There are various definitions for acids and bases including the Arrhenius and the Lewis definitions. A less-commonly-used definition is the Hard and Soft Acid and Base Theory (HSAB Theory). This theory is used in a more qualitative context by labeling chemical compounds as ‘hard’ or ‘soft’ depending on various chemical properties. These qualities of hard and soft acids and bases are as follows:

This definition allows for some definitions of certain molecules to be defined as acids or bases that are normally not considered acids or bases. For example, Cr³⁺ is a hard acid while Hg²⁺ is a soft acid.

Within this definition, acids and bases generally interact in such a way that hard-hard interactions and soft-soft interactions are strongest. While this chemical theory is largely qualitative, it also includes a quantitative component. For instance, the ‘softness’ of a base B can be measured using the following reaction and measuring the magnitude of the equilibrium constant:

BH + CH₃Hg⁺ ↔ H⁺ + CH₃HgB

Here, the larger the equilibrium constant, the softer the base. A table of some soft and hard acids can be seen below:

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