Particle Model of Matter — AQA Combined Science: Trilogy
The particle model explains the properties of solids, liquids and gases, density, changes of state and internal energy.
Density
Density is the mass per unit volume of a substance:
$$\rho = \frac{m}{V}$$ (density in kg/m³ or g/cm³, mass in kg or g, volume in m³ or cm³)
The density depends on how closely packed the particles are:
- Solids are usually most dense (particles closely packed in a regular arrangement).
- Liquids are slightly less dense.
- Gases are least dense (particles far apart).
Required practical: determining the density of regular and irregular solids and of liquids — measuring mass with a balance and volume by calculation or by displacement of water (using a eureka can / measuring cylinder).
The particle model and states of matter
- Solid — particles in a fixed, regular arrangement, vibrating about fixed positions; strong forces between particles.
- Liquid — particles close together but free to move past each other; weaker forces.
- Gas — particles far apart, moving randomly at high speed; very weak forces.
Changes of state
Changes of state (melting, freezing, boiling, condensing, evaporating, sublimating) are physical changes — the substance keeps the same mass (mass is conserved) and can be reversed. They differ from chemical changes because no new substance is formed.
When a substance changes state, energy is transferred but the temperature stays constant during the change, because the energy is used to break (or form) the forces between particles, not to raise the temperature.
Internal energy
The internal energy of a system is the total kinetic and potential energy of all its particles.
- Heating a substance increases the internal energy, which either raises the temperature or causes a change of state.
Specific heat capacity
Energy needed to raise the temperature of 1 kg of a substance by 1 °C: $$\Delta E = m , c , \Delta\theta$$
Specific latent heat
The energy needed to change the state of 1 kg of a substance without changing its temperature: $$E = m \times L$$
- Latent heat of fusion — for melting/freezing.
- Latent heat of vaporisation — for boiling/condensing.
On a heating graph, the flat sections show changes of state (temperature constant while energy is added).
Particle motion in gases
The particles in a gas are in constant random motion. The temperature of a gas is related to the average kinetic energy of its particles — the hotter the gas, the faster the particles move on average.
Gas pressure
Gas particles collide with the walls of their container, exerting a force and therefore a pressure.
- Increasing the temperature (at constant volume) makes particles move faster and hit the walls harder and more often, so the pressure increases.
- Decreasing the volume (at constant temperature) means particles hit the walls more often, so the pressure increases. (At Higher/separate level: pressure × volume = constant — Boyle's law.)
Exam tips
- Learn the density equation and practise unit conversions (g/cm³ ↔ kg/m³).
- Explain the properties of each state using particle arrangement and forces.
- For heating graphs, explain why temperature is constant during a change of state (energy breaks forces between particles).
- Use ΔE = mcΔθ for temperature changes and E = mL for changes of state — don't mix them up.
- Link gas pressure and temperature to the speed and frequency of particle collisions with the walls.