The next class of oxides comprises those which may be termed neutral, because they can act either as bases or as acids, according as they are treated with an acid or with a base. Their hydroxides may be comprised in the same class. A case of this kind has already been explained on p. 70; it is there shown that aluminium hydroxide, when treated with acids, yields salts of aluminium, while with bases aluminates are formed.

Complexity

It appears probable that such oxides have molecular formulae more complex than those usually ascribed to them; for instance, aluminium oxide is certainly more complex than is implied by the usual formula A1₂O₃; it may be A1₄O₆ or A1₆O₉, but there is no means at present of determining the degree of complexity of the molecule. The argument in favour of this view is the very high melting-points and boiling-points of such oxides. It is a well-known fact that as the molecular weight of compounds increases the boiling-point rises. Examples to illustrate this are best drawn from carbon compounds, where " poly-merism " is not infrequent; that is, where compounds exist having the same percentage composition but molecular formulae, of which the higher ones are multiples of the lower one. We are acquainted with a series of compounds of carbon and hydrogen, of which the first member is ethylene, C₂H₄; bodies of the formulae C₄H₈, C₆H₁₂, C_gH₁₆, C₁₀H₂₀, etc, are also known ; and the boiling-point increases with the molecular weight. Now, the chlorides of the elements are, as a rule, easily volatile, and have low melting-points ; and where it happens that both chloride and oxide have a simple molecular formula, as, for example, carbon tetrachloride, CC1₄, and carbon dioxide, CO₂, the chloride has always a higher boiling-point than the oxide. It would appear to follow, therefore, that if the oxides of the metals had as simple molecular formulae as the chlorides they would show more volatility than the latter. As this is not the case, the presumption is that the oxides possess more complex formulae than we are in the habit of ascribing to them. This probability will be dealt with as occasion arises.

Among the oxides and hydroxides which exhibit the power of acting both as acid and basic compounds are cupric hydroxide, Cu(OH)₂, which dissolves in a concentrated solution of potassium hydroxide with a dark blue colour; zinc and cadmium hydroxides, which dissolve in excess of alkali; sodium zincate has been separated by addition of alcohol, and is precipitated in white needles of the formula Na₂ZnO₂.8H₂O; and aluminium hydroxide, which dissolves in alkali, forming an aluminate, MA1O₂; stannous and plumbous hydroxides, Sn(OH)₂ and Pb(OH)₂, dissolve in alkalies, forming compounds no doubt analogous to zincates. Chromous, ferrous, manganous, cobaltous, and nickelous hydroxides are not thus soluble. Chromic hydroxide, however, is soluble in soda, probably forming a compound like sodium aluminate; unlike the latter, chromium hydroxide is thrown down on boiling the solution.

But such compounds, when they do not contain sodium or potassium, are often insoluble in water, and then they cannot be prepared by the action of the one hydroxide on the other. The oxides combine when heated together in the dry condition, and sometimes when the compound formed is decomposed by water (hydrolysed) it is convenient to prepare it either from the oxide or from the carbonate.