Very early in the history of chemistry many substances were designated as acids, bases, and salts. Acids have a sour taste (e.g., citric acid gives lemon juice its sour taste); they dissolve certain metals; and they also dissolve carbonate minerals to produce carbon dioxide. Bases have a bitter taste (e.g., sodium carbonate); they feel slippery when touched; and they react with many dissolved metal salts to form precipitates. However, the most striking characteristic of bases is their ability to neutralize the properties of acids; when a base reacts with an acid a salt is produced.

The French chemist Lavoisier thought that all acids contain oxygen (the word oxygen means “acid former” in Greek). However, it was subsequently found that many acids contain no oxygen (e.g., hydrochloric acid, HC1), but that they all contain hydrogen.

Acids and bases figure prominently in the equilibrium of aqueous solutions, where they significantly enhance the electrical conductivity of water. In this chapter, we will explore some of the important properties of acids and bases in aqueous solutions. This will lead us to a discussion of several theories of acids and bases.

Some definitions and concepts

Equation (4.22) is valid for aqueous solutions as well as for pure water. A solution for which [H+(aq)] = [OH-(aq)] is said to be neutral; thus, pure water is neutral. If [H+(aq)] > [OH-(aq)] the solution is said to be acidic. If [OH-(aq)] > [H+(aq)] the solution is said to be basic.