this post was submitted on 17 Oct 2024
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My chemistry teacher once explained it to me like below. Does anyone know how much truth there is to this explanation?
Temperature as measured by a thermometer or your finger is an average. Not every single molecule has the same temperature. The molecules constantly bounce around, smashing into each other, transferring heat to each other. By chance, some molecules will get hit in just the right way by other molecules to reach a very high temperature and then it evaporates. So there is constantly a gradient of temperatures among the molecules and the ones with the highest temperature are the ones evaporating, until there is no liquid left at all.
As the average temperature increases, the chance of some molecules reaching a high enough temperature also increases, so warm water evaporates faster than cold water.
This also explains why evaporation cools down (like when you sweat): the molecules with the highest temperature are the ones evaporating, so the average temperature decreases as those high-temperature molecules leave the system. Only the relatively colder molecules are left behind - thus it cools as a whole.
I think it's much easier and truthful to stop talking about temperature and introduce speed in that context.
The average speed is what we percieve as temperature, but single molecules can be fast, so fast as to break the boundaries of the liquid pool and shoot up toward space.
Single unbounded molecules are what gas is.
But temperature is not just the speed of a molecule right? Isn't it also like the "energy" stored in the molecule, or its "wiggling" or something? Like a molecule moving very fast through space can still be at a very low temperature, right?
What you're thinking about is the relation between energy, temperature, and heat capacity. When you add energy to a system (e.g. heat) the amount of energy you need to heat it up a certain amount is described by its heat capacity. If your molecules can "wiggle" (i.e. they're multi-atomic) a portion of the energy you're adding will go to increasing the "wiggling" rather than the mean speed of the molecules.
What we perceive as temperature is related to the mean speed of the molecules, so because molecules that can "wiggle" more will require more heat to see the same increase in mean speed as non-wiggling molecules (because some of the heat is going to increasing the wiggling) they have higher heat capacity.
It should also be mentioned that even the concept of temperature is really a statistical concept, so it doesn't really make much sense to talk about the temperature of a single isolated molecule, or even a pair of them. Temperature as a concept starts to be fruitful to talk about in the thermodynamic limit which classically means "a whole shitload of molecules", but (relatively) recent research suggests "a whole shitload" can be as little as 10-30 molecules. Once you go below the thermodynamic limit, we're not really talking about the temperature of a system, but it's energy, which is still well defined (although definitions may vary depending on context). Depending on who you ask, it can make sense to define a temperature also for single-particle systems, but at that point we're talking about applying thermodynamic definitions that work (and are correct in the macroscopic limit) and no longer about what we classically perceive as temperature.
Thanks, that's useful!