Phase Change, Latent Heat, and Internal Energy
At fixed transition temperature and pressure, latent heat satisfies Q = mL; the internal-energy change is ΔU = Q − PΔV for quasistatic pressure-volume work, so latent heat and internal-energy change need not be identical.
Why this shows up in the exam
Melting ice at atmospheric pressure · Vaporizing water into steam · Freezing-evaporation energy balances
Learn the idea
During a phase change, heat can alter molecular binding and volume without changing temperature. At the melting or boiling temperature, supplied energy rearranges matter rather than increasing average molecular kinetic energy.
🧠 Memory hook: Latent heat changes phase; subtract expansion work to get ΔU.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- Q = mL — heat absorbed for a complete phase change of mass m at the transition condition
- ΔU = mL - P(V_f-V_i) — internal-energy change at constant external pressure using work-by convention
How to approach it
- 1Identify the phase-change mass and latent heat
- 2Determine whether boundary work matters
- 3Apply the first law with the stated pressure and volume change
Common slip-ups that cost marks
- •Setting ΔU equal to zero because temperature is constant
- •Using sensible heat mcΔT during a pure phase change
- •Ignoring the large vaporization volume change
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A gas absorbs 500 J of heat and does 200 J of work. What is the change in its internal energy?
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