Is it permissible to use Depilatories (creams or "shaving powders" which chemically dissolve hair) on Shabbos? and if not- why?
Har Tz'vi (Yore Dea 144) says it's forbidden (by Divine, not rabbinic, decree) as a part of the prohibition on removing hair on Shabas. (He notes that use of a chemical is one of the standard ways of removing hair, so this doesn't count as a case of kil'achar yad that would remove it from the Divine prohibition.) Sh'miras Shabas K'hilchasah 14:41 cites this as practical halacha.
The answer to this is probably based in the method by which most of those creams work. They are an acid, and essentially burn the hairs off of your face. This is the same basis as many chemical cleaners work off of. That is discussed a bit here. Would you use hydrogen peroxide to clean with on shabbat? (Other than for a medicinal purpose)