Anthocyanin, which is found in red cabbage, is a natural indicator that can be easily extracted.
Universal indicator goes from red (pH 4) to violet (pH10) as the pH of a substance changes. Adding NaOH to water starts the solution off at pH 8-9 (blue). When dry ice is added to water it forms carbonic acid, and lowers the pH, which is the reason for the color changes. The “fog” that we see is condensed water vapor though, not carbon dioxide gas.
This demo uses acid base chemistry and an indicator to make an ink that will appear or disappear depending on the solution added to it. Phenolphthalein, which is clear, reacts with a base, and then turns pink.
Under basic conditions glucose is oxidized while methylene blue is reduced, changing from blue to colorless. The reduction reaction can be reversed by shaking the flask which forces oxygen into solution, which oxidizes the methylene blue, thus turning it back to blue. If the flask is let sit, the oxygen will come out of solution making the solution colorless and ready to shake again.