1. Temperature change: a change in temperature is a key way to show a chemical reaction occurred because it is indicative of an energy exchange. All reactions exchange energy, whether that energy is lost by combining compounds or elements or gained by breaking bonds.
2. Precipitate forms: a precipitate is a solid particle that randomly forms in a liquid solution.
3. Change in color: each particle reflects and absorbs light differently (which produces colors), so when a reaction occurs the particle is altered and therefore so is the color.
4. Gas bubbles form: gas bubbles occur when a reaction occurs because the original compounds have been rearranged and changed into new compounds of a different phase, such as occurs during the formation of carbon dioxide or oxygen.
5. Light: this occurs mostly in combustion reactions, but when bonds are broken violently energy can be released quickly, resulting in light.
6. Change in odor or taste
7. Change in boiling or melting point: every compound has a unique melting point, so if this changes it is a key sign a reaction occurred.
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