Variables in Heat Transfer Practice Test

A student investigates: Which insulation material keeps water hot the longest? They wrap identical cups with either foam, fiberglass, or plastic. Each cup gets 500 mL of water starting at 80°C, and all cups sit in the same room. After 60 minutes, the student records the water temperature. What is the independent variable in this investigation?

A student investigates: How does the mass of water affect heating time? They heat 100 g, 200 g, and 400 g of water in the same pot on the same burner setting. All trials start at 20°C. They record how long it takes the water to reach 80°C. Which variable should be kept controlled to make this a fair test?