A chemist conducted an experiment in which they observed a chemical reaction under various temperatures. The chemist used the same substances in the same amounts for each reaction and only varied the heat applied for each reaction. The chemist recorded the duration and primary color of each reaction. What is a control variable in this experiment?

The primary reaction color is an outcome of the chemical reactions and therefore cannot be controlled. It is a dependent variable that changes in response to the experimental conditions, rather than a factor that is kept constant throughout the experiment.

The amount of heat applied is the independent variable in this experiment, meaning it is the factor that the chemist varied in order to observe its effects on the reaction. Since it is intentionally changed in the experiment, it does not qualify as a control variable.

The duration of each reaction is also a response variable that can change based on the amount of heat applied. While it may be measured consistently, it is not controlled or kept constant across the different temperature settings, making it an outcome rather than a controlled variable.