Two processes, which are taking place in opposite directions are in equilibrium. How to write equilibrium constant

(i) Equilibrium in a system having more than one phase is called heterogeneous equilibrium. If ethanol is placed in a conical flask, liquidvapour equilibrium is established.

C₂H₅OH_(l) ⇌ C₂H₅OH_(g)

For a given temperature,

K_C = \(\frac{[C_2H_5OH_{(g)}]}{[C_2H_5(OH)_{(l)}]}\)

But [C₂H₅OH_(l)] = 1

∴ K_C = [C₂H₅OH_(g)]

Thus,

At any given temperature, density is constant irrespective of the amount of liquid, and the term in the denominator is also constant.

ii. similarly, consider I_2(g) ⇌ I_2(g)

K_C = [I_2(g)]

iii. Thus, the expression for equilibrium constant does not contain the concentration of pure solids and pure liquids. 

That is because for any pure liquid and solid, the concentration is simply its density and this will not change no matter how much solid or liquid is used. 

Hence,

The expression for heterogeneous equilibrium only uses the concentration of gases and dissolved substances (aq.). 

Solids are pure substances with unchanging concentrations and thus equilibria including solids are simplified.