Salt is a compound of metal ions or ammonium ions combined with acid or nonmetal ions. Such as sodium chloride, calcium nitrate, ferrous sulfate and calcium sulfate, ammonium acetate, copper chloride, sodium acetate, etc., in general, salt is double decomposition reaction resultant salt and salt reaction generates new salt and salt, salt and alkali reaction generates new salt and alkali, salt reacts with an acid to form a new salt and acid, such as the production of sulfuric acid and sodium hydroxide, sodium sulfate and water, sodium chloride react with silver nitrate to produce silver chloride and sodium nitrate, etc. There are other reactions that produce salt, such as displacement reactions. The solution of the soluble salt is conductive, because there are free swimming ions in the solution, so it can be used as an electrolyte. Hydrochloric acid is not only the important product of salt chemical industry, but also the important raw material for producing silicon. Salt is a kind of crystal.