Answer
The concentration of a solid does not change because a solid does not expand to fill a container.Therefore volume does not change.So concentration of pure solid only depends upon it's density,which is constant.
Again for pure liquid concentration does not change as it’s density has a much weaker dependence on temperature (than gases) ,and it is regarded as approximately constant.
As the concentrations of solid and pure liquids do not change so they are omitted from equilibrium expressions.
