From A Past Life:   Teaching Chemistry:

I once told my chemistry class if they wanted evidence of a chemical reaction, all they had to do was eat hardboiled eggs and sauerkraut. Because they would produce:

1. Gas. Lots of small bubbles and maybe, even lots of big ones. You know, the painful, oh my god! My gut’s going to explode kind (See also-Boyle’s Law on pressure)

2. Odors! Especially the room evacuating kind.   Good evidence for fermentation and that their intestinal bacteria are alive and well. Or waging war with probiotics. In any case, their bacteria will be very, very busy.

3. Noises. And if they were really lucky, they’d make turbo-sounds. How’s that for particle velocity?

There! Three examples of evidence for a chemical reaction. : gas production, odor, and sound. And if you add color, not to mention quantity and consistency, you’ll have even more. And then there’s always the possibility of explosions…(energy release)