A sample is heated at approximately constant power. The recorded pairs (time in min, temperature in °C) are (0, -10), (2, 0), (4, 0), (6, 0), (8, 20), (10, 40). (a) Describe the three distinct regions. [3] (b) Calculate the warming rate before and after the plateau. [3] (c) Explain the plateau using a particle-energy model. [3] (d) Evaluate whether the record proves that heating power was perfectly constant and propose one improvement. [3]

Question

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The sample warms at 5 °C/min, remains at 0 °C from 2–6 min, then warms at 10 °C/min. During the plateau, supplied energy changes state rather than temperature. Six readings do not prove perfectly constant input power. (a) From 0–2 min temperature rises from −10 °C to 0 °C; from 2–6 min it remains at 0 °C; from 6–10 min it rises from 0 °C to 40 °C. (b) Initial warming rate = [0 − (−10)]/(2 − 0) = 5 °C/min. Final warming rate = (40 − 0)/(10 − 6) = 10 °C/min. (c) At 0 °C, energy is used to separate particles and change the phase mixture rather than increase mean kinetic energy, so temperature stays approximately constant while heating continues. (c) After the phase change, a different specific heat capacity and thermal-loss pattern can give a different rate even if heater power is similar. (d) The record supports an approximately steady process but does not directly measure electrical power; heat loss also changes with temperature. Log voltage and current continuously and collect more frequent temperature readings with repeats.

Interpret a phase-change heating record

What a complete response should do

Temperature during constant-power heating

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