1. Color change: One of the most common indicators of a chemical reaction is a change in color. This is because the reactants and products of a chemical reaction have different colors. For example, when iron is exposed to oxygen, it rusts and turns from a silvery-white color to a reddish-brown color.

2. Formation of Gas: A chemical reaction may also produce gas. For example, when baking soda and vinegar are mixed, they react to produce carbon dioxide gas, which causes a fizzing reaction and the formation of bubbles. The presence of bubbles or a change in pressure can indicate a chemical reaction.

3. Production of Heat or Light: Some reactions release energy in the form of heat or light. For example, when wood burns, it undergoes a chemical reaction with oxygen that produces heat and light. Similarly, the reaction between hydrogen and oxygen to form water releases heat and light in the form of a flame. A noticeable change in temperature or the emission of light can indicate a chemical reaction.

4. Formation of a precipitate: A precipitate is a solid that forms when two solutions are mixed. The formation of a precipitate is an indication that a chemical reaction has occurred. For example, when silver nitrate solution is added to sodium chloride solution, a white precipitate of silver chloride forms.