You are heating 1 kg of water. You add heat steadily to raise it from 10°C to 100°C. You continue adding heat at the same rate once it reaches 100°C. What happens to the water's temperature while it is actively boiling?
AIt continues rising at the same rate, since the same amount of heat is being added
BIt rises more slowly because steam has a lower specific heat than liquid water
CIt stays at 100°C until all the water has vaporized, then resumes rising
DIt drops slightly as the water expands into steam
During a phase transition, all added energy goes into breaking intermolecular bonds (increasing potential energy), not into raising molecular kinetic energy. Since temperature measures average kinetic energy, it doesn't change while the phase change is occurring. The temperature is locked at 100°C until all the water has vaporized; only then does adding heat raise the temperature of the steam. This plateau — not a rise or fall — is the defining signature of latent heat.
Question 2 Multiple Choice
A cook is burned by 50 g of steam at 100°C, and a coworker is burned by 50 g of boiling water also at 100°C. Both burns cover the same skin area. Which burn is more severe?
AThe boiling water burn — liquid transfers heat to skin more efficiently than gas
BThey are equally severe, since both are at 100°C and have the same mass
CThe steam burn — steam is hotter than 100°C when it first leaves the pot
DThe steam burn — the steam must first condense on the skin, releasing ~113 kJ of latent heat before the resulting water even begins to cool
Steam and boiling water are at the same temperature, so intuition suggests equal burns. But when steam contacts skin, it first condenses into liquid water, releasing L_v ≈ 2260 kJ/kg — for 50 g, that's about 113 kJ deposited instantly, before any cooling begins. Boiling water starts cooling immediately without any phase-change energy release. The latent heat of condensation is a massive one-time energy dump that dwarfs the sensible heat both substances carry. This is why steam scalds are disproportionately dangerous.
Question 3 True / False
Adding heat to water that is actively boiling at 100°C will cause its temperature to rise above 100°C.
TTrue
FFalse
Answer: False
While water undergoes a phase transition, all added heat goes into breaking intermolecular bonds (latent heat of vaporization) rather than increasing molecular kinetic energy. Temperature measures average kinetic energy, so it cannot rise during the phase change. The temperature stays locked at 100°C (at standard pressure) until all the liquid has vaporized; only then does further heating raise the steam's temperature above 100°C. The temperature plateau, not a continued rise, is what defines latent heat.
Question 4 True / False
The temperature of a substance can remain constant while it absorbs heat, because the energy is being stored as potential energy in changed molecular arrangements rather than as kinetic energy.
TTrue
FFalse
Answer: True
This is the essential physics of latent heat. The temperature plateau during a phase transition is not a violation of energy conservation — the energy is still being absorbed, but it goes into potential energy (breaking bonds, separating molecules) rather than kinetic energy (faster molecular motion). Temperature only measures the kinetic part, so it doesn't change until the phase transition is complete. This is why latent heat was historically called 'hidden' — it didn't register on a thermometer, yet the energy was genuinely stored in the changed molecular configuration.
Question 5 Short Answer
Why does steam at 100°C cause more severe burns than boiling water at 100°C, given that both are at the same temperature?
Think about your answer, then reveal below.
Model answer: When steam contacts skin, it first undergoes a phase transition — condensing from gas to liquid — and releases the latent heat of vaporization (L_v ≈ 2260 kJ/kg) before any temperature change occurs. For a given mass, this condensation alone deposits far more energy than the sensible heat carried by the same mass of boiling water. Boiling water begins cooling immediately without any phase-change energy release. The steam therefore delivers much more total energy per kilogram, causing more severe tissue damage despite starting at the same temperature.
The key is recognizing that temperature alone does not capture the total thermal energy content of a substance. Steam at 100°C and water at 100°C carry very different energy because steam stores the latent heat of vaporization in its molecular configuration. When steam condenses, that energy is released as additional heat — on top of whatever sensible heat the resulting water then transfers. Understanding this asymmetry requires grasping that latent heat is real energy stored in phase, not merely a temperature reading.