You are heating 100 g of water that has just reached 100°C at standard pressure. You add 10,000 J of heat. What happens?
AThe temperature rises to about 124°C as the water absorbs sensible heat
BThe water remains at 100°C while approximately 4.4 g vaporizes; the remaining ~95.6 g stays as liquid
CAll 100 g instantly vaporizes because it is already at the boiling point
DTemperature rises and vaporization occur simultaneously in equal proportions
At the boiling point, added heat goes into latent heat of vaporization (L_v ≈ 2260 J/g for water), not into raising temperature. 10,000 J can vaporize about 10,000/2260 ≈ 4.4 g; the remaining liquid stays at 100°C. Temperature will not rise until all liquid is vaporized. Temperature is constant during a phase transition — this is the defining property of latent heat.
Question 2 Multiple Choice
Which requires more energy per gram: melting ice at 0°C, or vaporizing water at 100°C?
AMelting ice — overcoming a rigid crystal lattice requires the most energy
BThey require equal energy — both occur at fixed temperatures, so the energy input is the same
CVaporizing water — molecules must be completely separated from all neighbors, requiring far more energy than disrupting long-range lattice order
DMelting ice — because it starts at a lower temperature and must absorb more energy to change state
L_v ≈ 2260 J/g vs. L_f ≈ 334 J/g for water — vaporization requires about 6.8 times as much energy as melting. Melting only disrupts long-range lattice order; liquid molecules still attract one another at short range. Vaporization must overcome all short-range attractions, fully separating each molecule from all neighbors. This requires far more energy, even though both transitions occur at constant temperature.
Question 3 True / False
During a phase transition, adding heat to a system does not cause its temperature to rise.
TTrue
FFalse
Answer: True
This is the defining property of latent heat. During a phase change — melting, boiling, sublimation — added energy goes into breaking or forming intermolecular bonds and reorganizing matter, not into increasing molecular kinetic energy. Since temperature measures average kinetic energy, it remains constant until the entire sample has transitioned. Temperature resumes rising only after the phase change is complete.
Question 4 True / False
To calculate the total heat needed to convert 50 g of ice at −10°C to steam at 120°C, you can use a single equation Q = mcΔT with ΔT = 130°C.
TTrue
FFalse
Answer: False
This is the classic calorimetry error — treating a phase change as just another temperature step. The process requires five separate calculations: (1) warming ice from −10°C to 0°C; (2) melting ice at 0°C (q = mL_f); (3) warming water from 0°C to 100°C; (4) vaporizing water at 100°C (q = mL_v); (5) warming steam from 100°C to 120°C. No single q = mcΔT captures the full process, because latent heat operates at constant temperature with a different equation.
Question 5 Short Answer
Explain at the molecular level why temperature stays constant during a phase change, and give one practical example that demonstrates the importance of this effect.
Think about your answer, then reveal below.
Model answer: Temperature reflects average molecular kinetic energy. During a phase change, added energy goes into overcoming intermolecular attractive forces — breaking lattice bonds in melting, or fully separating molecules in vaporization — rather than increasing molecular speed. Until the structural reorganization is complete, no energy is available to increase kinetic energy, so temperature stays flat. Practically: sweating exploits water's large latent heat of vaporization (≈2260 J/g). Each gram of water evaporating from skin carries away 2260 J without any temperature rise — far more cooling per gram than any sensible-heat mechanism over a few degrees.
The plateau on a heating curve is a direct experimental signature of latent heat. If you plot temperature vs. time for a constant heat input, you will see a flat line at the melting point and another at the boiling point — the energy is being absorbed without any temperature change during both phase transitions.