What signs indicate that a chemical reaction may have occurred?
- 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 :)