For bases, the concentration of OH^– must exceed the concentration of H₃O⁺ in the solution. This imbalance can be created in two different ways.

First, the base can be a hydroxide, which merely dissociates to yield hydroxide ions:

where M represents the cation, usually a metal. The most familiar bases are such hydroxides. (See Table 1.)

The second type of base acts by extracting a hydrogen ion from a water molecule, leaving a hydroxide ion:

An example of this second type of base that is not a hydroxide can be an ammonia molecule in water (aqueous ammonia):

Ammonia acts as a base by stripping a proton from a water molecule, leaving an increased OH^– concentration. Notice in the equilibrium reaction that and NH₃ are a conjugate acid-base pair, related by transferring a single proton. Similarly, water acts as an acid by donating a proton to ammonia. H₂O and OH^– are a conjugate acid-base pair, related by the loss of a single proton.

Alternatively, the base may be a particular kind of negative ion with a high attraction for a hydrogen ion:

In 1923, the English chemist Thomas Lowry and the Danish chemist Johannes Br??nsted defined an acid and base in another way. An acid is a substance that can donate a proton, and a base is a substance that can accept a proton.

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