activity (chemistry)

Activity in chemistry is a measure of how different molecules in a non-ideal gas or solution interact with each other.Activity effects are the result of interactions between ions both electrostatic and covalent. The activity of an ion is influenced by its surroundings. The activity 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.
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