2. _By the decomposition of a compound._ This decomposition may be brought about either by heat alone or by the combined action of heat and a reducing agent. Thus when the nitrate of a metal is heated the oxide of the metal is usually obtained. Copper nitrate, for example, decomposes as follows: Cu(NO_{3})_{2} = CuO + 2NO_{2} + O. Similarly the carbonates of the metals yield oxides, thus: CaCO_{3} = CaO + CO_{2}. Most of the hydroxides form an oxide and water when heated: 2Al(OH)_{3} = Al_{2}O_{3} + 3H_{2}O. When heated with carbon, sulphates are reduced to sulphides, thus: BaSO_{4} + 2C = BaS + 2CO_{2}. 3. _Methods based on equilibrium in solution._ In the preparation of compounds the first requisite is that the reactions chosen shall be of such a kind as will go on to completion. In the chapter on chemical equilibrium it was shown that reactions in solution may become complete in either of three ways: (1) a gas may be formed which escapes from solution; (2) an insoluble solid may be formed which precipitates; (3) two different ions may combine to form undissociated molecules. By the judicious selection of materials these principles may be applied to the preparation of a great variety of compounds, and illustrations of such methods will very frequently be found in the subsequent pages. 4. _By fusion methods._ It sometimes happens that substances which are insoluble in water and in acids, and which cannot therefore be brought into double decomposition in the usual way, are soluble in other liquids, and when dissolved in them can be decomposed and converted into other desired compounds. Thus barium sulphate is not soluble in water, and sulphuric acid, being less volatile than most other acids, cannot easily be driven out from this salt When brought into contact with melted sodium carbonate, however, it dissolves in it, and since barium carbonate is insoluble in melted sodium carbonate, double decomposition takes place: Na_{2}CO_{3} + BaSO_{4} = BaCO_{3} + Na_{2}SO_{4}. On dissolving the cooled mixture in water the sodium sulphate formed in the reaction, together with any excess of sodium carbonate which may be present, dissolves. The barium carbonate can then be filtered off and converted into any desired salt by the processes already described. 5. _By the action of metals on salts of other metals._ When a strip of zinc is placed in a solution of a copper salt the copper is precipitated and an equivalent quantity of zinc passes into solution: Zn + CuSO_{4} = Cu + ZnSO_{4}. In like manner copper will precipitate silver from its salts: Cu + Ag_{2}SO_{4} = 2Ag + CuSO_{4}. It is possible to tabulate the metals in such a way that any one of them in the table will precipitate any one following it from its salts. The following is a list of some of the commoner metals arranged in this way: Zinc Iron Tin Lead Copper Bismuth Mercury Silver Gold