Changes of State and the Particle Model (P3.1)
The particle model
All matter is made of particles (atoms, molecules, ions). The arrangement and movement of these particles explains macroscopic properties.
| State | Arrangement | Movement | Properties |
|---|---|---|---|
| Solid | Close, regular, fixed | Vibrate in place | Fixed shape, fixed volume, incompressible |
| Liquid | Close, random | Slide past each other | Fixed volume, no fixed shape, incompressible |
| Gas | Far apart, random | Move rapidly in all directions | No fixed shape, no fixed volume, compressible |
Changes of state
| Change | Name | Energy change |
|---|---|---|
| Solid → Liquid | Melting | Energy absorbed (endothermic) |
| Liquid → Solid | Freezing/solidifying | Energy released (exothermic) |
| Liquid → Gas | Boiling/evaporation | Energy absorbed (endothermic) |
| Gas → Liquid | Condensation | Energy released (exothermic) |
| Solid → Gas | Sublimation | Energy absorbed |
| Gas → Solid | Deposition | Energy released |
During a change of state: temperature remains CONSTANT — all energy goes into breaking (or forming) intermolecular bonds, not into increasing kinetic energy. This is latent heat.
Density
density (ρ) = mass (m) / volume (V)
Units: kg/m³ or g/cm³. 1 g/cm³ = 1,000 kg/m³.
Worked example: A block of wood has mass 800 g and volume 1,000 cm³. ρ = 800/1000 = 0.8 g/cm³. Will it float? (water is 1.0 g/cm³) Yes — it is less dense.
Objects float if their density is less than the fluid they are in.
Measuring density
- Regular solid: measure dimensions with ruler; calculate volume (V = l×w×h); mass with balance.
- Irregular solid: mass with balance; immerse in water in measuring cylinder; volume = rise in water level.
- Liquid: pour known volume into measuring cylinder; mass with balance.
Gas density vs solid/liquid
Gases are much less dense than solids/liquids — particles are far apart. Gas density changes significantly with temperature and pressure.
Common exam errors
- Saying temperature rises continuously during melting — it stays constant at the melting point.
- Mixing up boiling and evaporation: boiling occurs at a fixed temperature throughout the liquid; evaporation happens at the surface at any temperature below the boiling point.
- Using the wrong units for density — always check that mass and volume units are compatible.
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