CBSE Class 10 Chemistry Carbon and its Compounds MCQs Set 36

Answer: (c) ketone

Explanation:
1. The suffix –one denotes a ketone, with the carbonyl group C=O between two carbon atoms.
2. Butanone is CH₃COCH₂CH₃.

Teacher's Note:
1. –al is an aldehyde, –ol an alcohol and –oic acid a carboxylic acid.
2. Propanone (acetone) is the first ketone.

Answer: (b) ethene

Explanation:
1. An alkene needs a C=C double bond, so at least two carbon atoms are needed.
2. The first member is ethene, C₂H₄.

Teacher's Note:
1. Methene does not exist.
2. General formula of alkenes: CnH₂n.

Answer: (d) glycerides (fats and oils)

Explanation:
1. Fats and oils are glycerides, esters of glycerol with long-chain fatty acids.
2. Boiling them with NaOH gives soap (sodium salt of a fatty acid) and glycerol.

Teacher's Note:
1. This reaction is alkaline hydrolysis of esters.
2. Glycerol is the by-product.

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

Explanation:
1. The three compounds form a homologous series because they have the same functional group, similar properties and successive members differ by a CH₂ unit, so A is true.
2. They are all hydrocarbons, so R is true, but being hydrocarbons alone does not make them a homologous series.

Teacher's Note:
1. Members of a homologous series differ by CH₂ (14 mass units).
2. Ethane and ethene are hydrocarbons but are not in the same series.

Answer: (a) pure acetic acid which freezes into ice-like crystals at about 17 °C

Explanation:
1. Pure (100%) ethanoic acid freezes at about 16.6 °C into ice-like crystals, so it is called glacial acetic acid.
2. Vinegar is the 5–8% solution.

Teacher's Note:
1. Glacial means ice-like.
2. It is a corrosive liquid.

Answer: (d) aldehydes, ketones and esters, all of these

Explanation:
1. The carbonyl group is present in aldehydes (–CHO), ketones (>C=O) and esters (–COO–).
2. So all of them contain it.

Teacher's Note:
1. Carboxylic acids also contain C=O.
2. Alcohols do not.

Answer: (a) CnH₂n and CnH₂n+1

Explanation:
1. An alkene has formula CnH₂n.
2. An alkyl group is an alkane minus one hydrogen, CnH₂n+1.

Teacher's Note:
1. Methyl is CH₃, ethyl is C₂H₅.
2. Alkane is CnH₂n+2.

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

Explanation:
1. In diamond each carbon is bonded to four others in a rigid three-dimensional network, so it is the hardest substance and bends light strongly.
2. It has no free electrons so it does not conduct electricity, and it does not mark paper.

Teacher's Note:
1. Graphite conducts and marks paper.
2. Diamond is used in cutting tools.

Answer: (a) it has a pi-bond that can break to add H and OH

Explanation:
1. The pi-bond is weaker and more reactive than a sigma bond and breaks easily.
2. The two carbons then bond to H and OH, giving ethanol.

Teacher's Note:
1. Unsaturated compounds give addition reactions.
2. Alkanes do not add water.

Answer: (d) exothermic, releasing heat and light

Explanation:
1. Burning of carbon compounds in oxygen gives CO₂ and water along with heat and light.
2. They are therefore exothermic, which is why carbon compounds are used as fuels.

Teacher's Note:
1. CH₄ + 2O₂ → CO₂ + 2H₂O + heat.
2. Incomplete combustion gives soot and CO.

Answer: (a) glycerol

Explanation:
1. Fat + NaOH → soap + glycerol.
2. Glycerol is the alcohol part of the fat.

Teacher's Note:
1. This is an alkaline hydrolysis of an ester.
2. Glycerol is valuable in cosmetics.

Answer: (c) sodium ethoxide and hydrogen gas

Explanation:
1. 2CH₃CH₂OH + 2Na → 2CH₃CH₂ONa + H₂.
2. Hydrogen is evolved, and the product is sodium ethoxide.

Teacher's Note:
1. The reaction shows the weakly acidic hydrogen of alcohol.
2. Sodium reacts more slowly than with water.

Answer: (a) 2, 3 and 5

Explanation:
1. Butane has two isomers (n-butane, isobutane).
2. Pentane has three and hexane has five.

Teacher's Note:
1. The number of isomers increases rapidly with carbon number.
2. C₇H₁₆ has nine.

Answer: (d) sodium hydroxide solution (caustic soda)

Explanation:
1. The process is saponification: fat + NaOH → soap + glycerol.
2. Common salt is added later to precipitate (salt out) the soap.

Teacher's Note:
1. The soap floats as a curd.
2. Potassium hydroxide gives soft soap.

Answer: (c) oxygen is added (or hydrogen is removed)

Explanation:
1. CH₃CH₂OH + 2[O] → CH₃COOH + H₂O.
2. Gain of oxygen means oxidation, and the oxidising agent here is alkaline KMnO₄ or acidified K₂Cr₂O₇.

Teacher's Note:
1. Oxidising agents give oxygen to other substances.
2. Reverse: reduction.

Answer: (a) detergents form no scum with hard water, whereas a soap does

Explanation:
1. Detergents lather even in hard water as their Ca and Mg salts are soluble.
2. So no scum is formed to show the hardness; soap is used for the test.

Teacher's Note:
1. Hardness is due to dissolved Ca²⁺ and Mg²⁺ salts.
2. Scum is calcium or magnesium soap.

Answer: (a) It turns blue litmus red and liberates CO₂ with NaHCO₃

Explanation:
1. Ethanoic acid is a weak acid: it turns blue litmus red and reacts with NaHCO₃ to give CO₂.
2. Ethanol does not change litmus and does not liberate CO₂.

Teacher's Note:
1. Both react with sodium to give hydrogen.
2. Ethanoic acid freezes at 16.6 °C and ethanol at –114 °C.

Answer: (b) C₄H₆ and C₆H₁₀

Explanation:
1. The series starts with ethyne, C₂H₂ (n = 2), then C₃H₄, C₄H₆, C₅H₈, C₆H₁₀.
2. The third member is C₄H₆ and the fifth is C₆H₁₀.

Teacher's Note:
1. Alkynes: CnH₂n–2.
2. Count members starting from n = 2.

Answer: (d) pentane, 2-methylbutane and 2,2-dimethylpropane

Explanation:
1. The three isomers of C₅H₁₂ are n-pentane, 2-methylbutane (isopentane) and 2,2-dimethylpropane (neopentane).
2. The other names are not valid.

Teacher's Note:
1. All isomers have the same formula and molar mass.
2. Branching lowers the boiling point.

Answer: (c) detergents lather in hard water and do not form scum

Explanation:
1. Detergents work even with hard water since their calcium and magnesium salts are soluble.
2. Soaps form insoluble scum with hard water, reducing cleaning.

Teacher's Note:
1. Detergents are made from petroleum chemicals.
2. Biodegradability depends on the structure.