This "Science note" arose out of practical work involving the dilution of ethanoic acid, the measurement of the pH of the diluted solutions and calculation of the acid dissociation constant, K[subscript a], for each diluted solution. The students expected the calculated values of K[subscript a] to be constant but they found that the calculated K[subscript a] decreased with dilution. In this "Science note," the authors aim to provide the necessary theory to explain these observations.