You will know that a chemical reaction has occurred because new substances will be made with new physical properties. For example, burning paper doesn't produce burnt paper, it produces smoke, ash, and carbon gasses.
Short of knowing that new substances have been produced, there are some clues to alert a person to a chemical reaction's occurrence. The following things can occur when a chemical reaction takes place.
- A change in color
- A change in temperature
- An emission of gas. This could be identified by the production of bubbles. For example, mixing vinegar and baking soda.
- A smell or odor may be produced
- The reaction may cause a sound to occur
- The reaction might generate light. This would be case for things like Halloween glow sticks or any form of bio luminescence.