BiCl_{3} + 3H_{2}O <--> Bi(OH)_{3} + 3HCl the addition of more water will result in the formation of more bismuth hydroxide and hydrochloric acid. The addition of more hydrochloric acid will convert some of the bismuth hydroxide into bismuth chloride. 2. _Formation of insoluble substances._ When one of the products of hydrolysis is nearly insoluble in water the solution will become saturated with it as soon as a very little has been formed. All in excess of this will precipitate, and the reaction will go on until the acid set free increases sufficiently to bring about an equilibrium. Thus a considerable amount of bismuth and antimony hydroxides are precipitated when water is added to the chlorides of these elements. The greater the dilution the more hydroxide precipitates. The addition of hydrochloric acid in considerable quantity will, however, redissolve the precipitate. ~Partial hydrolysis.~ In many cases the hydrolysis of a salt is only partial, resulting in the formation of basic salts instead of the free base. Most of these basic salts are insoluble in water, which accounts for their ready formation. Thus bismuth chloride may hydrolyze by successive steps, as shown in the equations BiCl_{3} + H_{2}O = Bi(OH)Cl_{2} + HCl, BiCl_{3} + 2H_{2}O = Bi(OH)_{2}Cl + 2HCl, BiCl_{3} + 3H_{2}O = Bi(OH)_{3} + 3HCl. The basic salt so formed may also lose water, as shown in the equation Bi(OH)_{2}Cl = BiOCl + H_{2}O. The salt represented in the last equation is sometimes called bismuth oxychloride, or bismuthyl chloride. The corresponding nitrate, BiONO_{3}, is largely used in medicine under the name of subnitrate of bismuth. In these two compounds the group of atoms, BiO, acts as a univalent metallic radical and is called _bismuthyl_. Similar basic salts are formed by the hydrolysis of antimony salts. EXERCISES 1. Name all the elements so far studied which possess allotropic forms. 2. What compounds would you expect phosphorus to form with bromine and iodine? Write the equations showing the action of water on these compounds. 3. In the preparation of phosphine, why is coal gas passed into the flask? What other gases would serve the same purpose? 4. Give the formula for the salt which phosphine forms with hydriodic acid. Give the name of the compound. 5. Could phosphoric acid be substituted for sulphuric acid in the preparation of the common acids? 6. Write the equations for the preparation of the three sodium salts of orthophosphoric acid. 7. Why does a solution of disodium hydrogen phosphate react alkaline? 8. On the supposition that bone ash is pure calcium phosphate, what weight of it would be required in the preparation of 1 kg. of phosphorus? 9. If arsenopyrite is heated in a current of air, what products are formed? 10. (a) Write equations for the complete combustion of hydrosulphuric acid, methane, and arsine. (b) In what respects are the reactions similar?