Equilibrium in Closed Systems
The requirement that no matter enters or leaves for a true chemical equilibrium to be established.
Unit
Dimensionless
In depth
If a product escapes — as a gas from an open vessel, for instance — the reverse reaction can never build up and the reaction proceeds to completion instead of reaching equilibrium. This is why thermal decompositions run further in an open crucible than in a sealed tube.
Examples in the real world
Limestone decomposing completely in an open kiln but reaching a fixed CO₂ pressure in a sealed vessel.