|{ 2FeCl_{3},2HCl,8H_{2}O }| | | | | | | X |{ 2FeCl_{3},12H_{2}O; }| -12.5° | 22.14 | 16.69 |{ 2FeCl_{3},2HCl,12H_{2}O }| | | | | | | Q | 2FeCl_{3},2HCl,8H_{2}O | -3° (melting point) ------------------------------------------------------------------------- Basic Salts.--Another class of systems in the study of {297} which the Phase Rule has performed exceptional service, is that of the basic salts. In many cases it is impossible, by the ordinary methods of analysis, to decide whether one is dealing with a definite chemical individual or with a mixture. The question whether a solid phase is a chemical individual can, however, be answered, in most cases, with the help of the principles which we have already learnt. Let us consider, for example, the formation of basic salts from bismuth nitrate, and water. In this case we can choose as components Bi_{2}O_{3}, N_{2}O_{5}, and H_{2}O; since all the systems consist of these in varying amounts. If we are dealing with a condition of equilibrium at constant temperature between liquid and solid phases, three cases can be distinguished,[371] viz.-- 1. The solutions in different experiments have the same composition, but the composition of the precipitate alters. In this case there must be two solid phases. 2. The solutions in different experiments can have varying composition, while the composition of the precipitate remains unchanged. In this case only one solid phase exists, a definite compound. 3. The composition both of the solution and of the precipitate varies. In this case the solid phase is a solid solution or a mixed crystal.