A beaker of water is boiling steadily at 100°C at atmospheric pressure. You turn up the burner to add heat faster. What happens to the temperature of the boiling water?
AIt rises above 100°C because more energy is being added per second
BIt stays at 100°C because all added energy goes into the phase transition, not into increasing molecular speed
CIt drops slightly below 100°C because faster evaporation absorbs heat from the remaining liquid
DIt fluctuates between 95°C and 105°C due to increased turbulence from faster boiling
This is the defining feature of phase transitions: temperature remains constant during the transition because energy is absorbed to break intermolecular forces (enthalpy of vaporization) rather than to increase kinetic energy. The extra heat just makes water evaporate faster, not hotter. Temperature only rises again once all the liquid has converted to gas.
Question 2 Multiple Choice
For the same substance, the enthalpy of vaporization is always larger than the enthalpy of fusion. What is the best physical explanation?
AVaporization requires the substance to first melt, so it includes the fusion energy as a subset
BThe gas phase is hotter than the liquid phase and requires additional thermal energy to maintain that temperature
CVaporization must fully separate molecules from all remaining intermolecular contact, while melting only disrupts the fixed lattice while molecules remain in close proximity
DVaporization requires more energy because the gas phase has greater gravitational potential energy than the liquid phase
In melting, molecules gain enough energy to slide past their neighbors, but they remain in close contact — intermolecular forces still act. In vaporization, molecules must be fully separated from the bulk liquid so that intermolecular forces become negligible. This requires overcoming all remaining attractive interactions, not just the lattice structure, which demands far more energy.
Question 3 True / False
During a phase transition, the temperature of a substance remains constant because no energy is actually being transferred into or out of the substance.
TTrue
FFalse
Answer: False
Energy IS being transferred — it just isn't going into kinetic energy (which would raise temperature). Instead, it breaks intermolecular bonds (during melting or vaporization) or releases energy from forming bonds (during freezing or condensation). The constant temperature reflects that all energy input is consumed by the phase change itself, not that energy transfer has stopped.
Question 4 True / False
A substance with stronger intermolecular forces will generally have a higher boiling point than a similarly sized substance with weaker intermolecular forces.
TTrue
FFalse
Answer: True
Boiling point is determined by how much kinetic energy molecules need to escape the liquid phase. Stronger intermolecular forces (e.g., hydrogen bonds vs. London dispersion forces) hold molecules together more tightly, requiring higher temperature — and thus greater kinetic energy — to achieve separation. This is why water (strong hydrogen bonds) boils at 100°C while methane (only weak London forces) boils at −161°C, despite methane having a higher molecular mass.
Question 5 Short Answer
Why doesn't the temperature of water rise above 100°C while it is actively boiling at atmospheric pressure, even if you increase the heat input?
Think about your answer, then reveal below.
Model answer: During boiling, all added energy goes into breaking the remaining intermolecular forces holding water molecules in the liquid phase (the enthalpy of vaporization), rather than increasing the kinetic energy of the molecules. Temperature is a measure of average kinetic energy, so if energy goes into bond-breaking instead, temperature stays constant. Only once all the liquid has vaporized can additional energy raise the temperature of the steam.
This constant-temperature behavior is the experimental signature of a first-order phase transition. The enthalpy of vaporization for water (40.7 kJ/mol) must be supplied to every mole of water converted to steam. Until that energetic 'debt' is paid for all the liquid present, the temperature cannot rise. Increasing heat input just pays off that debt faster — the water boils away more quickly at the same 100°C.