Building Blocks: States of Matter — AQA Combined Science: Synergy
This topic uses the particle model to explain the three states of matter, density, gas pressure, changes of state and the meaning of purity.
The particle model
Matter is made of tiny particles. The particle model represents them as small spheres and explains the three states:
- Solid — particles in a regular, fixed arrangement, vibrating in place. Fixed shape and volume.
- Liquid — particles close together but able to move past each other. Fixed volume, takes the container's shape.
- Gas — particles far apart, moving quickly in random directions. Fills the container.
The stronger the forces between particles, the higher the melting and boiling points. A limitation of the simple model is that it ignores forces between particles and treats them as solid spheres.
Density
Density is mass per unit volume: $$\rho = \frac{m}{V}$$ Solids are usually densest (particles closely packed), gases least dense (particles spread out). Measured in kg/m³ or g/cm³.
Required practical: find the density of regular and irregular solids and of liquids — measure mass on a balance and volume by calculation or water displacement.
Gas pressure
Gas particles collide with the walls of their container, creating pressure.
- Increasing temperature makes particles move faster, hitting the walls harder and more often → higher pressure.
- Decreasing the volume means more frequent collisions → higher pressure.
Heating and changes of state
Changes of state (melting, freezing, boiling, condensing) are physical changes — mass is conserved and they are reversible, with no new substance formed.
When a substance changes state, energy is transferred but the temperature stays constant, because the energy breaks (or forms) the forces between particles rather than raising their kinetic energy. Stronger forces mean higher melting and boiling points.
Meanings of purity
In chemistry a pure substance is a single element or compound, not mixed with anything else.
- A pure substance melts and boils at a specific, sharp temperature.
- Impurities lower the melting point and broaden the range over which it melts.
Measuring melting point is therefore a test of purity.
Exam tips
- Link each state's properties to particle arrangement and forces.
- Learn ρ = m/V and convert units carefully.
- Explain why temperature is constant during a change of state.
- A pure substance has a sharp melting point; impurities broaden and lower it.
- Relate gas pressure to the frequency and force of particle collisions with the walls.