Heterogeneous Equilibrium
An equilibrium involving species present in more than one phase.
Unit
Dimensionless
In depth
Pure solids and pure liquids have constant activity and are therefore omitted from the equilibrium expression, so their amounts do not affect the equilibrium position provided some of each remains. This is why adding more limestone does not increase the CO₂ pressure above it.
Examples in the real world
CaCO₃(s) ⇌ CaO(s) + CO₂(g), where Kp equals the CO₂ pressure alone.