Aside from considerations based on its ionization, water may be shown, by its chemical behavior, to have the functions of an acid and of a base, and the conclusions reached are in complete accord with those reached with the aid of the theory of ionization. «Water is An Acid.»—If the oxide of a metal such as copper, lead or calcium, is treated with an acid, a salt is formed by the combination of the two; for instance, we have PbO + HCl → Pb(OH)Cl, Pb(OH)Cl + HCl → PbCl_{2} + H_{2}O, PbO + 2 HCl → PbCl_{2} + H_{2}O, CaO + 2 HCl → CaCl_{2} + H_{2}O. Water will combine with a number of oxides very much in the same manner and sometimes with such vigor, that considerable heat is evolved, as in the slaking of lime (‹exp.›): CaO + HOH → Ca(OH)_{2}. Water in this, and similar actions, takes the place of and plays the rôle of, an ‹acid›, and ‹the metal hydroxides or bases appear as its salts›.[364] It is a ‹very weak› acid, which can easily be driven out of its salts by any stronger acid (neutralization of bases), but that does not alter the conclusions reached. Considered from the point of view of the theory of ionization, the relation [p178] would be expressed by saying that in the common bases the positive hydrogen ion of water has been replaced by some other positive or metal ion. The salt of any acid could be defined in exactly the same way. «Water as a Base.»—Acid oxides, such as carbon dioxide, silicon dioxide, arsenious oxide, combine more or less readily with bases, such as sodium hydroxide, to form salts: CO_{2} + NaOH → NaHCO_{3}, As_{2}O_{3} + 2 NaOH → 2 NaAsO_{2} + H_{2}O. A number of acid oxides combine with water in exactly the same manner, and sometimes with such tremendous vigor, that great care must be taken in bringing the two together, as is the case when sulphur trioxide or phosphorus pentoxide are added to water (‹Exp.›). We have P_{2}O_{5} + HOH → 2 HPO_{3}. It is evident that in such actions water may take the place of, and play the rôle of, an ordinary base, forming the ‹acids›, which may well be defined as hydrogen salts.[365] It is true that the basic properties of water are so weak, that the metal ion of even a weak base, like ammonium hydroxide, will replace the hydrogen-ion in its salts, the acids, quite readily (HCl + NH_{4}OH ⥂ NH_{4}Cl + H_{2}O). But such a weak base, in turn, will have to give way, of course, to still stronger bases; for instances, NH_{4}Cl + NaOH ⥂ NaCl + NH_{4}OH. From the point of view of the theory of ionization, the hydrogen-ion is positive, like all the other metals ions whose hydroxides are bases.