Functional groups

The governing concept is the characteristic physical property and use of esters. Many esters have pleasant, sweet, fruity odours, so option A is correct. This property makes selected esters useful in perfumes, flavouring agents and artificial fragrances. Their smell is not generally metallic, and esters are not universally odourless. A pungent or foul odour is not the usual characteristic taught for esters, although individual compounds may vary in intensity.

A homologous series contains organic compounds with the same functional group and a common general formula. Each successive member differs from the previous one by a –CH₂– unit, called a methylene group. Consequently, the relative molecular mass generally increases by 14 units, because carbon contributes 12 and two hydrogens contribute 2. The other options are unrelated particles and do not define the series’ regular progression.

The functional group is the reactive part of an organic molecule and largely determines its characteristic chemical reactions. Members of a homologous series possess the same functional group, so they undergo similar types of reactions, even though their carbon-chain lengths and some physical properties may differ. Colour is not the governing principle, these compounds generally contain carbon, and members of a homologous series need not be ionic.

Alcohols are organic compounds in which a hydroxyl group, –OH, is attached to a saturated carbon atom. This group gives alcohols characteristic reactions, such as reaction with sodium and oxidation under suitable conditions. A carboxyl group, –COOH, defines carboxylic acids; an aldehyde group, –CHO, defines aldehydes; and a ketone contains a carbonyl group within the carbon chain. Therefore A is correct.

The governing idea is that compounds with the same functional group show related chemical behaviour and belong to the same homologous family. Methanol, CH3OH, and ethanol, C2H5OH, both contain the hydroxyl group (–OH), so both are alcohols. Ethanol has –OH, ethanoic acid has –COOH, ethene has a C=C bond, and methane has no such functional group. Hence option A is the only correct pair.

The governing concept is the role of chlorophyll in photosynthetic nutrition. Chlorophyll is the green pigment in chloroplasts that absorbs suitable wavelengths of light and makes that energy available for converting carbon dioxide and water into food. Thus option A is correct. Water removal is not its main function, and blood formation and digestion are not plant functions performed by chlorophyll.

Ethanol is a good solvent, so it is used to dissolve substances in many medicines and perfumes. It is not commonly used in cement manufacture, glass making, or iron extraction. Exam tip: Remember the main uses of ethanol: as a solvent, a fuel, and a component of alcoholic beverages.