I stored some Klean-Strip denatured alcohol in a test tube and capped it with a rubber stopper. I did this in preparation for a chemistry demonstration. When I opened my kit a few days later at the event, I was surprised to find that the ethanol in the tube had turned a definite urine yellow. It burned just fine but let off nearly imperceptible wisps of smoke. Even more interesting was when I added water into the tube, which caused an almost instant white precipitate to form. I had to improvise the demonstration and play it off as turning flammable urine into semen, which it closely represented (because it was meant to be a comedic demonstration for a mature audience). The next few days I ran an experiment of using the same denatured ethanol in a jar over the black rubber stopper. By the end of the experiment, the solution exhibited the same properties and characteristics as the accidental batch. Now, not even to save my own skin, I cannot come up with a reasonable explanation for why the ethanol turns yellow, what causes the discoloration, and how does it turn milky when water is added.
Why does this happen. Chemically, what is happening. And would this be a good indicator for a non-anhydrous chemical? For example, I can add this to methanol or NaOH, measure the "turned whitedness" and determine whether or not that chemical is truly anhydrous?