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

Explanation

In writing the expression for the equililbrium constant, molar concentration of the species involved in the reaction are taken into account. We all know that the molar concentration of a substance implies number of moles per unit volume `("moles"// "volume") " or " ("mass// "volume")`. Since mass divided by volume represents density of the substance therefore molar concentration of a substance is directly related to its density.
Molar concentration `=(" No. of moles ")/(" Volume ") prop (" Mass ")/(" Volume ") (prop " density ")`
Density as we know, is intensive property and does not depend upon the mass of the substance. Consequently , molar concentration of a pure substance (solid or liquid) has always the same value and it can be ignored while writing the value of the equilibrium constant. However , for substances in gaseous state or in aqueous solution, the amount in a given volume can vary and their molar concentration does not remain constant and cannot be ignored while writing the expression for the equilibrium constant.