Question

Why do we describe a liquid-vapor equilibrium as a dynamic equilibrium?

Answer 1

Because there is no way to prevent bits of the substance from moving from one phase to another. We cannot establish some kind of magic "force field" that keeps every part of a liquid in the liquid state, or every part of a gas in the gaseous state.

What happens, then, is that bits of the liquid will, indeed, go into the gaseous state all the time. Bits of the gas will go into the liquid state. This happens all the time, at any temperature, under any conditions. (The temperature just determine how often these fluctuations occur, and how large they are -- how many molecules at once make the transition.)

So, for example, in a glass of water, water molecules will be escaping from the water surface and going into the water vapor above the glass. Conversely, water molecules from the gas phase will be crashing into the liquid surface and becoming part of the liquid. This happens constantly. So technically you might even say this system is not "in equilibrium" because on the microscopic, atomic level, it is not at all motionless. It is in fact in constant motion, and bits of the system are flipping back and forth between one phase and the other all the time.

However, on the macroscopic level -- what we can see with our unaided eyes and ears -- the system is in equilibrium, meaning it isn't changing. The net amount of water and water vapor doesn't change. The system *looks* quiescent, peaceful, unchanging. And we'd like to distinguish that from a more actively changing system, such as a glass of water boiling, or some water vapor condensing as rain.

What we do is call the system in which a system is not changing, on the macroscopic level, because two or more microscopic changes are conspiring to cancel each other out, a system in dynamic equilibrium. It means a system that is not changing (on the macroscopic level) only because microscopic changes ("dynamic" means "changing") are canceling each other out. In the case of a glass of water, the constant evaporation of water molecules from the liquid surface is being canceled out by the constant condensation of water molecules from the gas phase. (Provided the water is not in fact boiling or condensing.)

An analogy would be your bank account: the bank balance might be in equilibrium (not changing) for one of two reasons: you are writing no checks and making no deposits -- i.e. nothing is happening. This would be ordinary equilibrium. But it might also be that you are writing lots of checks and making lots of deposits (so a lot of change is happening) but it just so happens that the checks and deposits are exactly canceling each other so that your balance stays constant. This would be a dynamic equilibrium.