Janet learned in science class that the amount of salt that can dissolve in a certain amount of water depends on the temperature of the water. she decides to do an experiment to test this. she carefully adds salt, 100 mg at a time, to a cup of water and stirs until all the salt dissolves. when she has added so much salt that it will not dissolve after 5 minutes, she measures the temperature of the water. she records the temperature and the final amount of salt. she does this with cold water, cool water, room-temperature water, warm water, and hot water. what is the test variable (independent variable) in this experiment?