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The Idea of Equilibrium If you are at a party and there are 100 other people there you may notice the following: as the evening goes on there are always a hundred people present but the identities of the people change. New people are constantly arriving, and others are leaving. The population at the party is said to be in dynamic equilibrium. If new people no longer show up, but those already there continue to leave, the party is no longer at equilibrium and eventually the population will decrease to zero. If you place a glass of water on a window sill and leave it uncovered, all the water will evaporate. If you place a saucer over the glass however, the water won't dry up. Instead, water molecules will leave the liquid, bounce around, and return, much like the same guests being recycled through the same party again and again. The water and the entrapped water vapor above it are said to be in dynamic equilibrium. We have equilibrium when the composition of some system stays the same indefinitely. We have dynamic equilibrium when the mechanism for this stability is the balancing of departure and arrival of one or more components of the system.

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