A salt is a chemical compound formed by replacing all or part of the hydrogen atoms of an acid with one or more metal ions or other positive ions from a base. The vast majority of minerals are salts. Typically, salts are crystalline ionic compounds such as sodium chloride, Na+Cl- (common or table salt), and ammonium nitrate, NH4+NO3–. They are good electrolytes; those of weak acids or bases undergo partial hydrolysis in water. Salts may be classified as normal (fully neutralized), acid (containing some acidic hydrogen, e.g., bicarbonates), or basic (containing hydroxide ions). They may alternately be classified as single salts, double salts (two simple salts combined by regular substitution in the crystal lattice), including alums and complex salts (containing complex ions: see ligand).