Density and particle motion
Density definition
Density is mass per unit volume.
Equation: density = mass / volume, or rho = m / V.
Units: kg/m³ (SI) or g/cm³. The two are related by 1 g/cm³ = 1000 kg/m³.
Measuring density
- Regular solid: measure mass with a balance, calculate volume from dimensions (V = l × w × h for a cuboid).
- Irregular solid: use a displacement (eureka) can — the volume of water displaced equals the object's volume.
- Liquid: measure mass of liquid in a measuring cylinder (mass of cylinder + liquid minus mass of cylinder); read volume directly.
Densities and particle arrangement
| State | Particle arrangement | Typical density |
|---|---|---|
| Solid | Tightly packed, regular | High (e.g. iron 7900 kg/m³) |
| Liquid | Close together, no order, can flow | Slightly less than solid |
| Gas | Far apart, fast random motion | Very low (e.g. air 1.2 kg/m³) |
Most substances become less dense when they melt because particles spread slightly. Water is the famous exception — ice is less dense than liquid water.
Particle motion
- Solids vibrate about fixed positions.
- Liquids move past each other, vibrating and translating.
- Gases move rapidly in random directions, colliding with each other and the container.
Higher temperature means more kinetic energy and faster particle motion.
WJEC exam tip
Density questions almost always involve unit conversion. Convert cm to m (divide by 100) BEFORE calculating volume to avoid factor-of-million errors. Always quote the unit on your final density answer.
AI-generated · claude-opus-4-7 · v3-wjec-combined-science-leaves