[Xplo]

Plumbers no more design industrial chemistry processes than welders design aircraft carriers.

. . .

Chemistry is a difficult skill to adjudicate. People who aren't familiar with it IRL won't necessarily know what to do with it, other than "I want to improvise a powerful explosive using the contents of this lab/household kitchen/ janitor's closet." (I don't have my book with me, but as I recall, you actually want Explosives skill for this - and then I think it's some variant of Engineer to actually make a working bomb from the substance! Ridiculous...)

People who are familiar with it IRL are going to want to use it for making bombs, fuels, fire accelerants, acids, and poisons (oh, SO many poisons), performing diagnostic laboratory medicine, analyzing strange materials and forensic evidence, possibly even creating weird weapons or other devices that utilize exotic states of matter (though this borders Physics), and pretty much any other thing you can imagine having to do with studying or building materials or substances. And they probably should be able to do all of these things, but without appropriate equipment or skills specializing in those tasks they ought to be at huge penalties at least some of the time. A cinematic chemistry whiz (skill 25+) might be able to do almost anything with chemistry that the script can handwave an explanation for, but your typical Master or PhD chemist (skill 15-18-ish) probably won't...

The worst problem is when the player knows (small-c) chemistry and the GM doesn't, and then they argue over what the skill should be able to do, and either the player feels cheated by an ignoramus or the GM ends up getting steamrolled. In this case, the player can buy all the various engineering, medical, and forensic skills that make specific practical uses of chemistry, and justify his PC's broad competence that way, but at that point it starts to feel like redundancy and skill bloat, and the player might well wonder why he's paying points for Chemistry at all, or what it's going to do for him that one of the other skills doesn't, especially if his GM doesn't understand what Chemistry itself is actually good for...

The second worst problem is when neither the GM or the player knows chemistry, and the game is not a highly cinematic one, and Chemistry ends up being a useless skill because no one really understands what it's good for, and the best the player can hope for is that once in a while the game uses a chemical substance or formula as some kind of puzzle or clue so that the player will have a contrived excuse to roll against their PC's Chemistry skill that they paid points for.

Of course, it could be used for job rolls, if the PC is employed as a chemist, and being able to make money at a somewhat lucrative profession is probably worth investing a few CP, but that's also boring...