A pure substance starts fully liquid and cools at constant pressure. Its (time in s, temperature in °C) data are (0, 58), (30, 50), (60, 42), (90, 42), (120, 42), (150, 34). It is fully solid by the last reading. (a) Estimate its freezing temperature and the duration of the recorded constant-temperature interval. (b) Explain the energy changes before and during this interval. (c) State one measurement improvement.

Question

Accepted Answer

42 °C; recorded plateau lasts 60 s; energy is released during freezing although temperature stays constant. The repeated temperature 42 °C marks the freezing interval. The recorded plateau extends from 60 s to 120 s, so its duration is 120 − 60 = 60 s. Before the plateau the liquid cools: its particles' average kinetic energy decreases along with temperature. Thermal energy is transferred to cooler surroundings, so the substance's internal energy decreases. During freezing, liquid and solid coexist while the temperature stays constant. Energy continues to leave the substance as particle arrangements and their potential energy change; a horizontal graph does not mean there is no energy transfer. Take temperature readings more frequently near the start and end of the plateau to improve its estimated duration. Keep the thermometer bulb in the substance, not touching the container, and stir gently while liquid remains so it measures a representative temperature.

Cooling a pure substance 1

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Cooling measurements

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Cooling measurements — supplied values
Time / s	Temperature / °C
0	58
30	50
60	42
90	42
120	42
150	34