In chemistry, Le Chatelier's Principle applies to systems at, or approaching, equilibrium. It states that changing one of the conditions of the equilibrium (applying a "stress" to it) will cause the equilibrium to shift in a direction that compensates for the change or stress. For example, in the equilibrium

A + B <-> C + D

(where the concentrations of ingredients A, B, C, and D are constant) if the equilibrium is "stressed" by adding more ingredient A, the equilibrium will shift to the right, producing more C and D.

The principle applies to heat as well as concentrations of substances. If in the above example, the reaction of A + B is exothermic (A + B --> C + D + heat), then raising the temperature will drive the equilibrium to the left, increasing the concentrations of A and B and reducing the concentrations of C and D.

Similarly, if ingredient B is a gas and all others are solids or liquids, then increasing the overall pressure will drive the equilibrium to the right since gas pressure is equivalent to concentration.