Ans: The valency of carbon is 4.

Ans: Diamond and Graphite are two allotropes of carbon.

Ans: Saturated hydrocarbons have only single C–C bonds (alkanes). Example: Ethane (C₂H₆). Unsaturated hydrocarbons have double or triple C–C bonds. Example: Ethene (C₂H₄) — double bond; Ethyne (C₂H₂) — triple bond.

Ans: Soap molecules have a hydrophilic head (attracts water) and hydrophobic tail (repels water, attracts oil). In water, tails surround oil droplets while heads face outward, forming micelles. These micelles keep oil suspended in water and are washed away.

Ans: A homologous series is a group of organic compounds with same functional group, differing by –CH₂– unit and 14 mass units. They show gradual change in physical properties. Alkanes: CH₄, C₂H₆, C₃H₈ (CₙH₂ₙ₊₂). Alkenes: C₂H₄, C₃H₆ (CₙH₂ₙ). Alkynes: C₂H₂, C₃H₄ (CₙH₂ₙ₋₂). Functional groups determine chemical behaviour.

Ans: Carbon forms an extremely large number of compounds due to two unique properties: (1) Catenation: Carbon atoms can bond with other carbon atoms to form long chains, branched chains, and ring (cyclic) structures. No other element shows this property to such an extent. (2) Tetravalency: Carbon has 4 valence electrons and can form 4 covalent bonds — with hydrogen, oxygen, nitrogen, sulphur, halogens, and other carbon atoms — giving rise to millions of stable compounds (the basis of organic chemistry).

Ans: A homologous series is a series of organic compounds with the same functional group and similar chemical properties, where each successive member differs by a –CH₂– unit (14 u) in molecular mass. Example: Alkanes — CH₄, C₂H₆, C₃H₈, C₄H₁₀... Characteristics: (1) Same general formula (alkanes: CₙH₂ₙ₊₂). (2) Same functional group → similar chemical properties. (3) Physical properties (BP, MP, density) change gradually with increasing carbon chain length. (4) Each member is called a homologue; consecutive members differ by 14 u. (5) Can be prepared by similar methods. Example: Alcohols: CH₃OH, C₂H₅OH, C₃H₇OH... (–OH group, formula CₙH₂ₙ₊₂O).

Ans: Esterification: Reaction between a carboxylic acid and an alcohol in the presence of an acid catalyst (conc. H₂SO₄) and heat to form an ester and water. CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O (ethyl ethanoate/ethyl acetate). Esters have a sweet fruity smell — used in perfumes, flavouring agents, solvents. Saponification: Hydrolysis of an ester using a base (NaOH) to give back the alcohol and sodium salt of the acid (soap). CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH. On a larger scale: Fat + NaOH → Soap + Glycerol. This is how soaps are manufactured (alkaline hydrolysis of fats/oils).

Ans: Diamond: Each C is bonded to 4 other C atoms in a 3D tetrahedral structure. Hardest natural substance. Does not conduct electricity (no free electrons). Very high MP. Used in cutting tools, jewellery. Graphite: Each C is bonded to 3 other C atoms in flat hexagonal layers. Soft and slippery (layers slide). Conducts electricity (1 free electron per C). Used as lubricant, pencil leads, electrodes. Key difference: Diamond (sp³ hybridisation, no free electrons) vs Graphite (sp² hybridisation, delocalised electrons in each layer).

Ans: Hard water contains Ca²⁺ and Mg²⁺ ions. Soap (sodium stearate, C₁₇H₃₅COONa) reacts with these ions to form insoluble calcium/magnesium salts (scum): 2C₁₇H₃₅COONa + CaCl₂ → (C₁₇H₃₅COO)₂Ca↓ + 2NaCl. This white scum is deposited on clothes and wastes soap — no lather forms until all Ca²⁺/Mg²⁺ are precipitated. Solution: Detergents are used instead. Detergents have a sulphonate group (–SO₃Na) instead of carboxylate — their calcium and magnesium salts are soluble, so they work in hard water.

Ans: Catenation is the property of an element to form bonds with its own atoms, giving long chains, branched chains and rings. Carbon shows catenation to the maximum extent.

Ans: The third member of the alkane series is propane, C₃H₈ (after methane CH₄ and ethane C₂H₆).

Ans: CH₃COOH contains the carboxylic acid group (–COOH). CH₃CHO contains the aldehyde group (–CHO).

Ans: In methane (CH₄) carbon (2, 4) shares each of its four valence electrons with one hydrogen atom, and each hydrogen shares its one electron with carbon. Thus carbon forms four single covalent bonds with four hydrogen atoms, and each atom gets a stable configuration (carbon an octet, hydrogen a duplet).

Ans: Carbon compounds are formed by covalent bonds, in which electrons are shared and not transferred. No ions are formed, so there are no free charged particles to carry current.

Ans: Add sodium hydrogencarbonate (or sodium carbonate) to each. Ethanoic acid gives brisk effervescence of CO₂, which turns lime water milky; ethanol does not. Ethanoic acid also turns blue litmus red, while ethanol is neutral to litmus.

Ans: (a) Propan-1-ol (propanol). (b) Propanal. (c) Propanone. (d) Propanoic acid. (e) Propene.

Ans: Hydrogenation is the addition of hydrogen to unsaturated hydrocarbons in the presence of a catalyst such as nickel or palladium to give saturated compounds: CH₂=CH₂ + H₂ → CH₃–CH₃ (Ni catalyst). It is used industrially to convert vegetable oils (unsaturated) into solid saturated fats such as vanaspati ghee.

Ans: Combustion: CH₄ + 2O₂ → CO₂ + 2H₂O + heat and light; saturated compounds burn with a blue flame, unsaturated with a sooty yellow flame. Oxidation: alcohol is oxidised to carboxylic acid with alkaline KMnO₄ or acidified K₂Cr₂O₇: CH₃CH₂OH → CH₃COOH. Addition: unsaturated hydrocarbons add hydrogen in the presence of Ni: CH₂=CH₂ + H₂ → CH₃CH₃. Substitution: in sunlight, a hydrogen atom of a saturated hydrocarbon is replaced: CH₄ + Cl₂ → CH₃Cl + HCl.

Ans: With sodium: 2CH₃CH₂OH + 2Na → 2CH₃CH₂ONa + H₂ (sodium ethoxide and hydrogen gas). With hot conc. H₂SO₄ at 443 K (dehydration): CH₃CH₂OH → CH₂=CH₂ + H₂O (ethene). Uses: as a solvent for medicines, tinctures and cough syrups; as a fuel additive (power alcohol). Harmful effects: drinking it affects the nervous system, causes addiction and liver damage; pure methanol is poisonous and can cause blindness or death, so industrial alcohol is denatured.