The Memory Trick
💡 Temperature Freezes During a Phase Change
During any phase change — melting, freezing, vaporization, condensation, or sublimation — temperature stays completely constant, even while heat continues flowing in or out. All of that added (or removed) energy goes entirely into breaking or forming intermolecular bonds, not into raising or lowering the kinetic energy that temperature actually measures. The energy required is Q = mL, where L is the substance's latent heat for that specific phase change.
Why It Works
Temperature is a measure of average molecular kinetic energy — but during a phase change, added energy is instead reorganizing how tightly molecules are bonded to each other (their potential energy), not how fast they're moving. Until every bond has finished breaking (or forming), the kinetic energy — and therefore temperature — simply cannot change.
Step by Step
Working With Latent Heat
1
Latent heat of fusion — solid ↔ liquid
This is the energy required to melt a solid into a liquid (or the energy released when a liquid freezes into a solid), at a fixed melting/freezing temperature.
Ice at exactly 0°C absorbs energy while melting into water still at 0°C — the thermometer doesn't budge until every bit of ice has fully melted.
2
Latent heat of vaporization — liquid ↔ gas
This is the energy required to vaporize a liquid into a gas (or released when a gas condenses into a liquid), at a fixed boiling/condensation temperature — typically much larger than the latent heat of fusion for the same substance.
Water at exactly 100°C absorbs a large amount of energy while boiling into steam still at 100°C, before the temperature can rise any further.
3
Q = mL for calculating phase-change energy
Just like Q = mcΔT calculates heat for a temperature change, Q = mL calculates heat for a phase change — mass times the substance's specific latent heat for that particular transition.
Melting 1 kg of ice at 0°C requires Q = (1)(334,000 J/kg) = 334,000 J, using water's latent heat of fusion — none of this energy raises the temperature above 0°C until melting is complete.
🏥 Worked Example
How much energy is required to completely vaporize 0.5 kg of water already at its boiling point of 100°C? (Latent heat of vaporization for water, L_v = 2,260,000 J/kg)
1
Apply Q = mL: Q = mL_v.
2
Plug in values: Q = 0.5 × 2,260,000.
3
Solve: Q = 1,130,000 J = 1.13 MJ — and notably, the water's temperature remains at exactly 100°C throughout this entire process, only becoming steam once all of it has vaporized.
📌 Exam Application
Exams test correctly applying Q = mL for phase-change energy calculations (distinct from Q = mcΔT for temperature-change calculations), and explaining WHY temperature stays constant during a phase change.
⚠️ Most Common Phase Changes Mistakes
The most common trap is using Q = mcΔT (the temperature-change formula) during a phase change, or assuming temperature continues rising steadily while a substance melts or boils — temperature is completely flat during the phase change itself; only Q = mL applies until the transition is fully complete.
✓ Quick Self-Test
1) What happens to temperature during a phase change, even while heat continues being added or removed? It stays constant. 2) Where does the added energy go during a phase change, if not into raising temperature? Into breaking (or forming) intermolecular bonds — changing potential energy, not kinetic energy. 3) Write the formula for the energy required for a phase change. Q = mL, where L is the latent heat for that specific transition. 4) What is latent heat of fusion, specifically? The energy required to melt a solid into a liquid (or released when it freezes), at a fixed temperature. 5) Is the latent heat of vaporization for water typically larger or smaller than its latent heat of fusion? Larger — vaporization requires substantially more energy per kilogram than melting for most substances, including water.
→
← All Thermodynamics Lessons