Many chemical reactions can happen in both forward and reverse directions at the same time. We say the system is at equilibrium when both directions run at the exact same speed. The amounts of reactants and products stay completely stable during this balanced state.
French chemist Henri Le Chatelier studied what happens when you disturb this peaceful balance. He found that the system will always try to undo whatever change you just made. If you add more starting chemicals, the system will instantly speed up the forward reaction.
It uses up those extra chemicals to create more of the final product. If you add extra heat to a reaction, the entire system will try to cool down. It shifts the reaction toward the side that naturally absorbs the extra heat energy.
If you increase the pressure on a gas, the system tries to relieve that pressure. It shifts toward the side of the reaction that creates fewer molecules of gas. This powerful rule applies to temperature, pressure, volume, and chemical concentration changes.
Students sometimes think the system perfectly returns to its exact original starting state. It actually finds a brand new balance point that minimizes the effect of your change. This clever idea is vital for chemical factories trying to make as much product as possible.