A chemist studied how stirring speed affects the time required for 5.0 g of sugar to dissolve in 100 mL of water. She used a magnetic stir plate and tested four stirring speeds: 0 rpm, 200 rpm, 400 rpm, and 600 rpm. The water temperature was kept constant at 25°C using a water bath, and the same beaker type and sugar brand were used each time. For each trial, the chemist started a stopwatch when sugar was added and stopped it when no visible crystals remained. Which of the following is the independent variable?
- Water temperature (25°C)
- Mass of sugar added
- Stirring speed (rpm) (correct answer)
- Time for the sugar to dissolve
Explanation: In this experiment, the independent variable is the stirring speed (0 rpm, 200 rpm, 400 rpm, and 600 rpm), the dependent variable is the time for the sugar to dissolve, and controlled variables include the mass of sugar (5.0 g), water volume (100 mL), water temperature (25°C), beaker type, and sugar brand. The independent variable is the factor the experimenter changes on purpose, the dependent variable is what is observed and recorded as the result, and controlled variables are maintained the same in all trials to prevent them from affecting the outcome. Choice C correctly identifies stirring speed as the independent variable because it was deliberately varied to study its effect on dissolution time. Choice A is a distractor as it represents a controlled variable, the water temperature kept at 25°C, not the manipulated factor.