CBSE Class 10 Chemistry Carbon and its Compounds MCQs Set 32

Answer: (b) 7 covalent bonds

Explanation:
1. Ethane has one C–C bond and six C–H bonds.
2. The total is 1 + 6 = 7 covalent bonds.

Teacher's Note:
1. Each carbon forms four single bonds.
2. In an alkane CnH₂n+2 the number of bonds is 3n + 1.

Answer: (a) MgCl₂

Explanation:
1. Magnesium chloride is formed by transfer of electrons from Mg to Cl, so it has ionic bonds only.
2. HCl, CH₄ and CO₂ are made by sharing of electrons.

Teacher's Note:
1. Ionic compounds are formed between metals and non-metals.
2. Covalent compounds are formed between non-metals.

Answer: (c) 95 percent ethanol

Explanation:
1. Rectified spirit is the constant-boiling mixture of about 95.6% ethanol and 4.4% water.
2. It cannot be separated further by simple distillation.

Teacher's Note:
1. Absolute alcohol is 100% ethanol.
2. Denatured alcohol has poisonous substances added.

Answer: (b) Both A and R are true, but R is not the correct explanation of A.

Explanation:
1. Carbon forms long chains because of catenation, which comes from its small size and strong C–C bonds, so A is true.
2. Carbon is tetravalent, which allows it to bond with four other atoms, so R is also true, but the tetravalency is not the reason for chain formation.

Teacher's Note:
1. Catenation is the self-linking property of carbon.
2. Silicon also shows catenation to a smaller extent.

Answer: (a) acidic in nature (very weakly)

Explanation:
1. Sodium displaces hydrogen from ethanol, which shows that the O–H hydrogen is weakly acidic.
2. Ethanol does not change litmus, so it is only very weakly acidic.

Teacher's Note:
1. The reaction is similar to sodium and water.
2. The product is sodium ethoxide.

Answer: (b) two hydrogen atoms less than the corresponding alkene

Explanation:
1. An alkyne is CnH₂n–2 and the corresponding alkene is CnH₂n.
2. So an alkyne has two hydrogen atoms less than the alkene and four less than the alkane.

Teacher's Note:
1. Example: ethane C₂H₆, ethene C₂H₄, ethyne C₂H₂.
2. Each additional pi bond removes two H.

Answer: (b) 2-methylpentane-2,4-diol

Explanation:
1. The longest chain containing both OH groups has five carbons.
2. Both numbering directions give OH locants 2,4; the methyl then gets the lower locant 2 when numbered from the end carrying C(CH₃)(OH).
3. Name: 2-methylpentane-2,4-diol.

Teacher's Note:
1. Rule: lowest locants to functional groups first, then to substituents.
2. Diol: two OH groups.

Answer: (a) (ii), (iii) and (iv)

Explanation:
1. Ethene has a double bond, ethyne has a triple bond and benzene has alternating double bonds in its ring.
2. Ethane has only single bonds.

Teacher's Note:
1. Saturated: only single bonds.
2. Unsaturated: at least one multiple bond.

Answer: (b) ethene

Explanation:
1. Dehydration is the removal of water, the reverse of hydration.
2. C₂H₅OH → CH₂=CH₂ + H₂O with concentrated sulphuric acid at 443 K, so ethene is formed.

Teacher's Note:
1. Hydration and dehydration are opposite processes.
2. Conc. H₂SO₄ is the dehydrating agent.

Answer: (c) prefix (chloro), as in chloroethane

Explanation:
1. Halogen atoms are always named as prefixes in the IUPAC system: chloro, bromo, iodo.
2. So CH₃CH₂Cl is chloroethane.

Teacher's Note:
1. Alcohols, ketones and acids use suffixes (–ol, –one, –oic acid).
2. Halogens never form a suffix.

Answer: (b) the sulphonate group, –SO₃– Na+ (or sulphate)

Explanation:
1. Detergents are sodium salts of long-chain alkylbenzene sulphonic acids or alkyl sulphates.
2. The –SO₃– group is the polar (water-loving) head.

Teacher's Note:
1. Soaps have the carboxylate head, –COO–.
2. The long hydrocarbon tail is non-polar.

Answer: (b) covalent, XY₂ (such as CS₂)

Explanation:
1. Neither element can easily lose or gain four electrons, so they share electrons.
2. X shares four electrons and each Y shares two, so the compound is XY₂ with double bonds, for example S=C=S.

Teacher's Note:
1. Both are non-metals.
2. Covalent compounds are formed by sharing.

Answer: (c) many are non-biodegradable and pollute water

Explanation:
1. Detergents with branched hydrocarbon chains are not broken down by bacteria and remain in water causing pollution and foaming.
2. Their advantage is that they work in hard water.

Teacher's Note:
1. Modern detergents use straight chains, which are biodegradable.
2. Phosphates cause algal bloom.

Answer: (c) ethanol and ethanoic acid

Explanation:
1. Ethanol is used as an antiseptic.
2. Oxidation gives ethanoic acid: CH₃CH₂OH + 2[O] → CH₃COOH + H₂O.

Teacher's Note:
1. Alkaline KMnO₄ or acidified K₂Cr₂O₇ are oxidising agents.
2. Both compounds have two carbons.

Answer: (d) –COO–, >C=O, –CHO and –NH₂

Explanation:
1. The ester group is –COO–, the carbonyl group >C=O, the aldehyde group –CHO and the amino group –NH₂.
2. So the first option matches.

Teacher's Note:
1. Carbonyl group is common to aldehydes, ketones, acids and esters.
2. Amines are derivatives of ammonia.

Answer: (d) converting vegetable oils into solid fats (vanaspati ghee)

Explanation:
1. Unsaturated vegetable oils are liquid; adding hydrogen to the double bonds gives saturated fats that are solid.
2. The product is vanaspati ghee.

Teacher's Note:
1. Ni is the catalyst at about 473 K.
2. Unsaturated fats are healthier than saturated ones.

Answer: (a) having the same functional group and general formula, with successive members differing by a –CH₂– unit

Explanation:
1. Members of a homologous series, for example CH₄, C₂H₆, C₃H₈, have similar chemical properties and show a gradation in physical properties.
2. Successive members differ by CH₂ (14 u).

Teacher's Note:
1. Example: alkanes CnH₂n+2.
2. Properties such as boiling point increase with the chain length.

Answer: (b) the ionic heads dissolve in water while the hydrocarbon tails clump together away from water; micelles are not formed in ethanol

Explanation:
1. In water the hydrocarbon tails avoid water and aggregate inside while the ionic heads face the water.
2. In ethanol, which dissolves the hydrocarbon tail, the soap does not form micelles.

Teacher's Note:
1. Micelles are spherical clusters.
2. They scatter light so the soap solution appears cloudy.

Answer: (a) C₂H₄, because it has a C=C double bond

Explanation:
1. C₂H₄ (ethene) contains a carbon–carbon double bond, so it is unsaturated.
2. Methane has only single bonds.

Teacher's Note:
1. Unsaturated compounds decolourise bromine water.
2. Saturated compounds have only single bonds.

Answer: (d) C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O + heat and light

Explanation:
1. Ethanol burns in excess oxygen to carbon dioxide and water, releasing heat and light.
2. Balancing: 2 C, 6 H and 7 O atoms on each side.

Teacher's Note:
1. Balance C first, then H, then O.
2. Ethanol is used as a fuel in spirit lamps.