A sample of a pure substance is heated from -100°C to 300°C at 1 atm. Its temperature remains constant at -30°C for a period, then rises, then remains constant at 150°C. Which of the following correctly describes the changes of state that occur?
Show the answer and explanation
Correct answer
A. Melting at -30°C and boiling at 150°C
Principle or equation
During heating at constant pressure, a pure substance's temperature remains constant at phase transition points. The first plateau corresponds to melting (solid to liquid) and the second to boiling (liquid to gas).
Why this answer is correct
The substance is initially solid. At -30°C, the temperature stops rising, indicating a phase change from solid to liquid (melting). After that, temperature rises until 150°C, where it plateaus again, indicating a phase change from liquid to gas (boiling).
Example
For water, heating from -20°C to 120°C shows a plateau at 0°C (melting) and at 100°C (boiling).
This published item includes a stored explanation and passed the platform’s publication workflow. It is independent preparation material, not a claim of an official or recalled examination question.