Question

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

Answer 1

For a pure substance (both solids and liquids),

${\displaystyle \left[\text{Pure substance}\right]=\frac{\text{Number of moles}}{\text{Volume}}}$

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

${\displaystyle =\frac{\text{Mass}}{\text{Volume x Molecular mass}}}$

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

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.