Chemical activities should be used to define chemical potentials, where the chemical potential depends on the temperature , pressure  and the activity  according to the formula:
where  is the gas constant and  is the value of  under standard conditions. Note that the choice of concentration scale affects both the activity and the standard state chemical potential, which is especially important when the reference state is the infinite dilution of a solute in a solvent. Chemical potential has units of joules per mole (J/mol), or energy per amount of matter. Chemical potential can be used to characterize the specific Gibbs free energy changes occurring in chemical reactions or other transformations.
Formulae involving activities can be simplified by considering that:
* For a chemical solution:
** the solvent has an activity of unity (only a valid approximation for rather dilute solutions)
** At a low concentration, the activity of a solute can be approximated to the ratio of its concentration over the standard concentration: 
Therefore, it is approximately equal to its concentration.
* For a mix of gas at low pressure, the activity is equal to the ratio of the partial pressure of the gas over the standard pressure:  Therefore, it is equal to the partial pressure in atmospheres (or bars), compared to a standard pressure of 1 atmosphere (or 1 bar).
* For a solid body, a uniform, single species solid has an activity of unity at standard conditions. The same thing holds for a pure liquid.
The latter follows from any definition based on Raoult's law, because if we let the solute concentration  go to zero, the vapor pressure of the solvent  will go to . Thus its activity  will go to unity. This means that if during a reaction in dilute solution more solvent is generated (the reaction produces water for example) we can typically set its activity to unity.
Solid and liquid activities do not depend very strongly on pressure because their molar volumes are typically small. Graphite at 100 bars has an activity of only 1.01 if we choose  as standard state. Only at very high pressures do we need to worry about such changes. Activity expressed in terms of pressure is called fugacity.