CBSE Class 10 Chemistry Carbon and its Compounds MCQs Set 39

Answer: (c) 6

Explanation:
1. Each of the two carbon atoms in CH₃–CH₃ is attached to three hydrogen atoms.
2. So there are 3 + 3 = 6 C–H bonds.

Teacher's Note:
1. The seventh bond of ethane is the C–C bond.
2. Ethane is C₂H₆.

Answer: (a) four carbon atoms

Explanation:
1. Branching means a side chain on the main chain.
2. With three carbons there is no way to attach a carbon to a middle carbon without making the main chain longer, so the minimum is four (isobutane).

Teacher's Note:
1. Butane has two isomers, n-butane and isobutane.
2. Pentane has three isomers.

Answer: (a) combining atoms share electron pairs

Explanation:
1. A covalent bond is formed by the mutual sharing of electrons between atoms to achieve a stable configuration.
2. Electrostatic attraction between ions gives an ionic bond.

Teacher's Note:
1. Single, double and triple bonds share 1, 2 or 3 pairs.
2. Carbon completes its octet by sharing.

Answer: (a) A-S, B-P, C-Q, D-R

Explanation:
1. Alkane CnH₂n+2: C₁₆H₃₄ (S). Alkene CnH₂n: C₁₂H₂₄ (P).
2. Alkyne CnH₂n–2: C₈H₁₄ (Q). Alcohol CnH₂n+1OH: C₈H₁₇OH (R).

Teacher's Note:
1. Check general formulas.
2. Alkynes have two H less than alkenes.

Answer: (a) it melts at 800 °C

Explanation:
1. Graphite has a very high melting point (above 3500 °C), so melting at 800 °C is not its property.
2. It is slippery, conducts electricity because of free electrons and marks paper.

Teacher's Note:
1. Graphite is used as a lubricant and in electrodes.
2. Pencil lead is graphite.

Answer: (b) 2-methylpropan-2-ol

Explanation:
1. t-Butyl alcohol is (CH₃)₃C–OH.
2. The longest chain with the OH carbon has three carbons with a methyl on C-2, so the name is 2-methylpropan-2-ol.

Teacher's Note:
1. The OH is on the tertiary carbon.
2. Butan-2-ol is secondary butyl alcohol.

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

Explanation:
1. Ethane gives a substitution reaction with chlorine in sunlight, and burns to CO₂ and water.
2. It is saturated and cannot be hydrogenated.
3. Ethane and isobutane are both alkanes, so they belong to the same homologous series.

Teacher's Note:
1. Hydrogenation needs a double or triple bond.
2. Alkanes: substitution and combustion.

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

Explanation:
1. Soaps are made from glycerides, which are present in vegetable oils such as cottonseed, linseed and soyabean oil.
2. Diesel is a hydrocarbon mixture and does not saponify.

Teacher's Note:
1. Animal fats are also used.
2. Mineral oils are not esters.

Answer: (b) poor conductors of electricity, since they contain no ions

Explanation:
1. Covalent compounds are formed by sharing electrons and have no free ions or electrons.
2. So they do not conduct electricity (graphite and some acids in water are exceptions).

Teacher's Note:
1. Ionic compounds conduct when molten or dissolved.
2. Covalent compounds have low melting and boiling points.

Answer: (b) CH₃COOH

Explanation:
1. Ethanoic acid is a carboxylic acid and gives H+ in water, so it turns blue litmus red.
2. Alcohols, aldehydes and ketones are neutral.

Teacher's Note:
1. The carboxyl group is acidic.
2. Mineral acids are much stronger.

Answer: (b) the enzymes in yeast get denatured and stop working

Explanation:
1. Fermentation is brought about by the enzymes zymase and invertase of yeast.
2. These enzymes work best at 25–35 °C and are denatured by high temperature.

Teacher's Note:
1. Enzymes are proteins.
2. Very low temperatures also stop the enzymes.

Answer: (d) sodium ethanoate and ethanol; saponification

Explanation:
1. CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH.
2. The alkaline hydrolysis of an ester is called saponification.

Teacher's Note:
1. Soap making is the same reaction on a fat.
2. Esterification is the reverse process.

Answer: (b) a 5–8% solution of ethanoic acid in water

Explanation:
1. Vinegar is dilute acetic acid, made by bacterial oxidation of dilute ethanol.
2. It is used as a preservative in pickles.

Teacher's Note:
1. Glacial acetic acid is pure.
2. It has a pH of around 3.

Answer: (c) in diamond each carbon is joined to four others in a rigid 3D network, while in graphite layers are held by weak forces

Explanation:
1. Diamond has a tetrahedral network of strong covalent bonds in all directions.
2. Graphite has hexagonal layers held to each other only by weak van der Waals forces, so the layers slide over one another.

Teacher's Note:
1. Graphite conducts since each carbon has one free electron.
2. Both are allotropes of carbon.

Answer: (b) supply oxygen or remove hydrogen (or accept electrons)

Explanation:
1. An oxidising agent oxidises other substances by giving them oxygen or taking hydrogen or electrons.
2. Examples: alkaline KMnO₄, acidified K₂Cr₂O₇.

Teacher's Note:
1. The oxidising agent is itself reduced.
2. Reducing agents do the opposite.

Answer: (d) 3

Explanation:
1. Pentane (C₅H₁₂) has three isomers: n-pentane, 2-methylbutane (isopentane) and 2,2-dimethylpropane (neopentane).
2. So the number is three.

Teacher's Note:
1. Butane has two and hexane has five.
2. Branched isomers have lower boiling points.

Answer: (b) a hydrophilic ionic head that dissolves in water and a hydrophobic hydrocarbon tail that dissolves in oil or grease

Explanation:
1. The hydrocarbon tail embeds in the oily dirt while the ionic head stays in water.
2. Micelles are thus formed, which carry the dirt away when rinsed.

Teacher's Note:
1. Agitation helps break the dirt and form micelles.
2. Micelles scatter light.

Answer: (a) A is a gas (butane) and B is a liquid (hexane)

Explanation:
1. The first four alkanes (C-1 to C-4) are gases at room temperature.
2. C-5 to C-17 are liquids, so butane is a gas and hexane is a liquid.

Teacher's Note:
1. Melting and boiling points rise with molar mass.
2. Higher alkanes (above C-17) are solids.

Answer: (a) poisonous substances such as methanol, pyridine or a dye

Explanation:
1. Ethanol for industrial use is denatured by adding methanol, copper sulphate or pyridine so it cannot be drunk.
2. Drinking methanol damages the optic nerve and can cause death.

Teacher's Note:
1. This is to prevent misuse and to avoid tax.
2. Methylated spirit is denatured alcohol.

Answer: (d) C₂H₅OH, CH₃COOH and SO₂

Explanation:
1. Ethanol, ethanoic acid and sulphur dioxide are made of non-metals and have only covalent bonds.
2. CaCl₂, NaNO₃ and NaHCO₃ contain ions and therefore ionic bonds.

Teacher's Note:
1. Salts of oxyacids contain both ionic and covalent bonds.
2. Metal + non-metal gives ionic.