Final answer:
The occurrence of a color change or production of gas during a mixture's reaction strongly suggests that a chemical change has taken place.
Step-by-step explanation:
When determining if a chemical change has occurred, there are several indicators to consider. Among these, the mixture changing color or the production of gas are strong indicators of a chemical reaction. A color change often implies that new substances with different properties are formed, and the production of gas that is not a result of boiling suggests a new gas has formed as a product of the reaction.