When a chemical reaction takes place in a container which prevents the entry or escape of any of the substances involved in the reaction, the quantities of these components change as some are consumed and others are formed. Eventually this change will come to an end, after which the composition will remain unchanged as long as the system remains undisturbed. The system is then said to be in its equilibrium state, or more simply, "at equilibrium"
Mechanical equilibrium describes what happens when there are no unbalanced forces in a system or with a system and its surroundings. This means that the forces must be equal in a system and in a system and its surroundings. One such force is pressure. If the pressure is the same in the system and with the system and its surroundings, mechanical equilibrium is reached. If there is no mechanical equilibrium, the system will endeavor to reach equilibrium.
In chemical equilibrium, it is also necessary that matter should not be moving from one area to another, such as what happens when diffusion is taking place.