Final answer:
Signs of a chemical reaction include the change in color, formation of bubbles (indicating gas production), production of light, and formation of a solid, all of which imply the creation of new substances.
Step-by-step explanation:
The observation that was a sign that a chemical reaction was occurring in the experiment can be any of the following: formation of a solid, change in color, formation of gas bubbles, and production of light. These are all events that indicate a chemical reaction is taking place because they involve the creation of new substances with different properties from the reactants. Specifically, a change in color typically indicates that new substances are being produced, the formation of bubbles often signifies the production of a gas, and the production of light is usually associated with exothermic reactions.