500
A student wants to know if temperature affects the amount of salt that can be dissolved in a pot of water. He gathers three identical pots and writes down these steps:
1. Add 400 mL of water to each pot.
2. Let the water in each pot heat for ten minutes on separate water heaters.
3. Measure and record water temperature in each pot.
4. Add salt to each pot, one spoonful at a time, until no more salt will dissolve.
5. Record the number of spoonfuls of salt added to each pot.
The student is missing one step. Which step must be added to ensure the data will help him answer his question?
Set each water heater to a different temperature (Ex: Set one water heater to 30°C, one to 50°C, and one to 70°C.)