Simulation of an Equilibrium

Two beakers are set up to show what happens with the reactants and products in a chemical areaction.  In the beginning, the reactant beaker is full and the product beaker is empty.  Water is simultaneously scooped from one beaker to the other.  Each time, the water level that results is recorded.  At first, the rate in the forward direction, from reactants to products, is high and water builds up quickly in the products beaker.  Since there is no product at the start, initially there is no reverse reaction at all.  However, as the concentration of products increases, so does the rate at which water can be transferred back into the reactants beaker.  The rate of the reverse reaction speeds up.

Eventually, after a number of trials, the rate at which water can be moved from reactants to products becomes the same as the the rate at which products turn back into reactants.  When the forward and reverse rates are equal, the system has reached an equilibrium, with unchanging amounts of water in the two beakers, even though water continues to be transferred back and forth the whole time.

Together, these two beakers represent an equilibrium system.  As long as no material enters or leaves the system – the system is closed – in due course the rate of the reaction in the forward and reverse directions will become the same.  At this point, we have reached an equilibrium recognized by its unchanging observable properties.