Activity in chemistry is a measure of how different molecules in a non-ideal gas or solution interact with each other.

It is given by the equation $a\_i\; =\; \backslash phi\_i\backslash \; x\_i$

where $\backslash phi\_i\backslash$ is the activity coefficient, which must be measured for different substances, and $x\_i\backslash$ is the mole fraction. The activity coefficient of ions in solution can be estimated with the Debye-Hückel equation, the Davies equation or the Pitzer equation. Activity effects are the result of interactions between ions both electrostatic and covalent. The activity of an ion is influenced by its surroundings. The reactivity of an ion in a cage of water molecules is different from that in the middle of a counterion cloud.

This type of activity is relevant throughout chemistry from reaction rates to equilibrium constants. For instance large deviations can exist between the calculated hydrogen ion concentration of a strong acid in solution and the hydrogen activity derived from a pH meter or a pH indicator pH Paradoxes: Demonstrating That It Is Not True That pH ≡ -log^* Christopher G. McCarty and Ed Vitz Journal of Chemical Education Vol. 83 752 2006. Two examples serve to illustrate this point.

In a solution of potassium hydrogen iodate at 0.02M the activity is 40% lower than the calculated hydrogen ion concentration resulting in a much higher pH than expected.

When to a 0.01M hydrochloric acid solution containing Malachite green indicator is added a 5M solution of lithium chloride the color changes from green to yellow indicating increasing acidity when in fact the solution is diluted. Although at low ionic strength (<0.1M) the activation coefficient decreases with increasing ionic strength this coefficient can actually increase with ionic strength in a high ionic strength regime. For hydrochloric acid solutions the minimum is around 0.4M.

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