Discussion
A liquid of heat capacity 2 kilojoules per kilogram kelvin is heated from 20 to 80 degrees Celsius and then vaporised with a latent heat of 300 kilojoules per kilogram. What is the total enthalpy change per kilogram?
Answer: B
Correct answer: 420 kilojoules
Sensible heat first, then latent: \[ 2 \times (80-20) + 300 = 120 + 300 = 420\ \mathrm{kJ/kg}. \] The two contributions must be added, and the latent heat is the larger by a factor of two and a half - which is the usual pattern and the reason a phase change dominates any balance containing one. Omitting either term gives the 300 or the 120 offered.
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