This Demonstration illustrates the law of mass action, which is an example of Le Chatelier's principle, that if a system in chemical equilibrium is disturbed it tends to change in such a way as to counteract the disturbance.[more]
The final concentrations in a reaction are related to initial concentrations by the equation for the equilibrium constant for a generic reaction with all stoichiometric coefficients set equal to 1 and fixed temperature :
This allows us to obtain the final concentrations by using:
, , and .
The concentrations at the equilibrium are set as:
, , and
so is equal to 1. The domain for a physically valid solution (all the final concentrations must assume positive values) is determined by and as a consequence of the solution of the system of inequalities: .
As the concentrations change, the plot is shifted (brown plot) following Le Chatelier's principle. A red line shows the magnitude of the shifting and the value.
Selecting "reaction behavior" shows the numerical values and which side is favored after altering the concentrations.[less]
Snapshot 1: system at the equilibrium; the initial concentration fulfils the equilibrium constant equation so there is no variation
Snapshot 2: Le Chatelier's principle: raising one reactant concentration and lowering one product concentration in the equilibrium state causes a shift toward the side of products ()
Snapshot 3: raising reactant concentrations and lowering product concentrations in the equilibrium state causes a shift toward the side of products ()
 C. H. P. Lupis, Chemical Thermodynamics of Materials, New York: North-Holland, 1983.
 S. Z. Lavagnino. Chemical Equilibrium [Video]. (Jun 25, 2020) www.youtube.com/watch?v=TDBQOF7M-W8&list=PLswwssc6Q2yac7AM3x5UjmesLQaye-xMP&index=3.