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.

Esterification is a reversible condensation reaction between an alcohol and a carboxylic acid, producing an ester and water. Concentrated sulphuric acid acts mainly as a dehydrating agent, removing or absorbing water and shifting the equilibrium toward ester formation; it also provides acidic catalysis. Therefore option A is correct. The other options describe transformations that do not occur in esterification.

Ethanol contains a hydroxyl (–OH) group whose hydrogen can be displaced by a sufficiently reactive metal such as sodium. The reaction is 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂. Thus sodium ethoxide is formed and hydrogen gas is released. Carbon dioxide is associated with combustion or reactions involving carbonates, oxygen is not the product here, and chlorine is unrelated.

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 concept is that a functional group is the atom or group of atoms responsible for the characteristic chemical behaviour of an organic compound. Carboxylic acids contain the –COOH group, called the carboxyl group. It combines a carbonyl group (C=O) with a hydroxyl group (–OH), but the complete –COOH group gives the acid its identity. Therefore option A is correct; hydroxyl alone represents alcohols, while a double bond or halogen is not the defining group here.

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 that the functional group controls the characteristic reactions of a carbon compound. For example, an –OH group gives alcohols their typical reactions, –COOH gives carboxylic acids acidic behaviour, and a C=C bond allows addition reactions in alkenes. The carbon skeleton may remain similar while changing the functional group changes chemical properties substantially. Therefore option A is correct; colour, mass reduction, and metallic character are not its defining functions.

Systematic naming begins by identifying the parent carbon chain, usually the longest continuous chain containing the principal functional group. Its number of carbon atoms gives the root name, such as meth-, eth-, or prop-. The functional group determines the suffix and the compound’s class, for example -ol for an alcohol or -oic acid for a carboxylic acid. Colour, smell, and mass do not establish the IUPAC name, so A is correct.

Geographical isolation separates populations by barriers such as mountains, rivers, or distance and reduces gene flow between them. Because each group faces its own mutations, variations, selection pressures, and environmental conditions, different changes can accumulate over generations. Reproductive differences may eventually lead to speciation, so option A is correct. Isolation does not make organisms instantly identical, erase chromosomes, or always remove food.

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.

The governing concept is the structural and functional organisation of the excretory system. Each nephron performs the essential stages of urine formation: filtration of blood at the glomerulus, selective reabsorption of useful water and solutes, and secretion of some wastes into the tubule. Because these processes together carry out the kidney’s basic filtering function, the nephron is called its functional unit. Thus option A is correct; the other options describe digestive, respiratory, or plant functions.

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.