Class 10 Science Chapter 4: Important & Previous Year Questions – Bihar Board

Correct answer: (D) C₆H₆

Benzene is a ring of six carbons with alternate single and double bonds — C₆H₆ (lesson 3, Fig. 4.10). C₂H₂ is ethyne, C₂H₄ ethene, CH₄ methane.

Correct answer: (C) Single bond

Each H has one electron; together they form one shared pair — a single bond, H—H (lesson 1, Fig. 4.1, 4.2).

Correct answer: (D) C₂H₄

C₂H₆, CH₄ and C₃H₈ are saturated alkanes. C₂H₄ (ethene) has a C=C double bond — unsaturated compounds are more reactive than saturated ones (lesson 3).

Vinegar = a 5–8% solution of acetic acid (ethanoic acid, CH₃COOH) in water.

Uses:

• as a preservative in pickles;
• to give a sour taste to food;
• acetic acid is a weak acid, so it is safe to use in food in small amounts.

The reaction of a carboxylic acid with an alcohol in the presence of an acid catalyst (conc. H₂SO₄) to form an ester is esterification.

Ethanoic acid + ethanol → ethyl ethanoate (ester) + water. Esters smell sweet — used in perfumes and flavourings.

Chemical bond: the force that holds atoms together in a molecule or compound. Atoms bond to fill their outer shell (noble gas configuration). Two main types:

1. Ionic (electrovalent) bond — by transfer of electrons (Chapter 3). Na (2, 8, 1) loses one e⁻ → Na⁺; Cl (2, 8, 7) gains it → Cl⁻; their attraction = NaCl. High melting point, soluble in water, conducts when molten or dissolved.

2. Covalent bond — by sharing electrons (lesson 1 of this chapter). Single: H—H, CH₄; double: O=O; triple: N≡N. No ions form, so usually poor conductors; weak intermolecular forces, low melting and boiling points.

👉 Carbon can neither lose nor gain four electrons easily, so it forms covalent bonds.

Carbon compounds number in millions — more than those of all other elements put together. Reasons:

1. Catenation: carbon links with other carbon atoms to form long straight, branched or ring chains; C–C bonds may be single, double or triple. The C–C bond is very strong and stable. Silicon forms chains of only 7–8 atoms, and those compounds are very reactive.

2. Tetravalency: with valency 4, carbon bonds with four other atoms (C, H, O, N, S, Cl…) — each element gives compounds with new properties.

3. Small size: the nucleus holds the shared electrons strongly, so bonds are strong and compounds are stable.

4. Isomerism: one formula can have several structures, e.g. two for C₄H₁₀.

👉 Other unique features: allotropes (diamond, graphite, C₆₀); all living structures are carbon based.

Correct answer: (D) Carbonyl group

>C=O is the carbonyl group; between two carbons it is a ketone (suffix ‘-one’), e.g. propanone (lesson 4, Table 4.3). The aldehyde group is —CHO and the alcohol group —OH.

Correct answer: (B) Ethyne

Ethyne (C₂H₂) is burnt with oxygen — complete combustion gives a very high temperature (lesson 5; NCERT in-text question, page 71).

Correct answer: (D) 14

Carbon (2, 4) has 4 electrons in its outer shell — group 14 of the modern periodic table. Note: 6 is carbon's atomic number, not its group.

Correct answer: (C) 3

Ethyne H—C≡C—H has a triple bond between the two carbons — 3 shared electron pairs (lesson 3). (In the paper's English options, (D) is misprinted as 3; the Hindi options read 4.)

Pentane C₅H₁₂ has three structural isomers:

• n-pentane (straight chain): CH₃—CH₂—CH₂—CH₂—CH₃
• isopentane / 2-methylbutane: CH₃—CH(CH₃)—CH₂—CH₃
• neopentane / 2,2-dimethylpropane: C(CH₃)₄ — four CH₃ on the central carbon

All three have the same formula (C₅H₁₂) but different carbon skeletons.