Intermolecular forces, hydrogen bonding, evaporation, vapour pressure, boiling and the physical properties of liquids — exam-focused for the BIEK / Sindh Board paper. Read it straight through, or open the interactive lecture and play with the vapour-pressure simulator.
1 — The liquid state
In a liquid the molecules are close together (like a solid) but still free to move and slide past one another (like a gas) — an intermediate state.
General properties of liquids
- Definite volume but no fixed shape — a liquid takes the shape of its container.
- Fluid (flows) and can be poured.
- Much denser than gases; only slightly compressible.
- Diffuse slowly (molecules move, but are held by attractions).
- Evaporate, and exert a vapour pressure.
Liquids exist because of intermolecular forces strong enough to hold molecules together but weak enough to let them move.
2 — Intermolecular forces (van der Waals)
Weak attractions between molecules (much weaker than the covalent bonds within them). Collectively called van der Waals forces.
| Force | Acts between | Relative strength |
|---|
| Dipole–dipole | polar molecules | moderate |
| Dipole–induced dipole | polar + non-polar | weak |
| London (dispersion) | all molecules | weakest, but adds up |
| Hydrogen bonding | H–F, H–O, H–N | strongest of these |
3 — Dipole–dipole & London forces
- Dipole–dipole force — between polar molecules; the δ+ end of one attracts the δ− end of another. e.g. HCl.
- London (dispersion) force — a temporary, instantaneous dipole in one molecule induces a dipole in its neighbour. Present in all molecules; it grows with the number of electrons (molar mass / size).
That is why bigger non-polar molecules (e.g. I₂ vs F₂) have higher boiling points — stronger London forces.
4 — Hydrogen bonding
- Hydrogen bond — a strong dipole–dipole attraction between a hydrogen atom covalently bonded to a small, highly electronegative atom (F, O or N) and a lone pair on another such atom.
Consequences of hydrogen bonding
- Abnormally high boiling points of H₂O, HF and NH₃ compared with their group hydrides.
- Ice is less dense than water — an open hydrogen-bonded cage; so ice floats.
- High solubility of polar/H-bonding substances (sugar, alcohol) in water.
- Holds the two strands of DNA together.
5 — Evaporation
- Evaporation — the escape of higher-energy molecules from the surface of a liquid into the vapour phase, at any temperature below the boiling point.
Factors affecting the rate of evaporation
- Temperature — higher T → more molecules with enough energy to escape.
- Surface area — larger area → faster.
- Intermolecular forces — weaker forces → more volatile → faster.
- Air movement / lower humidity → faster.
Evaporation causes cooling: the fastest molecules leave, so the average energy (temperature) of those remaining falls — that is why sweating cools you.
6 — Vapour pressure
In a closed container, evaporation and condensation reach a dynamic equilibrium (rate of escape = rate of return).
- Vapour pressure — the pressure exerted by the vapour in equilibrium with its liquid at a given temperature.
It depends on
- Temperature — vapour pressure rises (steeply) with temperature.
- Nature of the liquid — weaker intermolecular forces → higher vapour pressure (more volatile).
Volatile liquids (ether, petrol) have high vapour pressure; non-volatile liquids (glycerine) have low vapour pressure.
7 — Boiling point
- Boiling point — the temperature at which the vapour pressure of the liquid equals the external (atmospheric) pressure; bubbles of vapour form throughout the liquid.
Effect of external pressure: in a pressure cooker the higher pressure raises the boiling point, so food cooks faster. At high altitude the lower pressure lowers the boiling point, so water boils below 100 °C and food cooks slowly. Normal boiling point of water = 100 °C at 1 atm.
8 — Surface tension
- Surface tension — the inward pulling force on the surface molecules of a liquid, which makes the surface behave like a stretched elastic skin and minimise its area.
Molecules inside are pulled equally in all directions, but surface molecules are pulled only inward and sideways — giving a net inward force.
Effects: droplets are spherical (smallest area), small insects walk on water, and water rises in a capillary tube. Surface tension decreases as temperature rises.
9 — Viscosity
- Viscosity — the internal resistance of a liquid to flow, caused by friction between layers of molecules.
Stronger intermolecular forces and larger molecules → higher viscosity (honey, glycerine). Viscosity decreases as temperature rises (molecules move faster and slip past each other).
10 — Capillary action
- Capillarity — the rise (or fall) of a liquid in a narrow tube, due to the balance of adhesion (liquid–tube) and cohesion (liquid–liquid).
Water rises in a glass capillary (adhesion > cohesion, concave meniscus); mercury falls (cohesion > adhesion, convex meniscus). Capillarity moves water up plant stems.
11 — Energy of phase changes
- Molar heat of vaporisation (ΔH_vap) — heat needed to convert 1 mole of liquid to vapour at its boiling point. Water: 40.7 kJ/mol.
- Molar heat of fusion (ΔH_fus) — heat needed to melt 1 mole of solid to liquid. Water: 6.0 kJ/mol.
During a phase change the temperature stays constant — the energy goes into breaking intermolecular forces, not raising temperature.
12 — Worked & reasoning questions
reasoning
Why does water have a much higher boiling point than H₂S, although both are group-16 hydrides?
Water forms strong hydrogen bonds (O is small & very electronegative); H₂S has only weak dipole–dipole forces → water boils far higher (100 °C vs −60 °C)
heat of vaporisation
Heat to vaporise 2 moles of water at 100 °C? (ΔH_vap = 40.7 kJ/mol)
Q = n × ΔH_vap = 2 × 40.7 = 81.4 kJ
reasoning
Why does food cook faster in a pressure cooker?
Higher internal pressure raises the boiling point above 100 °C → water gets hotter, cooking faster
13 — Exam recap
- Properties of liquids; role of intermolecular forces.
- van der Waals: dipole–dipole, London, and hydrogen bonding.
- Hydrogen bonding and its consequences (water, ice).
- Evaporation (factors, cooling); vapour pressure (dynamic equilibrium).
- Boiling point & the effect of external pressure.
- Surface tension, viscosity, capillarity; heats of vaporisation/fusion.