A pure substance is heated at a constant pressure of 1 atm. Its temperature rises steadily from -60°C to -10°C, then remains constant at -10°C for several minutes while some solid melts, then rises again until 90°C, where it remains constant until all liquid vaporizes. Which of the following statements correctly describes this behavior?
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Correct answer
C. The substance is a pure substance because it has sharp melting and boiling points.
Principle or equation
A pure substance has characteristic sharp melting and boiling points at a given pressure. During a phase change, temperature remains constant despite continued heating.
Why this answer is correct
The temperature plateaus at -10°C (melting) and at 90°C (boiling), indicating two distinct phase changes. A pure substance has well-defined phase transition temperatures, whereas a mixture typically melts or boils over a range. The substance is not necessarily an element or compound; it could be either, but the sharp transitions indicate purity.
Example
Pure water at 1 atm melts at 0°C and boils at 100°C, with constant temperature during each phase change.
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