Equilibrium represents a balance between opposing reactions. How sensitive is this balance to changes in the conditions of a reaction? What outside perturbations will change the equilibrium state? These are very practical questions if, for example, one is trying to increase the yield of a useful product in a reaction.

The easiest way to perturb a reaction and obtain more products is to remove the products as fast as they are formed. This means that the reaction is kept off-balance and that equilibrium is never achieved.

More and more reactants interact in a vain effort to maintain a balance, as products are taken away. For example, is quite soluble in water, whereas and are only slightly soluble. Therefore washing the ammonia reaction mixture with a spray of water dissolves and removes most of the , and leaves behind the unused and for further reaction.

This sort of product removal does not change the actual conditions of equilibrium; it only ensures that more and react than would be the case if , , and were allowed to come to equilibrium without interference. The same trick of keeping a reaction off-balance can be used if one of the products is a gas that bubbles out of the reaction mixture, or a solid that precipitates.