Which of the following is a sign that a chemical reaction has occurred?

A temperature change is a strong indicator that a chemical reaction has taken place. During a chemical reaction, bonds between atoms are broken and formed, which often releases or absorbs heat. This results in a change in temperature in the surroundings. Observing an increase in temperature might suggest an exothermic reaction, while a decrease indicates an endothermic reaction.

In contrast, a change in state is not exclusively indicative of a chemical reaction, as physical changes (like melting, freezing, or boiling) can similarly cause state alterations without any reaction taking place. Change in size can occur due to various reasons that may or may not involve a chemical reaction, and “no reaction” clearly implies that no change is taking place. Therefore, while there are several indicators of a chemical process, a temperature change is a clear and direct sign of chemical reactivity.