CBSE Class 10 Science Carbon and its Compounds MCQs Set 14

Answer: (d) 4. Carbon has four valence electrons, so it shares four electrons to complete its octet.

Teacher's Note:
1. Carbon neither loses nor gains four electrons easily.
2. It therefore forms covalent bonds.

Answer: (b) 7. There are six C–H bonds and one C–C bond.

Teacher's Note:
1. Draw the structure to count the bonds.
2. Each line is one shared pair of electrons.

Answer: (c) 6. There are four C–H bonds and two bonds in C=C.

Teacher's Note:
1. The double bond contains one sigma and one pi bond in advanced terms.
2. At this level, count it as two bonds.

Answer: (b) CₙH₂ₙ₊₂. Alkanes are saturated hydrocarbons, with n = 1 giving CH₄.

Teacher's Note:
1. Alkenes follow CₙH₂ₙ and alkynes CₙH₂ₙ₋₂.
2. Check by writing methane, ethane and propane.

Answer: (c) C₄H₁₀. For n = 4, 2n + 2 = 10, so the formula is C₄H₁₀.

Teacher's Note:
1. Butane has two structural isomers.
2. Its straight and branched forms have the same formula.

Answer: (d) carboxylic acid. –COOH is the carboxyl group present in ethanoic acid.

Teacher's Note:
1. –OH is alcohol, –CHO is aldehyde and the C=O group is ketone.
2. Learn the four with examples.

Answer: (b) ethanal. It has two carbons and the aldehyde group, so the name is ethanal.

Teacher's Note:
1. The suffix -al is used for aldehydes.
2. Replace the final -e of the alkane name with it.

Answer: (b) propan-1-ol. It is a three-carbon alcohol with –OH at carbon 1.

Teacher's Note:
1. Alcohols take the suffix -ol.
2. Ketones end in -one and acids in -oic acid.

Answer: (b) 2. n-butane and 2-methylpropane (isobutane) are the two isomers.

Teacher's Note:
1. Isomers have the same molecular formula but different structures.
2. Branching is the usual reason.

Answer: (a) C₂H₂. Ethyne is the simplest alkyne with a carbon–carbon triple bond.

Teacher's Note:
1. Its common name is acetylene.
2. It is used in welding.

Answer: (b) hydrogenation with nickel catalyst. Unsaturated oils add hydrogen in the presence of nickel to become saturated.

Teacher's Note:
1. This is an addition reaction.
2. Saturated fats are solid at room temperature.

Answer: (b) a yellow sooty flame. A high percentage of carbon leads to incomplete combustion and soot.

Teacher's Note:
1. Saturated hydrocarbons burn with a clean blue flame in plenty of air.
2. Soot formation indicates unburnt carbon.

Answer: (b) they form insoluble scum with calcium and magnesium salts. The calcium and magnesium salts of fatty acids are insoluble and separate as scum.

Teacher's Note:
1. Detergents do not form scum with hard water.
2. They are therefore preferred in such areas.

Answer: (b) micelle formation. The non-polar tails attach to dirt and the ionic heads face water, forming micelles.

Teacher's Note:
1. The micelle traps oily dirt and is rinsed away.
2. This action explains why soap needs water.

Answer: (a) sodium ethoxide and hydrogen. 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂.

Teacher's Note:
1. Hydrogen gas is released as brisk bubbles.
2. This test shows the –OH group of an alcohol.

Answer: (a) an ester with a sweet smell. CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O; ethyl ethanoate has a fruity smell.

Teacher's Note:
1. This is called esterification.
2. Concentrated sulphuric acid acts as a catalyst and dehydrating agent.

Answer: (b) saponification. Ester + NaOH → alcohol + sodium salt of the carboxylic acid (soap).

Teacher's Note:
1. The name comes from the Latin word for soap.
2. The alcohol obtained is glycerol in soap making.

Answer: (b) ethene. C₂H₅OH → C₂H₄ + H₂O; the acid acts as a dehydrating agent.

Teacher's Note:
1. This is a dehydration reaction.
2. Limited sulphuric acid at lower temperature gives ether.

Answer: (b) glacial acetic acid. In cold weather it forms ice-like crystals.

Teacher's Note:
1. Vinegar is a 5–8% solution of ethanoic acid in water.
2. The pure acid is highly corrosive.

Answer: (b) ethanoic acid. It adds oxygen to the alcohol, giving carboxylic acid. Oxidising agents like acidified K₂Cr₂O₇ do the same.

Teacher's Note:
1. The purple colour of KMnO₄ disappears as it is used up.
2. This is a test for oxidisable organic compounds.

Answer: (b) CH₂. Each member differs from the next by a –CH₂ unit, i.e. 14 u.

Teacher's Note:
1. Members show a gradation in physical properties.
2. Chemical properties stay similar due to the same functional group.

Answer: (b) diamond. Each carbon in diamond is bonded to four others in a rigid three-dimensional structure.

Teacher's Note:
1. Graphite has layers and is soft.
2. Fullerene C-60 resembles a football.

Answer: (b) low melting and boiling points. The intermolecular forces are weak, so little energy is needed to separate molecules.

Teacher's Note:
1. They are poor conductors as no ions are present.
2. Most dissolve in organic solvents.

Answer: (b) substitution reaction. A hydrogen atom of methane is replaced by chlorine.

Teacher's Note:
1. Saturated hydrocarbons typically undergo substitution.
2. Unsaturated ones undergo addition.

Answer: (b) CO₂ and H₂O with heat and light. Hydrocarbon + O₂ → CO₂ + H₂O + heat and light.

Teacher's Note:
1. Carbon compounds are widely used as fuels for this reason.
2. Incomplete burning gives carbon monoxide or soot.

Answer: (b) methanol. Methanol (poisonous) and dyes are added to ethanol to discourage misuse.

Teacher's Note:
1. Methanol can cause blindness and death.
2. Denatured alcohol is used as a solvent.

Answer: (b) propane. Adding a –CH₂ unit to C₂H₆ gives C₃H₈, propane.

Teacher's Note:
1. Homologues follow the same general formula.
2. Methane is the one before ethane.

Answer: (a) Both A and R are true and R is the correct explanation of A. Self-linking into chains and four bonds produce many structures, so R explains A.

Teacher's Note:
1. Catenation means self-linking of atoms.
2. Carbon does this far better than other elements.

Answer: (a) Both A and R are true and R is the correct explanation of A. The scum formed reduces cleansing, so R explains A.

Teacher's Note:
1. Hardness is caused by dissolved Ca²⁺ and Mg²⁺ salts.
2. Detergents avoid this problem.