In the question Pregancy Test on Shabbos a notion was raised in an answer that the chemical reaction is a problem due to
So what are the boundaries of this, in terms of chemical reactions being a problem, with the specific question of those glasses that dim in sunlight, and become dark like sunglasses. This is a reversible chemical reaction, and I have always wondered if it applies.
As a possible side note/question, what are the boundaries of tzovaya as would mixing vinegar and baking soda to make a volcano on Shabbos be allowed? A chemical reaction that makes a mess...