There are several observations which can be made which gives hints as to whether a chemical reaction has occurred. Those observable changes are:
1. When liquid or solid substances are combined and the evolution of gas occurs. eg: evolution of gas from an antacid in water or hydrogen peroxide in an open wound.
2. An insoluble precipitate is produced when chemicals are combined in aqueous solutions. eg: the formation of an insoluble scum in a tub after adding soap or bath salts.
3. The evolution of heat when chemicals are combined. eg: iron powder in a "pocket warmer" heating when exposed to air.
4. The production of energy in the form of sound or light. eg: an explosion.
5. A change in color which is not just caused by the combination of other colors. Combining red and yellow pigments making green pigment is not a reaction, however cooking pink, raw meat to a grey or brown color is.
6. A change in the magnetic or electric properties of the origianlreagents. eg: heating sulfur in iron cause the iron to lose its magnetic properties