Chemistry / Unit 10 Chem 1758 short lesson, about 20 seconds
Homogeneous vs. Heterogeneous Equilibria
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The idea
A homogeneous equilibrium has every species in one phase; a heterogeneous equilibrium spans phases, and its expression omits pure solids and pure liquids entirely.
In practice
For CaCO3(s) ⇌ CaO(s) + CO2(g), K = [CO2], since the two solids' concentrations are constant and folded into K.
Where people slip
Including a pure solid or liquid's "concentration" in the expression is a common construction error.
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