How do particle arrangements and motion explain the properties of solids, liquids, and gases?
The particle model is a powerful simplification: matter consists of tiny particles separated by empty space, and the state of matter depends on the arrangement and motion of those particles. In a solid, particles vibrate about fixed positions; in a liquid, particles can flow but remain close; in a gas, particles move rapidly and are far apart. Changes of state involve energy — specifically latent heat — transferred to break or form inter-particle bonds without changing temperature.
Gas pressure arises from the force of particles colliding with container walls. Increase the temperature and particles move faster, hit the walls harder and more frequently — so pressure increases. Decrease the volume and the same number of particles are colliding with less wall area, also increasing pressure. This is the basis of Boyle's Law and the pressure-temperature relationship, both of which can be derived from the particle model.
200 g of water at 20 °C is heated to boiling point (100 °C), then completely vaporised. Calculate: (a) the energy needed to raise the temperature to 100 °C; (b) the additional energy needed to vaporise the water. (Specific heat capacity of water = 4200 J/kg°C; specific latent heat of vaporisation = 2.26 × 10⁶ J/kg.)
Key equations
Density: ρ = m/V recall
Specific heat capacity: ΔE = mcΔθ equation sheet
Specific latent heat: E = mL equation sheet
What students must understand
Particle arrangement in solids, liquids, gases — and how this explains properties
Density: ρ = m/V; units kg/m³; how particle spacing explains density differences between states
Internal energy: total kinetic and potential energy of all particles in a system
Heating increases internal energy (particles move faster); changes temperature or state, not both at once
Specific heat capacity (SHC): energy per kg per °C; ΔE = mcΔθ — equation sheet
Latent heat: energy for change of state at constant temperature (breaking/forming bonds)
Specific latent heat of fusion (solid↔liquid) and vaporisation (liquid↔gas): E = mL — equation sheet
Heating/cooling graph: flat sections at melting and boiling points show latent heat being absorbed/released
Gas pressure from particle collisions with walls; pressure increases with temperature and decreases with volume