If the activity of a solid is one why does adding salt to water increase the salt ion concentration?

Suppose salt dissolves in water according to the equation NaCl(s) -> Na(aq) + Cl(aq). Since the activity of solid salt is 1, adding salt shouldn't affect sodium and chloride ions at equilibrium. However, if I were to add salt to an unsaturated solution, the salt would dissolve and increase the ion concentrations. How is it possible for the activity of solid salt to be 1 if it affects equilibrium in this way? My only theory is that an unsaturated solution is not at equilibrium, but I'm not sure this is correct. 




Yes, you are correct: an unsaturated solution is not in equilibrium. Equilibrium is the state when rate of dissolution is equal to rate of crystallisation. But in an unsaturated solution there is only dissolution of salt in the water and no crystallisation back to salt.


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