Salt refers to a kind of compound in which metal ions or ammonium ions are combined with acid ions, such as calcium sulfate, copper chloride and sodium acetate. In most cases, salt is the product of double decomposition reaction, such as sulfuric acid and sodium hydroxide to produce sodium sulfate and water, and other reactions can produce salt, such as displacement reaction. It exists widely in nature, such as alum, which is a natural and widely used double salt.