~Physical properties.~ Hydrosulphuric acid is a colorless gas, having a weak, disagreeable taste and an exceedingly offensive odor. It is rather sparingly soluble in water at ordinary temperatures, about three volumes dissolving in one of water. In boiling water it is not soluble at all. In pure form it acts as a violent poison, and even when diluted largely with air produces headache, dizziness, and nausea. It is a little heavier than air, having a density of 1.18. ~Chemical properties.~ The most important chemical properties of hydrosulphuric acid are the following: 1. _Acid properties._ Hydrosulphuric acid is a weak acid. In solution in water it turns blue litmus red and neutralizes bases, forming salts called _sulphides_. 2. _Action on oxygen._ The elements composing hydrosulphuric acid have each a strong affinity for oxygen, and are not held together very firmly. Consequently the gas burns readily in oxygen or the air, according to the equation H_{2}S + 3O = H_{2}O + SO_{2}. When there is not enough oxygen for both the sulphur and the hydrogen, the latter element combines with the oxygen and the sulphur is set free: H_{2}S + O = H_{2}O + S. 3. _Reducing action._ Owing to the ease with which hydrosulphuric acid decomposes and the strong affinity of both sulphur and hydrogen for oxygen, the substance is a strong reducing agent, taking oxygen away from many substances which contain it. 4. _Action on metals._ Hydrosulphuric acid acts towards metals in a way very similar to water. Thus, when it is passed over heated iron in a tube, the reaction is represented by the equation 3Fe + 4H_{2}S = Fe_{3}S_{4} + 8H. Water in the form of steam, under similar circumstances, acts according to the equation 3Fe + 4H_{2}O = Fe_{3}O_{4} + 8H. ~Salts of hydrosulphuric acid,--sulphides.~ The salts of hydrosulphuric acid, called sulphides, form an important class of salts. Many of them are found abundantly in nature, and some of them are important ores. They will be frequently mentioned in connection with the metals. Most of the sulphides are insoluble in water, and some of them are insoluble in acids. Consequently, when hydrosulphuric acid is passed into a solution of a salt, it often happens that a sulphide is precipitated. With copper chloride the equation is CuCl_{2} + H_{2}S = CuS + 2HCl. Because of the fact that some metals are precipitated in this way as sulphides while others are not, hydrosulphuric acid is extensively used in the separation of the metals in the laboratory. ~Explanation of the reaction.~ When hydrosulphuric acid and copper chloride are brought together in solution, both copper and sulphur ions are present, and these will come to an equilibrium, as represented in the equation Cu^{+} + S^{-} <--> CuS.