The formation of an insoluble salt by the interaction of two soluble substances is well illustrated in the preparation of Burgundy mixture, the most effectual remedy yet proposed for checking the spread of potato disease. This mixture contains copper carbonate, that is, the copper salt of carbonic acid. For its preparation we require copper sulphate and sodium carbonate (washing soda), a soluble carbonate. When these two substances, dissolved in separate portions of water, are mixed, copper carbonate is formed as a pale blue solid which is in such a state of fine subdivision that it remains suspended in the solution of sodium sulphate, the other product of the reaction.
The change is represented diagrammatically below. Each circle represents the atom or a group of atoms named therein. At the moment of mixing, these groups undergo re-arrangement.
Bordeaux mixture, which some gardeners prefer, is a similar preparation containing copper hydroxide instead of copper carbonate. It is made by mixing clear lime water (a soluble hydroxide) with copper sulphate.
Elements and Compounds. It is scarcely possible to discuss chemical processes without having from time to time to use terms which are not in everyday use. A few preliminary definitions and explanations of terms which will be frequently used may serve to simplify descriptions, and render it unnecessary to encumber them with purely explanatory matter.
Among the many different kinds of materials known, which in the aggregate amount to several hundreds of thousands, there are about ninety substances which up to the present time have not been broken up into simpler kinds. These primary materials are called "elements," the remainder being known as "compounds."
The following is a list of the commonest of these elements, together with the symbols by which they are represented in Chemistry.
METALS Aluminium Al. Antimony (_Stibium_) Sb. Barium Ba. Bismuth Bi. Cadmium Cd. Calcium Ca. Chromium Cr. Copper (_Cuprum_) Cu. Gold (_Aurum_) Au. Iron (_Ferrum_) Fe. Lead (_Plumbum_) Pb. Lithium Li. Magnesium Mg. Manganese Mn. Mercury (_Hydrargyrum_) Hg. Nickel Ni. Platinum Pt. Potassium (_Kalium_) K. Silver (_Argentum_) Ag. Sodium (_Natrium_) Na. Strontium Sr. Tin (_Stannum_) Sn. Zinc Zn.