So now I am utterly confused. Do evaporation and condensation occur at the same time, or is it one or the other but never both? I thought that I knew the answer to this question and I was prepared to rant a little bit about the state of science education and how we are doing a disservice to kids by not raising this central question (and many others like it), but now I feel the need to take a step back first.
What is evaporation? It is the transfer of particles from the liquid state to the gas state from the surface of the liquid. Okay. Seems clear enough and there is some level of consensus around this definition. And condensation? It is the transfer of particles from the gas state to the liquid state. So far, so good.
However, the problem with these definitions is that they do nothing to help me answer the question: Do evaporation and condensation occur at the same time, or is it one or the other but never both? There is too much ambiguity.
Here is what I need to know to clear up my confusion: Is evaporation (1) the transfer of a particle from the liquid state to the gas state from the surface of the liquid, or (2) the net transfer of particles from the liquid state to the gas state from the surface of the liquid?
The first definition focuses on a single particle. If one particle breaks away from the surface of the liquid state and enters the gas state, then evaporation occurred. The second definition focuses on the system as a whole. Evaporation only occurs if more particles are breaking away from the surface of the liquid state and entering the gas state than are leaving the gas state and entering the liquid state.
Let's consider a glass of water on a warm day. Water molecules are breaking away from the surface of the liquid and entering the gas state. Water molecules in the air are leaving the gas state and entering the liquid state. Because there are more water molecules entering the gas state than the liquid state, the amount of water in the glass is decreasing over time.
Now let's drop a bunch of ice cubes into the glass of water. Water molecules are still breaking away from the surface of the liquid and entering the gas state. Water molecules in the air are still leaving the gas state and entering the liquid state. However, the addition of the ice cubes has shifted the rates of these two parallel processes, and now more water molecules are entering the liquid state than the gas state, and the amount of water in the glass is increasing over time. (Droplets of liquid water are forming on the outside of the glass, but condensation is happening in the glass as well. It's simply easier to point out these visible droplets of water as evidence of condensation than measure the amount of liquid water in the glass.)
I don't particularly care how we define evaporation and condensation (okay, I do), but I do want kids and adults to understand evaporation and condensation at this level. We are teaching kids about evaporation and condensation, and they are building mental models about how the world works on an atomic and molecular level; yet, we aren't probing to help us or our students see if those mental models are accurate or robust. And what about the kids themselves? Why aren't they probing their own mental models? Why isn't it nagging at them that they don't really understand what is going on? Is it because they don't care, don't expect clarity, or don't expect their teachers to be able to provide clarity?
Well, I do expect clarity for myself. This is something that I expect to understand, and I think that I do on an atomic and molecular level. Where I feel myself on shaky ground is the actual definitions of evaporation and condensation.
Here is what one of my college physics textbooks says on the matter: "Air normally contains water vapor (water in the gas phase) and it comes mainly from evaporation. To look at this process in a little more detail, consider a closed container that is partially filled with water (it could just as well be any other liquid) and from which the air has been removed. The fastest moving molecules quickly evaporate into the space above. As they move about, some of these molecules strike the liquid surface and again become part of the liquid phase; this is called condensation." The implication is that evaporation and condensation occur on a per particle basis (definition 1).
Now let's take a look at Wikipedia: "Evaporation is a type of vaporization of a liquid that occurs from the surface of a liquid into a gaseous phase that is not saturated with the evaporating substance." The implication is that evaporation ceases when the gaseous phase is saturated and the system has reached equilibrium (definition 2).
And here is what I turned up when I googled for a scientific definition of evaporation:
"The change of a liquid into a vapor at a temperature below the boiling point. Evaporation takes place at the surface of a liquid, where molecules with the highest kinetic energy are able to escape. When this happens, the average kinetic energy of the liquid is lowered, and its temperature decreases."
"The process by which molecules undergo the spontaneous transition from the liquid phase to the gas phase. Evaporation is the opposite of condensation."
"It is the process whereby atoms or molecules in a liquid state (or solid state if the substance sublimes) gain sufficient energy to enter the gaseous state."
"Evaporation occurs when a liquid changes to a gas at a temperature below its normal boiling point."
"Evaporation is the slow vaporization of a liquid and the reverse of condensation. A type of phase transition, it is process by which molecules in a liquid state spontaneously become gaseous."
So is it definition 1 or definition 2? Has the scientific community reached consensus? If some of you can help me out, please weigh in with your comments.