Final answer:
Chemical reactions are indicated by several observable factors, such as temperature changes, light production, unexpected color changes, and bubbles from gas formation. These changes signify a transformation of substances and are usually harder to reverse as compared to physical changes.
Step-by-step explanation:
An unexpected change in temperature is a sign that a chemical reaction may have occurred. Substances giving off a smell or light while being combined is also an indicator of a chemical reaction. Observations that indicate a chemical change include changes like temperature increase or decrease, light emission, unexpected color changes (a new color appears, not a blend of original colors), and bubble formation when a gas is produced at the temperature of the starting materials, indicating that the reaction is not simply boiling.
Chemical changes often involve the formation or breaking of bonds between molecules or atoms, resulting in the transformation of substances with distinct properties. These changes are generally harder to reverse than physical changes. Additionally, the production of a solid called a precipitate or a visible energy transfer in the form of light or heat can signal a chemical change.