We have quite a lot of good questions on the topic, which is actually quite broad. For example, just yesterday we had this question: Does gravity affect the trajectory of gas particles?
Despite it being the kinetic theory of gases, kinetics is not applicable here, since that tag deals with rates of reactions. So, this is certainly ruled out.
gas-laws is probably our best bet right now. However, I don't know about you guys, but "do gas molecules travel in straight lines?" doesn't sound to me like a question about "gas laws" - rather more like "gas behaviours".
physical-chemistry is far too broad a tag and cannot be used alone.
statistical-mechanics does not quite apply here either, in my opinion. Kinetic theory is a prerequisite for stat mech. I can't quite think of an exact definition of stat mech, but I would associate it with the ideas of configurations, microstates, and partition functions, not about how gases are hard spheres and have elastic collisions.
In my opinion, gas-laws already has a well-established scope, mostly consisting of questions about the ideal gas law and its derivation, but also including (for example) Dalton's law of partial pressures, and Raoult's and Henry's laws. This is reflected in its tag wiki, which is brief but clear.
- Experts on kinetic theory: Partially. Just being conservative here.
- Studying kinetic theory now: Yes, certainly! I even have a set of notes on the topic!1
- Would I leave a question with only this tag? Yes. For example, for the linked question above, I would only add physical-chemistry. Considering that phys-chem is such a broad tag, I hope that does not mean that this should be a no.
- Will a question only be tagged because it is related to it? No. kinetics will always be confused with this especially with first-timers who are told that they "need a tag".
- Universally unambiguous meaning: Yes.
- Will it not be misinterpreted? Partially. Again, the potential for a mix-up with kinetics is there.
- Are there many questions about it? Yes.
- Will people search for questions to answer? Partially, at the very least, and quite possibly Yes, but I'm being conservative here.
- Will people add it to their favourite tags? Partially again, because I'm conservative.
- Can questions about kinetic theory also have other tags? Yes - ignoring phys-chem, there is obvious potential for overlap with, for example, gas-laws (derivation of ideal gas law), statistical-mechanics (equipartition), temperature, and more - maybe even astrochemistry (e.g. what is the mean free path in outer space!).
That gives it a total score of 12, and that's with me being conservative - if we upgrade Q1, Q8, and Q9 to Yes's, then we get a score of 15.
The main problem that I could see with having such a tag is obviously the potential for it to be confused with kinetics.
I'd love to hear what the community thinks about this.
1 I did those notes a long time ago, so none of my differential d's were enclosed in