At equilibrium the following mathematical relationship exists. Substances that are excluded from this equation are SOLIDS AND LIQUIDS. They CANNOT change concentrations (they are pure), so they can not be included in this equilibrium expression.
Example of equilibrium constants
What does K imply?
Large K > 1 products are "favored"
K = 1 neither reactants nor products are favored
Small K < 1 reactants are "favored"
What do equilibrium constants with solids look like?
The equilibrium produced on heating calcium carbonate
This equilibrium is only established if the calcium carbonate is heated in a closed system, preventing the carbon dioxide from escaping.
The only thing in this equilibrium which isn't a solid is the carbon dioxide. That is all that is left in the equilibrium constant expression.
Why you don't include the solids or liquids?
ANIMATION-Notice the change when more solid is added to the system. Nothing Changes. That is why it needs to be excluded.
More Practice Problems-