Properties of Liquids & Solids · Phase Changes

You’ll be able to

Connect IMF strength to boiling point, vapor pressure, viscosity, and surface tension
Classify phase changes as endothermic or exothermic and read a phase diagram
Explain how temperature and pressure determine the state of a substance

IMFs set the physical properties

The strength of a substance’s intermolecular forces controls its bulk behavior. Stronger IMFs mean a higher boiling and melting point (more energy to pull molecules apart), lower vapor pressure (fewer molecules escape into the gas phase), higher viscosity (molecules resist flowing past one another), and higher surface tension (the surface pulls tight like a skin). Water’s hydrogen bonding gives it an unusually high boiling point and surface tension for its small size.

Vapor pressure and boiling

Vapor pressure is the pressure of vapor sitting above a liquid once evaporation and condensation balance. It rises with temperature. A liquid boils when its vapor pressure equals the external (atmospheric) pressure. That is why water boils below 100 °C on a mountaintop — lower air pressure is easier for the vapor pressure to match. A volatile liquid has weak IMFs, high vapor pressure, and a low boiling point.

Phase changes and the phase diagram

Melting, vaporization, and sublimation add energy to overcome IMFs and are endothermic; freezing, condensation, and deposition release energy and are exothermic. A phase diagram plots pressure vs temperature, with lines marking where two phases coexist. The triple point is the single condition where solid, liquid, and gas all coexist; beyond the critical point the liquid and gas become indistinguishable (a supercritical fluid).

Boiling condition

liquid boils when: vapor pressure = external pressure

Lower the external pressure and the boiling point drops; raise it (a pressure cooker) and the boiling point climbs.

Worked example

Ethanol (bp 78 °C) and water (bp 100 °C) are both liquids at room temperature. Which is more volatile, and which has the higher vapor pressure at 25 °C?

1.Volatility tracks weak IMFs: water has extensive hydrogen bonding across two O–H bonds per molecule; ethanol hydrogen-bonds through only one O–H.
2.Weaker overall IMFs in ethanol mean molecules escape the surface more easily.
3.More escaping molecules → higher vapor pressure and a lower boiling point.
4.So ethanol is the more volatile liquid and has the higher vapor pressure at 25 °C, consistent with its lower boiling point.

Answer: Ethanol is more volatile and has the higher vapor pressure at 25 °C

Tip

A quick self-check: high boiling point, low vapor pressure, high viscosity, and high surface tension all point the same way — toward stronger IMFs. If your reasoning makes two of them disagree, re-examine the forces.

Checkpoint

Which phase change is endothermic (absorbs energy)?

Checkpoint

Two liquids are at the same temperature. Liquid X has a higher vapor pressure than liquid Y. Which statement is correct?

On the exam

Reading a phase diagram: moving right (heating) at constant pressure crosses from solid to liquid to gas; moving up (compressing) at constant temperature can push a gas into a liquid or solid. Locate the triple point and critical point first — they anchor the whole graph.

Answer the 2 checkpoints as you read.