Carbon and its Compounds — RBSE Class 10 (Science)

Every living thing you have ever met is built on carbon. One small atom, by sharing electrons and linking to itself in endless chains and rings, gives rise to millions of compounds — fuels, plastics, medicines, food, DNA. This chapter explains the two "superpowers" of carbon and introduces the organic chemistry that follows from them.

1. Why carbon forms so many compounds

Carbon has 4 valence electrons, so it needs 4 more for a full octet. Losing or gaining 4 electrons is energetically impossible, so carbon shares electrons — it forms covalent bonds. Two properties then explode the number of compounds:

• Catenation — carbon links to other carbon atoms in long chains, branches and rings (stronger and more extensive than any other element).
• Tetravalency — each carbon can bond to four atoms, allowing huge, varied structures.

Covalent compounds: low melting/boiling points, poor conductors (no ions), usually insoluble in water.

Bonds can be single (C–C), double (C=C) or triple (C≡C). Compounds with only single bonds are saturated (alkanes); those with double/triple bonds are unsaturated (alkenes/alkynes).

2. Homologous series and functional groups

A functional group is an atom/group that gives a compound its characteristic properties (–OH alcohol, –CHO aldehyde, –COOH carboxylic acid, –C=C– alkene, halogen).

A homologous series is a family of compounds with the same functional group and general formula, each member differing from the next by –CH₂– (14 u). Members show a gradual change in physical properties but similar chemical behaviour. Alkanes: $C_n{H}_{2n+2}$; alkenes: $C_n{H}_{2n}$; alkynes: $C_n{H}_{2n-2}$.

3. Naming carbon compounds (IUPAC)

1. Count the carbons → root (meth-1, eth-2, prop-3, but-4, pent-5).
2. Saturation → suffix (-ane / -ene / -yne).
3. Functional group → prefix or suffix (e.g. -ol for alcohol, -oic acid for carboxylic acid).

So $C{H}_{3}OH$ = methanol; $C{H}_{3}COOH$ = ethanoic acid; $C_2{H}_4$ = ethene.

4. Chemical properties

• Combustion — carbon compounds burn in air to give CO₂, water and heat: $C{H}_4+2O_2\to C{O}_2+2H_{2}O$. Saturated hydrocarbons give a clean blue flame; unsaturated ones a sooty yellow flame.
• Oxidation — alcohols are oxidised to carboxylic acids by oxidising agents (alkaline KMnO₄, acidified K₂Cr₂O₇).
• Addition — unsaturated hydrocarbons add H₂ (with Ni catalyst) to become saturated; used to hydrogenate vegetable oils into vanaspati.
• Substitution — saturated hydrocarbons react with Cl₂ in sunlight, one H at a time: $C{H}_4+C{l}_2\stackrel{\text{sunlight}}{\to }C{H}_{3}Cl+HCl$.

5. Two important compounds

Ethanol (C₂H₅OH): a liquid at room temperature, neutral, used in drinks, as a solvent and fuel. Reacts with sodium (→ H₂), and on heating with conc. H₂SO₄ dehydrates to ethene.

Ethanoic acid (CH₃COOH): "acetic acid"; 5–8% solution is vinegar. A weak acid — turns blue litmus red, reacts with carbonates (→ CO₂), with NaOH (→ salt + water), and with ethanol + acid catalyst to give a sweet-smelling ester (esterification).

6. Soaps and detergents

Soap is the sodium/potassium salt of a long-chain fatty acid, made by saponification (fat + NaOH → soap + glycerol). A soap molecule has a hydrophilic (water-loving) head and a hydrophobic (oil-loving) tail. In water the tails cluster inward around grease, forming a micelle, which lifts the dirt away.

Soaps fail in hard water (form scum with Ca²⁺/Mg²⁺); detergents work even in hard water because their calcium/magnesium salts are soluble.

7. Closing thought

Everything flows from carbon's catenation and tetravalency: covalent bonding → chains and functional groups → homologous series → named compounds with predictable reactions. Master the functional groups, the four reaction types, and the soap/micelle mechanism. In the RBSE board this is one of the highest-scoring chapters, with reliable questions on IUPAC naming, ethanol/ethanoic acid and soaps.