States of Matter and Kinetic Theory
Core concepts
Kinetic theory explains macroscopic behaviour through particle motion and energy. Solids vibrate in fixed positions, liquids flow while retaining volume, and gases expand to fill their container. The gas laws (Boyle, Charles, Gay-Lussac, Avogadro) combine into the ideal gas equation pV = nRT.
Candidates must interpret phase diagrams and heating curves, recognising that temperature is constant during a phase change while energy is absorbed as latent heat. The strength of intermolecular forces determines melting and boiling points, vapour pressure, and volatility.
Deviations from ideal behaviour occur at high pressure and low temperature, where particle volume and intermolecular attractions become significant.
Traps to watch
- Using degrees Celsius in gas-law calculations - always convert to kelvin.
- Assuming temperature rises during boiling; it does not until all liquid has vaporised.
- Mixing up latent heat of fusion (solid–liquid) with vaporisation (liquid–gas).
- Forgetting that pressure and volume are inversely related at constant temperature.
- Confusing vapour pressure with atmospheric pressure.
Worked example
A gas occupies 2.0 dm³ at 300 K. At constant pressure, what volume at 450 K? V₂ = V₁ × T₂/T₁ = 2.0 × 450/300 = 3.0 dm³. Rule of thumb: identify which variables are held constant, then use the simplified law that applies.