What occurs at the boiling point in terms of temperature and phase change?

• A. Temperature decreases.
• B. Phase changes do not occur.
• C. Temperature remains constant while phase change occurs.
• D. Temperature increases.

Answer: (C)

At the boiling point, heat goes into changing the substance from liquid to gas rather than raising its temperature. The added energy supplies the latent heat of vaporization, breaking intermolecular forces so bubbles form and the phase change proceeds. Because that energy is used for the transition, the temperature remains constant until all the liquid has vaporized. Once the liquid is fully turned into gas, further heating will raise the temperature of the gas.

So the statement that the temperature stays the same while a phase change occurs is the best description of what happens at the boiling point. The other ideas don’t fit: temperature doesn’t drop just as boiling starts, a phase change does occur, and the temperature doesn’t increase during the actual phase change itself.