In a reversible reaction, a chemical equilibrium is established when the concentrations of products and reactants don’t change. In this interactive, the equilibrium can be disturbed by changing the concentration of a reactant to see how a new equilibrium is reached according to Le Châtelier's principle. The change of concentrations can also be observed numerically as the ratio of products over reactants, called the reaction quotient Qr. Instructions: Drag the slider to see how reactants and products change over time. Select “Perturb the system by adding 0.002 M of reactant A at time T” to add more molecule A to the system at equilibrium.