Cooling Curve of an Unknown Mixture
A student heats 50 g of an unknown solid in a beaker over a hot plate while stirring and records temperature every 30 seconds. The data show: steady rise from 22 °C to 68 °C over 5 minutes; then the temperature holds at 68 °C for 3 minutes even though heating continues; then it rises again to 95 °C; then holds near 95 °C for 2 minutes while the material becomes fully liquid. The student claims the substance is a pure compound that melts at 68 °C. A classmate points out the second plateau and argues the sample is a mixture. The teacher asks the group to decide which claim the data support and what additional measurement would settle it.
1. Which claim (pure compound melting at 68 °C, or mixture) is better supported by the two plateaus, and why?
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The mixture claim is better supported. A pure crystalline substance melts over a narrow temperature range at a single melting point; while melting, added energy goes into breaking the crystal lattice rather than raising temperature, giving one plateau. This sample shows two plateaus (68 °C and 95 °C), which is consistent with at least two components with different melting points: the first component melts at 68 °C, and the second at 95 °C. The long 68 °C hold (3 min) is not just thermal lag - heating continued throughout.
Caveat: a single plateau alone cannot prove purity, and two plateaus cannot by themselves identify the components. The claim should be stated as 'the data are consistent with a mixture of at least two substances,' not as a confirmed identity.
2. Describe one additional measurement or test that would help distinguish a pure substance from a mixture, and explain what result would support each conclusion.
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A melting-point range test: slowly heat a small sample and record the temperature at which melting begins and ends.
- If the sample is a pure substance, melting should begin and end within a narrow range (roughly 1–2 °C) at a single temperature.
- If it is a mixture, melting should begin at a lower temperature and finish at a higher one, giving a broad range (e.g., begins ~66 °C, fully liquid by ~97 °C).
Other acceptable tests: chromatography (a mixture separates into multiple spots/bands; a pure substance gives one), or evaporating/cooling and checking whether a solid reappears at more than one temperature. In each case, the reasoning is the same: a pure substance has a single, sharp transition; a mixture shows a spread or multiple transitions.