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.
Alkalies become less alkaline when they are combined with acids. In essence, Boyle defined alkalies as substances that consume, or neutralize, acids. Acids lose their characteristic sour taste and ability to dissolve metals when they are mixed with alkalies. Alkalies even reverse the change in color that occurs when litmus comes in contact with an acid. In 1884 Svante Arrhenius suggested that salts such as NaCl dissociate when they dissolve in water to give particles he called ions .