LECTURE IV

WATER:  ITS CHEMISTRY AND PROPERTIES; IMPURITIES AND THEIR ACTION; TESTS OF PURITY—­Continued

In the last lecture, under the head of “Solution,” I mentioned that some salts, some chemical substances, are more soluble in water than others, and that their solubilities under different circumstances of temperature vary in different ways.  However, some salts and compounds are practically insoluble in water under any circumstances.  We now arrive at the important result known to chemists as the precipitation of insoluble compounds from solutions.  In order to define this result, however, we must, of course, first consider the circumstances of causation of the result.  Let us take a simple case of chemical decomposition resulting in the deposition or precipitation of a substance from solution in the insoluble state.  We will take a salt you are probably acquainted with—­sulphate of copper, or bluestone, and dissolve it in water, and we have then the sulphate of copper in solution in water.  Now suppose it is our desire to obtain from that solution all the copper by depositing it in some insoluble form.  We may accomplish this in several different ways, relying on certain methods of decomposing that sulphate of copper.  One of the simplest and most economical is that adopted in a certain so-called wet method of extracting copper.  It is based on the fact that metallic iron has a greater tendency to combine in water solutions, with the acids of copper salts, than the copper has in those salts.  We simply need to place some scraps of iron in the copper sulphate solution to induce a change which may be represented as follows:  Copper sulphate, consisting of a combination of copper oxide with sulphuric acid, yields with iron, iron sulphate, a combination of iron oxide with sulphuric acid, and metallic copper.  The metallic copper produced separates in the form of a red coating on the iron scraps.  But we may also, relying on the fact that oxide of copper is insoluble in water, arrange for the deposition of the copper in that form.  This we can do by adding caustic soda to a hot solution of copper sulphate, when we get the following change:  Copper sulphate, consisting of a combination of copper oxide with sulphuric acid, yields with caustic soda, sulphate of soda, a combination of soda with sulphuric acid and oxide of copper.  Oxide of copper is black, and so in this decomposition what is called a “black precipitate” of that oxide is produced on adding the caustic soda.  But it might not suit us thus to deposit the copper from our solution; we might desire to remove the sulphuric acid from the copper sulphate, and leave the copper dissolved, say in the form of a chloride.  We select, then, a compound which is a chloride, and a chloride of a metal which forms an insoluble combination with sulphuric acid—­chloride of barium, say.  On adding this chloride of barium to sulphate of copper