CBSE Class 10 Chemistry Carbon and its Compounds MCQs Set 42

Answer: (a) alkenes

Explanation:
1. Alkenes add water across the double bond in the presence of an acid catalyst to give alcohols.
2. For example CH₂=CH₂ + H₂O → CH₃CH₂OH.

Teacher's Note:
1. Addition of water is called hydration.
2. Alkanes are saturated and do not add water.

Answer: (c) propanone

Explanation:
1. A ketone has the carbonyl group between two carbon atoms, so it needs at least three carbons.
2. The first member is propanone, CH₃COCH₃ (acetone).

Teacher's Note:
1. The first aldehyde is methanal (HCHO).
2. Ketone suffix: –one.

Answer: (a) H₂

Explanation:
1. H₂ is formed by sharing an electron pair between two hydrogen atoms.
2. KCl, BaC₂ and Na₂S are made of a metal and a non-metal and are ionic.

Teacher's Note:
1. Elements made of two identical non-metal atoms are covalent.
2. Metal + non-metal gives an ionic compound.

Answer: (b) 4

Explanation:
1. Carbons joined by a triple bond are sp hybridised with a bond angle of 180°, so C-1, C-2, C-3 and C-4 lie in a line.
2. C-4 is sp² and the C-4–C-5 double bond is at 120°, so C-5 is off the line.

Teacher's Note:
1. Linear geometry: sp carbon.
2. Trigonal planar: sp₂ carbon.

Answer: (c) diamond

Explanation:
1. Diamond is a crystalline allotrope made only of carbon atoms.
2. Charcoal and coal contain impurities.

Teacher's Note:
1. Graphite is also pure but diamond is the standard answer.
2. Fullerenes are other pure forms.

Answer: (a) 3,4,4-trimethyloctane

Explanation:
1. The longest chain runs through the butyl group and has eight carbons: CH₃–CH₂–CH(CH₃)–C(CH₃)₂–CH₂–CH₂–CH₂–CH₃.
2. Numbering from the left gives methyl groups at C-3, C-4 and C-4 (locants 3, 4, 4).
3. Name: 3,4,4-trimethyloctane.

Teacher's Note:
1. Choose the longest chain, then the lowest locants.
2. Octane has eight carbons.

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

Explanation:
1. Finely divided nickel, palladium and platinum are the catalysts for adding hydrogen to unsaturated compounds.
2. Carbon is not a hydrogenation catalyst.

Teacher's Note:
1. Ni is cheapest and used for vanaspati ghee.
2. Pd and Pt work at lower temperatures.

Answer: (a) pentane and 2,2-dimethylpropane

Explanation:
1. C₅H₁₂ has three isomers: pentane, 2-methylbutane and 2,2-dimethylpropane.
2. 2,3-Dimethylpropane and 3-methylbutane are not valid names (the numbering is wrong).

Teacher's Note:
1. The chain should be numbered from the end giving the lowest locant.
2. Branched alkanes have lower boiling points.

Answer: (a) carboxylic acids

Explanation:
1. –COOH is called the carboxyl group.
2. Compounds containing it are carboxylic acids, such as ethanoic acid.

Teacher's Note:
1. –OH: alcohol, –CHO: aldehyde, >C=O: ketone.
2. Functional groups give the characteristic reactions.

Answer: (d) glycerol (propane-1,2,3-triol)

Explanation:
1. Glycerides are triesters of glycerol with three fatty acid molecules.
2. On saponification glycerol is released as the by-product.

Teacher's Note:
1. Glycerol has three –OH groups.
2. It is a viscous sweet liquid.

Answer: (d) NaCl and CCl₄

Explanation:
1. NaCl is formed by transfer of an electron from Na to Cl, so it is electrovalent.
2. CCl₄ is formed by sharing of electrons, so it is covalent.

Teacher's Note:
1. Metals form ionic chlorides.
2. Non-metals form covalent chlorides.

Answer: (d) ethyl ethanoate (a sweet-smelling ester) and water

Explanation:
1. CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O, in the presence of conc. H₂SO₄ as catalyst.
2. This is esterification, and esters are fruity smelling.

Teacher's Note:
1. Sulphuric acid also removes water.
2. The reverse reaction is hydrolysis.

Answer: (b) soaps form scum with hard water, while detergents do not

Explanation:
1. Soaps react with the Ca²⁺ and Mg²⁺ ions of hard water to form insoluble scum.
2. The calcium and magnesium salts of detergents are soluble, so they do not form scum.

Teacher's Note:
1. Detergents are made from petroleum products.
2. Soaps are biodegradable.

Answer: (a) C₄H₁₀ and C₁₀H₂₂

Explanation:
1. A saturated hydrocarbon (alkane) fits CnH₂n+2: C₄H₁₀ (n = 4) and C₁₀H₂₂ (n = 10).
2. The others have fewer hydrogens and therefore have multiple bonds.

Teacher's Note:
1. Unsaturated hydrocarbons have double or triple bonds or rings with double bonds.
2. C₆H₆ is benzene.

Answer: (a) red litmus turns blue (soap is basic), blue litmus is unchanged

Explanation:
1. Soap is the salt of a strong base (NaOH) and a weak fatty acid, so its solution is slightly alkaline.
2. It turns red litmus blue.

Teacher's Note:
1. Detergents are generally neutral.
2. Soap solution feels slippery.

Answer: (d) sodium hydrogencarbonate solution, which gives CO₂ with the acid but not with the alcohol

Explanation:
1. Carboxylic acids react with NaHCO₃ giving brisk effervescence of CO₂, which turns lime water milky.
2. Alcohols do not react.

Teacher's Note:
1. Another test is blue litmus: only the acid turns it red.
2. Alcohol is neutral.

Answer: (b) it helps the micelles to pull out and disperse the oily dirt from the fabric

Explanation:
1. Soap molecules surround the oily dirt but need mechanical energy to lift it from the fibres.
2. Agitation separates the dirt and keeps the micelles suspended in water for rinsing.

Teacher's Note:
1. The oil droplets are held in the centre of the micelle.
2. Rinsing removes the micelles.

Answer: (c) propane

Explanation:
1. The boiling point of the members of a homologous series increases with the molar mass because of stronger intermolecular forces.
2. Propane is the heaviest of the three.

Teacher's Note:
1. CH₄ –162 °C, C₂H₆ –89 °C, C₃H₈ –42 °C.
2. Longer chains have stronger van der Waals forces.

Answer: (b) to add sodium metal: hydrogen gas is evolved with brisk fizzing

Explanation:
1. Alcohols react with sodium to give hydrogen: 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂.
2. Alcohols do not give CO₂ with NaHCO₃ and do not affect litmus.

Teacher's Note:
1. The hydrogen burns with a pop sound.
2. Water also reacts with sodium, so the sample must be dry.