Strictly speaking an alkali is a base in aqueous solution and so the word is confined to soluble bases. As the word alkali is part of everyday language it is often used instead of the word base. Some texts talk in terms of acids and alkalis, others of acids and bases. The subtle difference between base and alkali need not be a problem in this text but both words must be used as the book sits somewhere between industrial chemistry and academic chemistry. As such, both words are used in a manner that reflects common usage rather than strict chemical definition.

It was seen above that water dissociates into hydrogen ions and hydroxide ions. In terms of simple definitions relating to aqueous chemistry: a substance that produces hydrogen ions as the only positive ions is an acid; a substance that reacts with hydrogen ions in a neutralization reaction is an alkali; usually alkalis react with hydrogen ions because they produce hydroxide ions.

Considering the dissociation of water we see that it behaves both as an acid and an alkali. It is amphoteric, a unique property that is also a feature of some surfactant molecules discussed later on in the surfactants section.

If we add an acid to water then extra hydrogen ions are introduced. For the ionic product to remain the same, as is required by the equilibrium principle, the number of hydroxide ions must diminish. Likewise, if a base is added to the pure water...