Density and States of Matter
The particle model of matter
The particle model describes matter as being made of tiny particles. It explains the three states of matter and their properties by how the particles are arranged and how they move.
| State | Arrangement | Movement | Shape/Volume |
|---|---|---|---|
| Solid | Regular, tightly packed | Vibrate in fixed positions | Fixed shape and volume |
| Liquid | Close but random | Move around each other | Fixed volume, takes container's shape |
| Gas | Far apart, random | Move quickly in all directions | Fills the container (no fixed shape/volume) |
Density
Density is the mass per unit volume — how much matter is packed into a space.
density = mass ÷ volume (ρ = m ÷ V)- Units: kg/m³ (or g/cm³). Solids are usually densest (particles closest); gases least dense.
- Required practical: find density by measuring mass (balance) and volume (regular shape: measure and calculate; irregular: displacement of water in a measuring cylinder).
Changes of state
Changes of state are physical changes — the substance is the same, just rearranged, so they are reversible and mass is conserved.
- Melting (solid → liquid) and freezing (liquid → solid).
- Boiling/evaporating (liquid → gas) and condensing (gas → liquid).
- Sublimation (solid → gas directly).
Internal energy and heating
Heating a substance increases its internal energy (the energy stored in its particles). This can:
- Raise the temperature — related by specific heat capacity (energy to raise 1 kg by 1 °C).
change in thermal energy = mass × specific heat capacity × temperature change
(ΔE = m × c × Δθ)- Change the state — during melting/boiling the temperature stays constant while energy breaks bonds between particles. This energy is the specific latent heat.
Worked example
A block has a mass of 240 g and a volume of 30 cm³. Find its density.
density = mass ÷ volume = 240 ÷ 30 = 8 g/cm³✓
Common mistakes
- Rearranging density wrongly — density = mass ÷ volume (mass on top).
- Saying temperature rises during a change of state — it stays constant while the state changes.
- Forgetting changes of state conserve mass and are reversible.
Exam tips
- Learn the particle arrangement/movement for each state and link to properties.
- Practise the density formula and unit conversions (g/cm³ ↔ kg/m³).
- Explain flat sections on a heating graph as changes of state (energy breaking bonds, not raising temperature).
Key facts to remember
- Solid/liquid/gas differ in particle arrangement and movement; explains shape/volume.
- Density = mass ÷ volume (ρ = m/V); solids densest, gases least dense.
- Changes of state are physical, reversible, conserve mass; during a change of state the temperature stays constant (energy goes into breaking bonds — latent heat).