Kate gathered three boxes of the same size made of different materials: glass, clear plastic, and aluminum painted black. she placed them on a window sill in the sun for an hour and then measured the warmth of the air in each box. in this experiment the warmth of the air is an?

The answer is Free factor  / Independent variable.

Explanation:

A free factor is a variable that is controlled to decide the estimation of a needy variable s. The reliant variable is what is being estimated in a trial or assessed in a scientific condition and the autonomous factors are the contributions to that estimation. An autonomous variable in a test setup is the controlled variable. As it were, the free factor is the variable that is being tried or modified by the experimenter.

yes it is weird but true

not only is it colder, it also becomes darker, less salty, and more dense.