Properties of liquids

MDCAT Chemistry · Chemistry · Liquids. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

In a liquid the molecules touch but can slide past one another, so a liquid flows and diffuses but can hardly be compressed. The fastest molecules at the surface escape: this is evaporation. It happens at any temperature and it cools the liquid left behind.

In a closed container, evaporation and condensation reach a balance. The pressure of the vapour at that point is the vapour pressure. It depends only on the liquid and its temperature.

A liquid boils when its vapour pressure equals the pressure outside. A change of state takes in or gives out heat while the temperature stays constant. That heat changes the forces between the molecules, not their speed.

Rules to remember

Vapour pressure rises with temperature along a curve; it does not depend on the amount of liquid, the surface area or the size of the container
Boiling point: the temperature at which vapour pressure = external pressure (100 °C for water at 760 torr); lower pressure gives a lower boiling point
Stronger intermolecular forces: lower vapour pressure, higher boiling point, larger heat of vaporisation
Evaporation is faster at a higher temperature, with a larger surface area and with weaker intermolecular forces
heat for a change of state = moles × molar heat of fusion (or of vaporisation); melting, boiling and sublimation absorb heat; freezing and condensation release it
Molar heat of vaporisation is larger than molar heat of fusion for the same substance
Surface tension pulls a drop into a sphere; water rises in a glass capillary (adhesion > cohesion), mercury is pushed down (cohesion > adhesion)

Common mistake

Students think stronger heating makes boiling water hotter. At a fixed pressure the temperature stays at the boiling point; extra heat only makes the water boil away faster.

Worked example

The molar heat of vaporisation of water is 40.7 kJ mol⁻¹. How much heat is needed to change 9.0 g of water at 100 °C into steam at 100 °C? (H = 1, O = 16)

Molar mass of H₂O = 2 + 16 = 18 g mol⁻¹.
Moles of water = 9.0 ÷ 18 = 0.50 mol.
Heat = 0.50 × 40.7 = 20.35 kJ.

Answer: About 20.4 kJ is absorbed

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