The Scientist's Essay for Grade 5, 2. Water to Vapor

What's important about evaporation and condensation?

Condensation provides another clue about where the water goes, by showing, indirectly, that there really is "water" in the air, even though we can't see it.

Evaporation really seems like magic. You leave some water in a glass, or a small puddle on a stone countertop, and when you come back later it's just disappeared. You didn't see it leave, and you can't see it anywhere now that it's gone, and it doesn't seem like you can get it back. Probably the most important thing about evaporation, then, is that in fact nothing has been lost, the matter is still around somewhere, and the process can be reversed. You can't see the water that used to be in the puddle, but those molecules still exist.

The evidence for these bold assertions is rather indirect, and may not be entirely convincing at the level of precision we can attain in the classroom. In a closed system — one that doesn't allow any matter to come in or go out — the water that "disappears" from one part of the system can "appear" somewhere else. We may not be able to see exactly how it got from one place to the other — in the vapor phase it was invisible to us — but we can infer that it must have been there. The weight of the system also remains constant in this process, showing that the total amount of "stuff" in the system didn't change. (Our two-bottle system isn't perfectly closed — water vapor can actually pass through the plastic at a slow but measureable rate, especially when the plastic is warm, so over the course of a few days the weight drops by a few grams. This caused us quite a bit of puzzlement until we figured out what was happening!)

Condensation provides another clue about where the water goes, by showing, indirectly, that there really is "water" in the air, even though we can't see it. Where else can the beads of water on the outside of a cold glass come from? It's interesting to think of tests, or experiences, to rule out other possibilities — for example, that it's the liquid from inside the glass that somehow appears on the outside.

These phenomena are not unique to water. Alcohol, for example, evaporates more readily than water, a phenomenon that's exploited in making distilled liquor. But everything evaporates, even solids — just the rate is usually extremely low at normal temperatures.

On the microscopic level we think of evaporation as a process in which an occasional molecule in the liquid (or solid) happens to get enough energy from random thermal motion to break its bonds to its neighbors and escape. It's not a collective phase transition, like melting, freezing, or boiling, so it happens at any temperature (just much faster at higher temperature). Condensation is the opposite — when a molecule in the vapor hits a cool surface, it can lose enough energy to reconnect with and be trapped by the neighboring molecules.

—Roger Tobin