A pure substance is heated at a constant pressure of 1 atm. Its temperature rises steadily from -50°C to 0°C, then remains constant at 0°C for several minutes, then rises again until 100°C, where it remains constant until all liquid has vaporized. Which of the following statements is correct?
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Correct answer
C. The substance is a pure substance with a melting point of 0°C and a boiling point of 100°C.
Principle or equation
A pure substance has sharp, constant melting and boiling points at a given pressure. The observed constant temperatures during phase changes indicate a pure substance, not a mixture.
Why this answer is correct
The temperature plateau at 0°C indicates a solid-to-liquid phase change (melting point), and the plateau at 100°C indicates a liquid-to-gas phase change (boiling point). These are characteristic of a pure substance. The identity (element or compound) cannot be determined solely from phase change temperatures.
Example
Pure water melts at 0°C and boils at 100°C at 1 atm; a salt solution would show a range of temperatures during freezing or boiling.
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