Ethanoic acid is the main acidic constituent of which of the following…

Concept: Ethanoic acid, CH3COOH, is the IUPAC name of the compound commonly called acetic acid — the two-carbon member of the carboxylic acid family, identified by the –COOH functional group. Every fermented food or household product is characterised by the particular acid, alcohol or salt that its method of production generates, so identifying a substance from its acid means asking which acid that process yields.

Application: Vinegar is a dilute aqueous solution of ethanoic acid. By mass it is mostly water, since table vinegar carries only about 5 to 8 per cent ethanoic acid, but that acid is its only significant acidic component and the compound by which the product is defined. Vinegar is produced by letting a fermented alcoholic liquid stand in contact with air so that Acetobacter bacteria oxidise its ethanol to the acid, and this acid gives vinegar its sharp sour taste, its preservative action and its pH of about 2.4 to 3.4.

Cross-check: the other everyday substances listed are generated by different processes and are therefore defined by different compounds.

• Soap — the sodium or potassium salts of long-chain fatty acids, made by boiling fats or oils with alkali (saponification); a soap solution is basic, not acidic.

• Wine — an alcoholic beverage whose defining compound is ethanol, C2H5OH, formed when yeast ferments grape sugars in the absence of air; ethanoic acid appears in wine only when it spoils and turns sour.

• Curd — milk set by Lactobacillus bacteria, which convert lactose into lactic acid, CH3CH(OH)COOH, a three-carbon hydroxy acid that is not ethanoic acid.

Hence the substance whose main acidic constituent is ethanoic acid is vinegar.