Identifying the Independent Variable in Chemical Reaction Experiment

Researchers conducted an experiment to study the impact of temperature on the rate of a chemical reaction. They set up two groups of identical reaction vessels, both containing the same reactants. Group 1 was kept at a low temperature of 10°C, while Group 2 was maintained at a higher temperature of 30°C. After one hour, the amount of product formed in each group was measured in grams. The experiment was repeated three times for accuracy.

What is the primary independent variable in this experiment?