What is Standard state, activity?

The activity $a_i$ of a species is a dimensionless number that measures, in the expression of equilibrium laws, how much of it is available; it is defined relative to a standard state, a reference state at the temperature considered and under the standard pressure $p^\circ = 1\,\mathrm{bar}$. In the ideal approximations used in this book:

• for a gas, $a_i = p_i/p^\circ$ (the standard state is the pure perfect gas at $p^\circ$);
• for a solute in a dilute solution, $a_i = c_i/c^\circ$ with $c^\circ = 1\,\mathrm{mol}/\mathrm{L}$;
• for the solvent of a dilute solution, and for a pure solid or a pure liquid alone in its phase, $a_i = 1$.