How do you know when a chemical reaction has occurred

The Substance Has Changed and Has Become Something Else

Explanation:

When a chemical reaction occurs it has a few characteristics, for example burning. The substance, paper, becomes something else, ash. The ash can no longer be turned back into paper. Another example is rust, once and object is rust due to oxidation it can no longer return back. A physical reaction on the other hand can be reversed, like freezing. To spot chemical change look for color changing, gas production, a change in temperature, or if you see any light.