I mean there is some stuff that makes water flammabe. What was it again? Pretty nasty stuff that explodes if it touches anything.
Yeah mean Sodium in water?
Nope, but good one too.
Fluorine? Or maybe chlorine trifluoride?

Maybe Trichloro silane? I think it was something with “tri”, reactive with almost anything and creates a poisonous gas then.

Probably one of them, i give up.

Water-Reactive Chemicals

I think it is chlorine trifluoride, which I only know about because of Sand Won’t Save You This Time
Sand Won't Save You This Time

Ah yes, this was it. Thanks!
Not an acid, but maybe francium?
Well technically all of the alkaline metals have pretty strong reactions when in water though to different degrees. So maybe one of them?