A pure substance is heated at a constant pressure of 1 atm. Its temperature increases from -100°C to 0°C, then remains constant at 0°C for several minutes while some solid remains, then rises again until 150°C, where it remains constant until all liquid has vaporized. Which of the following statements correctly describes the changes of state for this substance?

Correct answer

A. The substance melts at 0°C and boils at 150°C.

Principle or equation

At constant pressure, a pure substance shows plateaus in its heating curve at phase transition temperatures. A plateau at a lower temperature with solid present indicates melting (solid to liquid), and a plateau at a higher temperature with liquid present indicates boiling (liquid to gas).

Why this answer is correct

The temperature remains constant at 0°C while some solid remains, meaning solid and liquid coexist, so 0°C is the melting point. Later, the temperature remains constant at 150°C while liquid and gas coexist, so 150°C is the boiling point. Therefore, the substance melts at 0°C and boils at 150°C.

Example

For water at 1 atm, the heating curve shows a plateau at 0°C (melting) and at 100°C (boiling). Similarly, the described plateaus indicate melting and boiling points.