What signs indicate that a chemical reaction may have occurred?

1 Answer


  • colour change
  • gas being evolved
  • new substance forming
  • energy given off or consumed
  • production of an odour


These are very general guidelines and there are exceptions:

A colour change does not always signal a chemical reaction, and chemical changes are not always associated with a chemical change.

Gases can be evolved during chemical reactions or phase changes, for a chemical reaction, look for the formation of bubbles in a solution -- this indicates a gas as a product of the reaction.

Remember that phase changes are not chemical reactions!

Energy is often given off or consumed during a chemical reaction, exothermic reactions and endothermic reactions respectively. That being said this can also happen during physical changes that are not accompanied by a chemical change (eg. phase changes).

The other two are pretty foolproof.

Hope this helps :)