A sample of a pure substance is heated from -60°C to 220°C at 1 atm. Its temperature remains constant at -20°C for a period, then rises, then remains constant at 110°C. Which of the following correctly describes the changes of state that occur during the heating?
Show the answer and explanation
Correct answer
A. Melting at -20°C and boiling at 110°C
Principle or equation
During heating, a pure substance absorbs heat and changes state at its melting point (solid to liquid) and boiling point (liquid to gas). The temperature remains constant during these phase changes.
Why this answer is correct
Starting as a solid below -20°C, the temperature stays constant at -20°C while the solid melts to liquid. Then the liquid warms until 110°C, where it boils to gas. Thus, melting occurs at -20°C and boiling at 110°C.
Example
For water, heating from -10°C to 120°C shows a plateau at 0°C (melting) and another 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.