In 1661 Robert Boyle summarized the properties of acids as follows. Acids have a sour taste. Acids are corrosive. Acids change the color of certain vegetable dyes, such as litmus, from blue to red. Acids lose their acidity when they are combined with alkalies.
Vinegar tastes sour because it is a dilute solution of acetic acid in water.
Lemon juice tastes sour because it contains citric acid. Milk turns sour when it spoils because lactic acid is formed, and the unpleasant, sour odor of rotten meat or butter can be attributed to compounds such as butyric acid that form when fat spoils. In 1661 Boyle summarized the properties of alkalies as follows. Alkalies feel slippery. Alkalies change the color of litmus from red to blue.