Le Châtelier's principle
MCAT glossary · Chemical & Physical Foundations of Biological Systems
Definition
The rule that a system at equilibrium shifts to counteract an imposed change.
Le Châtelier's principle in plain English
Le Chatelier's principle says a balanced chemical reaction is like a stubborn seesaw: push it one way and it pushes back to undo your change. When a reaction is at equilibrium (forward and reverse happening at equal pace) and you disturb it, by adding more of something, removing something, or changing temperature or pressure, the system shifts in whichever direction cancels part of your disturbance. It doesn't fully return to where it started, but it always leans against the change you made, which lets chemists nudge reactions toward the products they want.
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