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 $]=\frac{\text { Number of moles }}{\text { Volume }}$

$=\frac{\text { Mass } / \text { molecular mass }}{\text { Volume }}$

$=\frac{\text { Mass }}{\text { Volume } \times \text { Molecular mass }}$

$=\frac{\text { Density }}{\text { Molecular mass }}$

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.