According to this table copper will precipitate bismuth, mercury, silver, or gold from their salts, and will in turn be precipitated by zinc, iron, tin, or lead. Advantage is taken of this principle in the purification of some of the metals, and occasionally in the preparation of metals and their compounds. ~Important insoluble compounds.~ Since precipitates play so important a part in the reactions which substances undergo, as well as in the preparation of many chemical compounds, it is important to know what substances are insoluble. Knowing this, we can in many cases predict reactions under certain conditions, and are assisted in devising ways to prepare desired compounds. While there is no general rule which will enable one to foretell the solubility of any given compound, nevertheless a few general statements can be made which will be of much assistance. 1. _Hydroxides._ All hydroxides are insoluble save those of ammonium, sodium, potassium, calcium, barium, and strontium. 2. _Nitrates._ All nitrates are soluble in water. 3. _Chlorides._ All chlorides are soluble save silver and mercurous chlorides. (Lead chloride is but slightly soluble.) 4. _Sulphates._ All sulphates are soluble save those of barium, strontium, and lead. (Sulphates of silver and calcium are only moderately soluble.) 5. _Sulphides._ All sulphides are insoluble save those of ammonium, sodium, and potassium. The sulphides of calcium, barium, strontium, and magnesium are insoluble in water, but are changed by hydrolysis into acid sulphides which are soluble. On this account they cannot be prepared by precipitation. 6. _Carbonates, phosphates, and silicates._ All normal carbonates, phosphates, and silicates are insoluble save those of ammonium, sodium and potassium. EXERCISES 1. Write equations representing four different ways for preparing Cu(NO_{3})_{2}. 2. Write equations representing six different ways for preparing ZnSO_{4}. 3. Write equations for two reactions to illustrate each of the three ways in which reactions in solutions may become complete. 4. Give one or more methods for preparing each of the following compounds: CaCl_{2}, PbCl_{2}, BaSO_{4}, CaCO_{3}, (NH_{4})_{2}S, Ag_{2}S, PbO, Cu(OH)_{2} (for solubilities, see last paragraph of chapter). State in each case the general principle involved in the method of preparation chosen. CHAPTER XXIII THE ALKALI METALS