7.7 Explain why pure liquids and solids can be ignored while writing the equilibrium constant expression?

For a pure substance (both solids and liquids), 
Pure substance=Number of molesVolume                               =Mass/molecular massVolume                               =MassVolume×Molecular mass                               =DensityMolecular mass

Now, the molecular mass and density (at a particular temperature) of a pure substance is always fixed and is accounted for in the equilibrium constant. Therefore, the values of pure substances are not mentioned in the equilibrium constant expression.