What scientific tool would be the most helpful when measuring the temperature of a substance that is undergoing a phase change?

A thermometer measures temperature directly, so it is the most helpful tool for tracking a substance during a phase change. The temperature may remain constant while the substance changes phase, but the thermometer can still show that temperature. A scale measures mass, while a spectrophotometer and the periodic table do not directly measure temperature.

AI-written and checked against the verified answer.