Five indicators of a chemical change are color change, temperature change, precipitate formation, gas bubble formation and smell or taste change. Since some chemical products are poisonous, detecting a chemical reaction via smell or taste change is not recommended.
Gas bubbles can form when a chemical solution is boiled or warmed. When two soluble reactants are mixed together in a solution, a precipitate form can occur. A chemical compound has its own color absorption spectrum. However, a color change does not mean a chemical reaction occurred. For example, although zinc oxide changes color from white to yellow, but there is no chemical change. Energy is required for breaking chemical bonds. When chemical bonds are formed, energy is released. If more breaking than forming of bonds happen, energy is absorbed, making the solution cooler. The surroundings become hotter when more bonds are made than when bonds are broken.