How do you know for sure that a chemical reaction has taken place?

A chemical change involves changes in the chemical composition of the reacting species. That is, the chemical composition of the material/s before and after the change are different.

The most certain way to know if a change is a chemical change is to determine the chemical composition of the products. However, since that may not always be possible, we can use indirect methods to confirm the chemical change. There are a number of observations associated with a chemical change and if one of them is there, we can be sure that the change is chemical in nature. Here is a small list of such observations:

• Generation of gases or bubbles


• Changes in temperature


• Formation of precipitate


• Generation of odor or smell


• Change in the color of the solution 

If one observes some of these, one can be fairly sure that a chemical change has taken place.

Hope this helps.