Members of both these classes may be regarded as hydroxyl, that is, water minus one atom of hydrogen, OH, in combination with elements, but they differ radically in that the true hydroxides ionise into element and hydroxyl, thus:

whereas acids ionise into hydrogen and a negatively charged radical, thus:

There are certain hydroxides in which the ionisation may take either form ; such compounds are said to be either "basic" or "acid" according to circumstances; thus, aluminium hydroxide, Al(-OH)₃, is basic; with hydrochloric acid it reacts in the following manner:

Alj(-OH)₈.Aq+ 3H|-Cl.Aq = Al|(-Cl)₃.Aq+3H₂O; on the other hand, when caustic soda is presented to aluminium hydroxide, it forms sodium aluminate, NaA1O₉. Aq, a derivative of the acid HA1O₂.Aq, which is formed from Al(OH)₃ by loss of water :

The ions in the latter case are H and -A1O₂, and the reaction takes place between HA1O₂ and NaOH, thus:

H|( -A1O₂). Aq + Na|-OH.Aq = Na|-A1O₂. Aq + H₂O.

It is generally the case that the acids are derived from hydroxides which have lost a portion of their hydrogen as water. They are, like 0=A10H, partly oxide, partly hydroxide.