This can be explained by considering density of pure solid or pure liquid. [Pure liquid] or [Pure solid]
= No. of moles/Volume in L
No of moles= Mass/ Mol. Mass = Mass / mol.mass/Volume
= Mass/Volume x 1/mol.mass = Density/mol.mass
• Density of a pure liquid or pure solid is constant at constant temperature and molecular mass is also constant, therefore, their molar concentrations are constant and are ignored into the equilibrium constant.