −CO−
Carboxylic acid: −COOH
Successive members differ by −CH₂− = 14 u

Homologous series: compounds with the same functional group where successive members differ by one −CH₂− unit
Members show similar chemical properties and a gradual change in physical properties such as boiling point

Naming Carbon Compounds

The name has a root word for the number of carbon atoms and a prefix or suffix for the functional group.

Root: meth 1, eth 2, prop 3, but 4, pent 5, hex 6
-ane, -ene, -yne (single, double, triple bond)
-ol alcohol, -al aldehyde, -one ketone
-oic acid carboxylic acid
chloro-, bromo- (halogen prefix)
CH₃CH₂CH₂OH = propanol
CH₃COCH₃ = propanone
CH₃CH₂Cl = chloroethane

Chemical Properties of Carbon Compounds

Saturated compounds burn with a clean blue flame, while unsaturated ones give a yellow sooty flame.

Combustion:
CH₄ + 2O₂ → CO₂ + 2H₂O + heat + light
Oxidation (alk. KMnO₄ or acidified K₂Cr₂O₇):
CH₃CH₂OH → CH₃COOH
Addition (Ni or Pd catalyst):
unsaturated oil + H₂ → saturated fat
Substitution (sunlight):
CH₄ + Cl₂ → CH₃Cl + HCl

Oxidising agents such as alkaline KMnO₄ convert alcohols into carboxylic acids
Hydrogenation turns vegetable oils into vegetable ghee

Ethanol, Ethanoic Acid and Soaps

Ethanol and ethanoic acid are two everyday carbon compounds. Esters formed from them smell sweet.

2Na + 2CH₃CH₂OH → 2CH₃CH₂ONa + H₂
CH₃CH₂OH → CH₂=CH₂ + H₂O
  (hot conc. H₂SO₄, 443 K)
CH₃COOH + CH₃CH₂OH → CH₃COOCH₂CH₃ + H₂O
  (acid catalyst, esterification)
Ester + NaOH → alcohol + sodium salt of acid
  (saponification)

Ethanoic acid: a 5 to 8% solution in water is vinegar; pure acid freezes in winter and is called glacial acetic acid
Soap molecules have an ionic (water-loving) head and a long hydrocarbon (oil-loving) tail; they form micelles that trap oily dirt
Soap forms scum with hard water (Ca²⁺ and Mg²⁺ ions); detergents work well even in hard water

Solved Examples

Example 1: Write the molecular formulae of the alkane with 7 carbon atoms and the alkene with 5 carbon atoms.

Alkane CₙH₂ₙ₊₂, n = 7 → C₇H₁₆
Alkene CₙH₂ₙ, n = 5 → C₅H₁₀
∴ C₇H₁₆ and C₅H₁₀

Example 2: How many covalent bonds are there in propane, C₃H₈?

C−C bonds = 2
C−H bonds = 8
∴ Total = 2 + 8 = 10 single covalent bonds

Example 3: Show that methanol (CH₃OH) and ethanol (C₂H₅OH) differ by 14 u.

CH₃OH = 12 + 4 + 16 = 32 u
C₂H₅OH = 24 + 6 + 16 = 46 u
46 − 32 = 14 u
∴ They differ by one −CH₂− unit (14 u)

Example 4: Name CH₃CH₂CHO, CH₃COCH₂CH₃ and CH₃CH₂CH₂COOH.

CH₃CH₂CHO: 3 C + aldehyde → propanal
CH₃COCH₂CH₃: 4 C + ketone → butanone
CH₃CH₂CH₂COOH: 4 C + acid → butanoic acid
∴ Propanal, butanone, butanoic acid

Example 5: Write the formula of the third member of the alkyne series.

1st: ethyne C₂H₂
2nd: propyne C₃H₄
3rd: CₙH₂ₙ₋₂ with n = 4
∴ Butyne, C₄H₆

Example 6: What forms when ethanoic acid reacts with sodium hydrogencarbonate?

CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂
∴ Sodium ethanoate, water and CO₂ (effervescence)

Example 7: A cooking oil and a sample of butter are tested with bromine water. Which one decolourises it, and why?

Answer: The oil contains C=C double bonds that take part in an addition reaction with bromine, so the oil (unsaturated) decolourises bromine water, while butter (mostly saturated) does not.

Common Mistakes to Avoid

Counting carbon atoms incorrectly while naming; include the carbon of −CHO or −COOH in the chain count.
Writing that diamond conducts electricity; only graphite conducts, because each carbon has one free electron.
Confusing addition with substitution: addition happens in unsaturated compounds, substitution in saturated ones.
Forgetting the catalyst or condition (Ni, sunlight, conc. H₂SO₄, 443 K) over the arrow; board answers expect it.

Quick Revision

Alkane CₙH₂ₙ₊₂, alkene CₙH₂ₙ, alkyne CₙH₂ₙ₋₂
Homologous series: difference −CH₂− = 14 u
CH₄ + Cl₂ → CH₃Cl + HCl (sunlight)
Acid + alcohol → ester + water