The complete lecture — liquids come alive in the live panel as you read. Scroll down; the animation keeps pace, and you can drive the vapour-pressure simulator yourself.
1 — The liquid state
A liquid has a definite volume but no fixed shape: molecules are close together yet free to move, held by intermolecular forces.
2 — Intermolecular forces
| Force | Strength |
|---|
| London (dispersion) | weakest (all molecules) |
| Dipole–dipole | moderate (polar) |
| Hydrogen bonding | strongest of these |
3 — Dipole–dipole & London forces
- Dipole–dipole — δ+ of one attracts δ− of another (polar).
- London — temporary induced dipoles; grow with size/mass.
4 — Hydrogen bonding
- Hydrogen bond — strong attraction when H is bonded to F, O or N. Gives water its high boiling point and makes ice float.
5 — Evaporation
- Evaporation — escape of high-energy molecules from the surface, below boiling point. Faster with heat, area & weaker forces.
Evaporation cools — that's why sweating cools you.
6 — Vapour pressure
- Vapour pressure — pressure of the vapour in dynamic equilibrium with its liquid; rises steeply with temperature.
Drag the temperature slider in the live panel: watch escaping molecules and the vapour pressure climb — until it reaches 1 atm.
7 — Boiling point
Boilingboils when vapour pressure = external (atmospheric) pressure
Pressure cooker → higher b.p. (faster cooking); high altitude → lower b.p.
8 — Surface tension
- Surface tension — inward pull on surface molecules; minimises surface area. Decreases as T rises.
9 — Viscosity
- Viscosity — resistance to flow from friction between layers; higher for stronger forces / bigger molecules.
10 — Capillary action
- Capillarity — rise/fall in a narrow tube from adhesion vs cohesion. Moves water up plant stems.
11 — Energy of phase changes
HeatsΔH_vap(H₂O) = 40.7 kJ/mol · ΔH_fus(H₂O) = 6.0 kJ/mol
12 — Worked questions
heat of vaporisation
2 mol water at 100 °C: Q = 2 × 40.7 = 81.4 kJ
reasoning
Water boils far above H₂S because of strong hydrogen bonding.
13 — Exam recap
- Liquid properties & intermolecular forces.
- Dipole–dipole, London, hydrogen bonding.
- Evaporation (factors, cooling); vapour pressure.
- Boiling point & external pressure.
- Surface tension, viscosity, capillarity.
- Heats of vaporisation & fusion.