Which of the following indicates heat has been produced in a chemical reaction?

The indication that heat has been produced in a chemical reaction is a change in temperature. When a chemical reaction occurs, it often involves changes in energy which can manifest as heat production or absorption. If the reaction releases heat, it is an exothermic reaction, which typically results in an increase in temperature of the surroundings. Therefore, a measurable change in temperature is a clear sign that heat has been produced.

While the formation of gases, change in color, and formation of a precipitate are all signs that a chemical reaction may have taken place, they do not directly indicate whether heat has been released or absorbed. For example, gases can be produced in reactions that are endothermic or exothermic, and color changes can also occur without any heat transfer. The formation of precipitates is similar in that it signifies a reaction has occurred but does not provide information about thermal energy changes. Thus, the change in temperature is the most direct evidence of heat production in a chemical reaction.