A system is in dynamic equilibrium when the rate of the forward reaction equals the rate of the reverse reaction. Though the reaction is still proceeding in both directions, the net concentrations of reactants and products remain constant.

At this point, if a reactant or product’s concentration is changed, for example by adding additional molecule A or by removing some molecule C, the equilibrium is disturbed. Such changes are stresses on the system.

According to Le Châtelier’s principle, when a system at equilibrium is stressed due to a change in concentration, a change in volume or pressure, or a change in temperature, the equilibrium position shifts in a direction to minimize the stress and restore equilibrium.

To understand a system’s response to changes in concentration, consider two connected water tanks, A and B. Initially, the water level in the two tanks is equal, and the system is in equilibrium. Then, water is added to tank B.

This stress raises the water level in tank B and disturbs the equilibrium. To offset this stress, some of the water flows into tank A to restore the equilibrium.

Conversely, if water is removed from tank B, the lowering of the water level also triggers stress.

Consequently, water from tank A flows into tank B to restore the equilibrium.

For a system at equilibrium, the reaction quotient equals the equilibrium constant. Adding more reactants to the equilibrium mixture decreases Q. According to Le Châtelier’s principle, the system minimizes this stress by shifting the equilibrium position towards the products to expend the added reactant and re-establish equilibrium. 

Likewise, removing a product from the equilibrium mixture will lead to a similar response in order to restore equilibrium.

In contrast, adding more products increases Q. Thus, the equilibrium position shifts towards the reactants to expend the added product and restore equilibrium.

A change in concentration shifts the equilibrium position without changing the value of K.