1. _Action as an acid._ In aqueous solution hydrochloric acid has very strong acid properties; indeed, it is one of the strongest acids. It acts upon oxides and hydroxides, converting them into salts: NaOH + HCl = NaCl + H_{2}O, CuO + 2HCl = CuCl_{2} + H_{2}O. It acts upon many metals, forming chlorides and liberating hydrogen: Zn + 2HCl = ZnCl_{2} + 2H, Al + 3HCl = AlCl_{3} + 3H. Unlike nitric and sulphuric acids it has no oxidizing action, so that when it acts on metals hydrogen is always given off. 2. _Relation to combustion._ Hydrochloric acid gas is not readily decomposed, and is therefore neither combustible nor a supporter of combustion. 3. _Action on oxidizing agents._ Although hydrochloric acid is incombustible, it can be oxidized under some circumstances, in which case the hydrogen combines with oxygen, while the chlorine is set free. Thus, when a solution of hydrochloric acid acts upon manganese dioxide part of the chlorine is set free: MnO_{2} + 4HCl = MnCl_{2} + 2H_{2}O + 2Cl. ~Aqua regia.~ It has been seen that when nitric acid acts as an oxidizing agent it usually decomposes, as represented in the equation 2HNO_{3} = H_{2}O + 2NO + 3O. The oxygen so set free may act on hydrochloric acid: 6HCl + 3O = 3H_{2}O + 6Cl. The complete equation therefore is 2HNO_{3} + 6HCl = 4H_{2}O + 2NO + 6Cl. When concentrated nitric and hydrochloric acids are mixed this reaction goes on slowly, chlorine and some other substances not represented in the equation being formed. The mixture is known as _aqua regia_ and is commonly prepared by adding one volume of nitric acid to three volumes of hydrochloric acid. It acts more powerfully upon metals and other substances than either of the acids separately, and owes its strength not to acid properties but to the action of the nascent chlorine which it liberates. Consequently, when it acts upon metals such as gold it converts them into chlorides, and the reaction can be represented by such equations as Au + 3Cl = AuCl_{3}. ~Salts of hydrochloric acid,--chlorides.~ The chlorides of all the metals are known and many of them are very important compounds. Some of them are found in nature, and all can be prepared by the general method of preparing salts. Silver chloride, lead chloride, and mercurous chloride are insoluble in water and acids, and can be prepared by adding hydrochloric acid to solutions of compounds of the respective elements. While the chlorides have formulas similar to the fluorides, their properties are often quite different. This is seen in the solubility of the salts. Those metals whose chlorides are insoluble form soluble fluorides, while many of the metals which form soluble chlorides form insoluble fluorides.