Which statement best describes the purpose of a melting/boiling point experiment?

• A. To measure the color changes during heating.
• B. To determine the rate of heat transfer.
• C. To determine the exact temperatures at which phase changes occur.
• D. To calibrate the thermometer.

Answer: (C)

Melting and boiling point experiments focus on pinpointing the temperatures where a substance changes from solid to liquid and from liquid to gas. As you heat a sample, the temperature rises until the solid melts; during the melting process, the temperature stays nearly constant, revealing the melting temperature (for a pure substance this is a sharp value). Similarly, once the liquid reaches its boiling point, the temperature plateaus while vaporization occurs, giving the boiling temperature. This information helps identify substances and assess purity, since impurities tend to broaden the melting range and can alter boiling behavior. The other choices miss the core purpose: color changes aren’t a reliable indicator of phase change, the rate of heat transfer isn’t what the test aims to measure, and thermometer calibration is a separate preparatory step rather than the main goal.