A sample of a pure substance is heated at a constant pressure of 1 atm. The temperature increases from -120°C to -50°C, then remains constant at -50°C for several minutes while some solid remains, then rises again until 60°C, where it remains constant until all liquid has vaporized. Which of the following statements correctly describes the changes of state?

Correct answer

A. The substance melts at -50°C and boils at 60°C.

Principle or equation

At constant pressure, a pure substance has characteristic phase transition temperatures. A plateau in the heating curve indicates a phase change: the first plateau (at -50°C) is melting (solid to liquid), and the second plateau (at 60°C) is boiling (liquid to gas).

Why this answer is correct

The temperature rises as the solid is heated. When it reaches -50°C, the temperature stops rising even though heat is added, meaning the substance is undergoing a phase change. Since some solid remains during the plateau, the solid is melting into liquid. After all solid melts, the temperature rises again until 60°C, where another plateau occurs while liquid is changing to gas (boiling). Thus, melting point = -50°C and boiling point = 60°C.

Example

For water at 1 atm, heating ice gives a plateau at 0°C (melting) and then at 100°C (boiling). Similarly, the first plateau here is melting and the second is boiling.