According to Le Chatelier's principle, a change in pressure affects the chemical equilibrium of the reaction system under what condition?

A change in pressure will affect the equilibrium position of the reaction if one or more of the reactants or products are in the gaseous phase.

Le Chatelier's principle states that whenever a change is brought about to a system under chemical equilibrium, the equilibrium shifts in a manner to reverse this change. So if the pressure of a reaction mixture is increased, the equilibrium will shift towards the side with the lower number of molecules, and if the pressure is reduced, the equilibrium will shift to the side with the greater number of molecules.