The macroscopic physical properties of matter, such as hardness, ability to flow, and compressibility, are all explained by the Kinetic Molecular Theory (KMT). The KMT asserts that all matter consists of discrete, microscopic particles in constant, randomised motion.
The Three States of Matter
Solids
- Particles vibrate in fixed positions within a regular lattice
- Strong intermolecular forces hold particles closely packed
- Fixed shape and fixed volume
Liquids
- Particles can slide past one another
- Weaker intermolecular forces than solids; particles close but not fixed
- No fixed shape, but fixed volume
Gases
- Particles move in constant, rapid, random motion
- Negligible intermolecular forces. Particles widely spaced
- No fixed shape and no fixed volume. Fills the container
Changes of State
Phase transitions occur when thermal energy is supplied or removed, providing the energy to overcome intermolecular forces or allowing those forces to dominate. You must be fluent in the correct scientific nomenclature:
| Phase Transition | From → To | Energy Transfer |
|---|---|---|
| Melting | Solid → Liquid | Endothermic. Energy absorbed to disrupt the lattice |
| Freezing | Liquid → Solid | Exothermic. Energy released as the lattice forms |
| Vaporisation | Liquid → Gas | Endothermic. Energy absorbed to overcome all intermolecular forces |
| Condensation | Gas → Liquid | Exothermic. Energy released as intermolecular forces engage |
| Sublimation | Solid → Gas | Endothermic. Direct transition, bypassing the liquid phase |
| Deposition | Gas → Solid | Exothermic. Direct transition, bypassing the liquid phase |
🔑 Key Concept. Energy During Phase Changes
During a change of state (e.g. Melting or boiling), the temperature remains constant even though energy is being supplied. All the energy goes into overcoming intermolecular forces, not increasing the kinetic energy of the particles. This is why heating curves show flat plateaus at melting and boiling points.
Evaporation vs Boiling
Both are forms of vaporisation, but the IB strictly differentiates between them. This is a common exam trap.
| Evaporation | Boiling | |
|---|---|---|
| Where? | Only at the surface of the liquid | Throughout the bulk of the liquid (bubbles form) |
| When? | At any temperature below the boiling point | Only at the boiling point |
| Rate | Slow, gradual process | Rapid process with continuous energy input |
| Mechanism | Particles at the surface with higher-than-average \(E_k\) escape | All particles have enough energy to overcome intermolecular forces |
⚠️ Examiner Tip: State Symbols
You must consistently use the correct state symbols in equations: (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous solutions. Examiners frequently assign marks specifically for correct state symbols. Remember: pure water is always (l), not (aq).