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If we change the concentration of either reactants or products, the value of equilibrium constant also changes.
Suppose you have an equilibrium established between four substances A, B, C and D

Let's say we decrease the concentration of 'C', technically the value of will reduce according to the above expression.
But, if you know Le Chatelier's principle, it states that when a system experiences a disturbance (such as concentration, temperature, or pressure changes), it will respond to restore a new equilibrium state.
This means that the system subjected to a change will try to move in a direction that reduces the effect of the change (image below).
When all this takes place, the value of does not change.
