Questions about photographic processing & chemical principals?

Hi you guys :) I'm doing a chemistry project and one of the things I have to do is show a brief illustration that shows a violation of a chemical principal. I was wondering if anyone could show me any examples or give me any suggestions? Also as for photography, what principle is being illustrated when developing photos? (You know, when you put it in the different chemicals and what not). Sorry if this sounds confusing (I'm not even sure if I asked the questions correctly). I'd appreciate the help!