A liquid sits between the order of a solid and the chaos of a gas. Its particles touch but slide past each other, and the fastest ones at the surface keep escaping, which is where vapour pressure and boiling points come from.
In a liquid the particles are close together but disordered, moving randomly and sliding past each other while intermolecular forces still hold them. Melting overcomes only some of those forces, but vaporisation separates the particles completely, so the enthalpy change of vaporisation is much larger than that of fusion (water: 41 against 6.0 kJ mol−1). In a closed container, evaporation and condensation reach a dynamic equilibrium; the pressure of the vapour then is the vapour pressure. It rises with temperature, and a liquid boils when its vapour pressure equals the external pressure.
Fig. 1: vapour pressure against temperature for ethanol and water
Section 01
Three states, one model.
Heat the solid in the simulator and watch the particles. Note when the lattice breaks, when particles start leaving the surface, and how the temperature stays constant while the state changes.
Solid
Liquid
Gas
arrangement
regular, close
random, close
random, far apart
motion
vibrate about fixed positions
move around each other
fast, in straight lines between collisions
forces
strong
still significant
very weak
During melting and boiling the energy supplied overcomes intermolecular forces instead of raising the kinetic energy, so the temperature stays constant.
Section 02
Vapour pressure.
Seal a liquid in a container. At first molecules evaporate faster than vapour condenses; soon the rates are equal and the pressure of the vapour stops changing.
Dynamic equilibrium: rate of evaporation = rate of condensation; both continue.
Temperature: heating gives more molecules enough energy to escape, so the vapour pressure rises steeply.
Intermolecular forces: the weaker they are, the more easily molecules escape, so the higher the vapour pressure and the lower the boiling point. At 20 °C ethoxyethane is far more volatile than water.
Stage 1 · Learn
Check what the sim just showed you
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Which describes the particles in a liquid?
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Why is the enthalpy change of vaporisation of water much larger than its enthalpy change of fusion?
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The vapour pressure of a liquid in a closed container is measured when:
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A liquid boils when:
Section 03
When a liquid boils.
Boiling happens throughout the liquid, when bubbles of vapour can form because the vapour pressure matches the pressure pushing down on the surface.
At sea level water boils at 100 °C because its vapour pressure reaches 101 kPa there (Fig. 1).
At altitude the air pressure is lower, so water boils below 100 °C.
In a pressure cooker the pressure is higher, so water boils above 100 °C and food cooks faster.
Examiner trap
Evaporation happens at any temperature, from the surface only; boiling happens at one temperature for a given pressure, throughout the liquid. And when explaining vapour pressure, say the equilibrium is dynamic: both processes continue at equal rates, they do not stop.
Stage 2 · Exam
Exam-style questions
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Why does water boil at about 70 °C near the top of a very high mountain?
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Which liquid has the highest vapour pressure at 20 °C?
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Why does the vapour pressure of a liquid increase with temperature?
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During boiling, the temperature of pure water stays constant at 100 °C at 101 kPa because:
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The liquid state, mastered.
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