For many strong acids, it is possible to form crystals of their hydronium salt that are relatively stable. These salts are sometimes called "acid monohydrates". As a rule, any acid with an ionization constant of or higher may do this. Acids whose ionization constants are below generally cannot form stable salts. For example, nitric acid has an ionization constant of , and mixtures with water at all proportions are liquid at room temperature. However, perchloric acid has an ionization constant of , and if liquid anhydrous perchloric acid and water are combined in a 1:1 molar ratio, they react to form solid hydronium perchlorate ().