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 moles"/"Volume"`
`= "Mass/molecular mass"/"Volume"`
`= "Mass"/("Volume x Molecular mass")`
`= "Density"/"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.
This is because molar concentration of a pine solid or liquid is independent of the amount present.
`"Molar concentration" = "No of moles"/"volume" xx "Mass"/"volume" xx "Density"`
Since density of pure liquid or solid is fixed and molar mass is also fixed. Therefore molar concentration are constant.