In a reversible reaction. is it true that two substances are in equilibrium if the concentration of each increases by the same amount but the equilibrium constant remains unaffected?
Yes, it is true that the equilibrium constant remains unaffected when you change any of the concentrations.
The equilibrium constant is a constant at a given temperature.
Consider the equilibrium
If you change any of the concentrations,
Instead, the system adjusts itself so that the new values of of