# Equilibrium involving water and solid component

$$\ce{A + B <=> C(s) + H2O(l)}$$

Why when $\ce{H2O}$ is liquid and $\ce{C}$ is solid, the equilibrium constant of the reaction is

$$K_\mathrm{eq} = \frac{1}{[\ce{A}][\ce{B}]} \quad ?$$

• I remember this being described as a "convention", as if to acknowledge that the role if a solid component of an equilibrium is ill - defined. Would appreciate hearing from others on the subject. – iad22agp Aug 22 '17 at 10:55