The kinetic molecular theory (KMT) explains the properties of solids, liquids and gases in terms of the behaviour of their particles. From this model you can understand melting, boiling, evaporation, and the shape of heating and cooling curves.
10.1 The Kinetic Molecular Theory
Postulates of the Kinetic Molecular Theory
- All matter is made of particles (atoms, ions or molecules) that are in constant motion.
- The particles of matter are attracted to each other by intermolecular forces.
- When temperature increases, the average kinetic energy of the particles increases.
- There are spaces between the particles of matter.
Solid vs Liquid
| Property | Solid | Liquid |
|---|---|---|
| Particle arrangement | Regular, fixed lattice — highly ordered | Random, close together — no fixed order |
| Particle motion | Vibrate about fixed positions only | Slide past each other — flow |
| Shape | Fixed shape — does not flow | Takes the shape of the container |
| Volume | Fixed volume | Fixed volume |
| Compressibility | Incompressible (particles already touching) | Incompressible (particles still close) |
In a gas, particles are far apart from each other — the spaces between particles are much larger than the particles themselves. Gas particles move rapidly in random directions, colliding with each other and with the walls of the container. Because of the large spaces between gas particles, gases are easily compressed (pressure reduces the spaces). Gases have no fixed shape or volume — they expand to fill any container. When temperature increases, gas particles move faster and collide more forcefully, increasing pressure (if volume is constant) or volume (if pressure is constant).
Practice Question
Use the kinetic molecular theory to explain why gases are compressible but solids are not.
(4 marks)
10.2 Phase Changes and Heating Curves
Definition
Melting point
The melting point is the temperature at which a pure substance changes from the solid state to the liquid state at standard pressure. The temperature remains constant during melting.
Definition
Boiling point
The boiling point is the temperature at which a pure substance changes from the liquid state to the gas state throughout the bulk of the liquid at standard pressure. The temperature remains constant during boiling.
Definition
Evaporation
Evaporation is the change of state from liquid to gas that occurs at the surface of a liquid, at temperatures below the boiling point. Only the fastest-moving (highest-energy) surface particles escape.
Definition
Condensation
Condensation is the change of state from gas (vapour) to liquid. Energy is released by the gas as the intermolecular forces pull particles back together.
Definition
Freezing
Freezing (solidification) is the change of state from liquid to solid. The liquid is cooled until its particles no longer have enough energy to slide past each other and they lock into a fixed lattice.
Definition
Sublimation
Sublimation is the change of state directly from solid to gas without passing through the liquid state. The reverse (gas directly to solid) is called deposition. Example: dry ice (solid CO₂) sublimes at −78°C.
During a phase change, the temperature of the substance remains CONSTANT even though heat is still being supplied. This is because the energy being added is used to break (or weaken) the intermolecular forces between particles — it increases the potential energy of the system, not the kinetic energy. Since temperature is a measure of average kinetic energy only, it does not change during a phase change. The energy absorbed during melting is called the latent heat of fusion; the energy absorbed during boiling is the latent heat of vaporisation.
Exam Tip
Exam tip: on a heating curve, a flat (horizontal) section always indicates a phase change. The lower flat section gives the melting point; the higher flat section gives the boiling point. The length of a flat section shows how much energy is needed for that phase change.
Worked Example
Water is heated from −20°C. Describe what happens at each flat section of the heating curve and state the temperature at which each flat section occurs.
Given
- Substance: water (H₂O)
- Starting temperature: −20°C
Find
Temperature and phase change at each flat section
Solution
- 1As water is heated from −20°C, the temperature rises (ice gains kinetic energy).
- 2First flat section at 0°C: melting — ice (solid) changes to water (liquid). Temperature stays at 0°C until all ice has melted.
- 3After melting, temperature rises again (liquid water gains kinetic energy).
- 4Second flat section at 100°C: boiling — water (liquid) changes to steam (gas). Temperature stays at 100°C until all water has boiled.
- 5After boiling, temperature rises again (steam gains kinetic energy).
Practice Question
A substance has a heating curve with flat sections at 30°C and 85°C. (a) What is the melting point of the substance? (b) What is the boiling point? (c) What is the state of the substance at 60°C?
(3 marks)