Concentration of pure solid and liquid is not included in the expression of equilibrium constant because

Explanation

[Pure liquid] or [Pure solid]=`("Number of moles")/("Volume of L")`
`=("Mass"//"Molecular mass")/("Volume")`
`=("Mass")/("Volume")xx1/("Molecular mass")`
`=("Density")/("Molecular mass")`
As density of a pure liquid or pure solid is constant at constant temperature and molecular mass is also constant, therefore, their molar concentration are constant and not included into the equilibrium constant.