Chemical Equilibrium is defined as a state of reaction where the rate of change in the reactants and the products remains constant. Note that the amounts of the different entities need not be constant, only their respective rates of change.
Because many chemical equilibria take place in the gaseous or aqueous phase, it is almost impossible to see the constant reaction that is going on, and to the naked eye, there appears to be no reaction. This is the phenomenon of equilibrium, and it is extremely vital to how our modern society functions.
Unfortunately, this concept can be difficult to show to students first learning chemistry. Luckily, there is a solution which exists that can show them how equilibrium actually occurs, and the only supplies that are required are a pair of graduated cylinders, some drinking straws, and some basic tap water.
Fill one graduated cylinder with 25 mL of water, and leave the other cylinder empty. Then have two people each take a drinking straw, but ensure that the two straws have different radii, and drop them into the graduated cylinders. Each person should then plug the top end of the straw with their thumb, so as to create a pseudo-vacuum, and then each person should transfer the amount of liquid they pick up to the other cylinder. Note that the first time, there will be no exchange from the empty cylinder. For each subsequent trial, the volume of the water in the cylinders will change by a differing number, because the amount of water picked up is dependent upon how much water is already in the containers.
Record the volume of each cylinder, and remember that both volumes should combine to a sum total of 25 mL. After a while, you’ll notice that equilibrium has been achieved, as the volume in either cylinder will not change regardless of how many transfers are performed. This is an analogy for chemical equilibrium, as the rate of change between the two containers is constant, and the “reactions” can be observed and understood.
While this demonstration is not a perfect analogy, it effectively demonstrates the process of equilibrium a concept that might otherwise be difficult to understand. Additionally, one could add extra water to a cylinder after the equilibrium has been reached in order to demonstrate how Le Chatelier’s principle governs changing quantities in equilibrium. This demonstration is effective even in higher-level chemistry courses, because of the clarity with which it presents the concepts and practical applications of equilibrium.