These words refer to sharp tastes or smells or to harsh dispositions and behaviour stemming from them. When used of tastes, sour refers to the characteristic sharpness produced by acids. Acid itself refers more directly to such a taste. Acidulous indicates a taste that is partially acid or mildly sour . Tart refers to a sharp taste that is pleasantly acid or piquant. Acrid pertains to any strong or sharp smell, but can apply also to sharp tastes produced either by acids or alkalis. Bitter is restricted to sharp taste produced mainly by alkalis, but can apply also to strong unpleasant smells as well. If the bitter taste or smell is mild it might be regarded as savoury or pleasant; if quite strong it might become unpleasant, or be capable of lingering on, causing discomfort. Caustic suggests a sharp smell such as a strong alkali might give off; it can be used of tastes only as a hyperbole, since it also refers quite literally to alkalinity intense enough to eat away or corrode organic tissues.

On their most literal level, these words are fairly clear in their neutrally descriptive distinctions from one another. In describing harsh disposition or behaviour, their shadings of meaning are rather more blurred. Sour applies almost solely to mood or disposition, suggesting a pessimistic, disenchanted or excessively solemn attitude: always wearing a sour expression that no pleasantry could soften; recalling her former naïve idealism with a sour smile; having no confront the sour face of the superintendent. Bitter suggests an even fiercer gloominess arising from a sense of having been unjustly treated or from a deep-seated anger that smoulders without catching fire: a bitter man who saw nothing worthy of admiration no matter where he looked; bitter accusations concerning the unfair division of money between them. Tart applies more appropriately to particular instances of behaviour and gives a different tone altogether; it suggests impertinence or cheekiness: giving a tart answer to the teacher’s scolding. Acid and acidulous are difficult to distinguish except for the greater formality of the latter; acidulous might sometimes suggest an abiding mood and acid an actual expression: an acidulous temper; an acid remark. In any case, acid seems considerably stronger in its suggestion of harsh or gratuitous hostility: Bitter at his own lack of success as a playwright, the critic was negative and acid in all his comments.

Acrid and caustic are the most intense of these words in pointing to harsh dispositions or expressions. Acrid applies best to mood: an acrid curtness of manner. But it can apply also to expressions: an acrid scowl disfiguring his face. Caustic perhaps suggests an even fiercer hostility than acrid, since it carries over here its implication of corrosive power: often filled with a caustic rage; caustic aspersions on his friend’s abilities; answers so caustic as to hint at some imbalance of mind.