Copper Hydroxide, Cu(OH)₂

From the solutions of cupric salts, strong bases precipitate cupric hydroxide, Cu(OH)₂, as a bright blue substance, which on being kept for some time under the solution, more quickly on heating, becomes dark brown, at the same time losing water and passing into cupric oxide, CuO. It may be asked how it is possible for a substance to lose water while lying under water, of which it has therefore as much at its disposal as it requires. The answer is that cupric hydroxide is not at all a stable compound at the ordinary and at higher temperatures, and the fact that it is produced before the form which is most stable under the conditions, viz. copper oxide and water, is a case of the law of the prior formation of the unstable forms.

Copper hydroxide is not soluble in alkalis except in very small amount, when the solvent is very concentrated. In the presence of many organic substances it dissolves with formation of complex compounds of dark blue colour. Ammonia also precipitates cupric salts with formation of hydroxide; an excess, however, again effects solution. The liquid thereby becomes of a dark corn-flower blue. This is a sign that a new ion has been produced; as a matter of fact, from the dark blue solutions salts can be obtained in the solid state containing the cation Cu(NH₃)₄^••.

Copper hydroxide is not a strong base; it is one of the weakest of the hydroxides of the divalent ions. This is shown in the distinct hydrolysis of its salts, in consequence of which the solutions of the salts of strong acids all react acid. Cupric salts of weak acids exhibit phenomena of decomposition; some, e.g. the carbonate, cannot be obtained at all in the normal condition, but only salts containing hydroxyl, or basic salts, are known.

Besides being formed by the decomposition of cupric salts, copper oxide is also obtained by the direct oxidation of copper in the air at a low red heat. Cuprous oxide, the anhydride of cuprous hydroxide (vide infra), is first formed, but this also passes into cupric oxide under the above conditions.

Copper oxide is very readily reduced to the metal by means of hydrogen with production of water. It has already been mentioned that this reaction was used in order to determine the ratio of combination between hydrogen and oxygen. This same property of ready reducibility conditions the use of copper oxide in organic elementary analysis. The substance to be investigated is mixed with excess of copper oxide, the mixture placed in a tube and the whole heated, after absorption apparatus for water (calcium chloride) and for carbon dioxide (caustic potash or soda lime) have been attached. By means of the oxygen of the copper oxide the carbon of the organic compound is burned to carbon dioxide, the hydrogen to water. These products are collected and weighed, and from this the amount of the above elements contained in the organic compound (also weighed) can be determined.

Any nitrogen which is present is evolved in the free state, and the amount can also be determined by collecting and measuring the gas.