Strong Base Definition and Examples

Chemistry Glossary Definition of Strong Base

3D illustration of sodium hydroxide.
Sodium hydroxide is an example of a strong base. Laguna Design / Getty Images

A strong base is a base that is completely dissociated in an aqueous solution. These compounds ionize in water to yield one or more hydroxide ion (OH-) per molecule of base.

In contrast, a weak base only partially dissociates into its ions in water. Ammonia is a good example of a weak base.

Strong bases react with strong acids to form stable compounds.

Examples of Strong Bases

Fortunately, there aren't very many strong bases.

They are hydroxides of the alkali metals and alkaline earth metals. Here is a table of the strong bases and a look at the ions they form:

BaseFormulaIons
sodium hydroxideNaOHNa+(aq) + OH-(aq)
potassium hydroxideKOHK+(aq) + OH-(aq)
lithium hydroxideLiOHLi+(aq) + OH-(aq)
rubidium hydroxideRbOHRb+(aq) + OH-(aq)
cesium hydroxideCsOHCs+(aq) + OH-(aq)
calcium hydroxideCa(OH)2Ca2+(aq) + 2OH-(aq)
barium hydroxideBa(OH)2Ba2+(aq) + 2OH-(aq)
strontium hydroxideSr(OH)2Sr2+(aq) + 2OH-(aq)

Note that while calcium hydroxide, barium hydroxide, and strontium hydroxide are strong bases, they are not very soluble in water. The small amount of compound that dissolves dissociates into ions, but most of the compound remains a solid.

The conjugate bases of very weak acids (pKa greater than 13) are strong bases.

Superbases

The Group 1 (alkali metal) salts of amides, carbanions, and hydroxides are called superbases. These compounds cannot be kept in aqueous solutions because they are stronger bases than the hydroxide ion.

They deprotonate water.