Physically, acids have a sour taste, are soluble in water, turn blue litmus paper red, are corrosive, have pH values less than 7 and conduct electricity. These physical properties of acids are a byproduct of the acids' chemical properties. Acids can be solutions or pure substances and can be either solid, liquid or gaseous in nature.
Acids are defined as substances that increase the concentration of hydrogen atoms, or more accurately, hydronium atoms (H30), in water. This ionization of acid, which is typical of all salts, is the reason acids conduct electricity. The pH values are defined as the negative logarithm of the concentration of hydronium atoms in a solution; thus acids give a pH value less than 7 because hydronium concentrations are greater than 10^-7 moles per liter. The strength of an acid, and thus its corrosiveness, is determined by how much an acid ionizes, or dissociates, into hydronium atoms in water, with strong acids dissociating completely and weak acids only partially. These hydronium atoms chemically react with metals and living tissue. This corrosive property is useful in the human body, as the acid in the stomach aids in the digestion of food. Weak acids are also used as additives in food to infuse a sour taste.