Activity of a Pure Solid or Liquid

[Patricia Sourinphoumy Dis1C]

I know that pure solids and liquids might need to be present in certain reactions in order for equilibrium to exist, but these do not appear in the expression for K because their activity is 1. Further, when a solution is very dilute, a change in solvent concentration due to the reaction is considered insignificant and so this solvent is treated as a pure substance and not included in the expression for K as well.

I didn't quite catch why the activity is 1 and exactly why we would not include pure solids, pure liquids, or solvents in the expression for K. Can someone explain?

Thanks!

[Abeni Liu 2B]

I don't think you have to worry about activity levels. Solids don't have a concentration and solvents are in such large excess that it will not be significant for the equilibrium expression.