Matter
This domain tests whether you can explain the behaviour of solids, liquids and gases using the particle model and choose a sensible separation technique for a mixture. The exam expects precise terms: melting, boiling, evaporation, condensation, sublimation, diffusion and pure substance.
The particle model
- In a solid, particles are close together in a regular arrangement and vibrate about fixed positions. In a liquid they are close but can slide past each other. In a gas they are far apart, moving randomly and fast.
- Heating increases the kinetic energy of particles, so they move or vibrate faster and the spacing between them increases. The particles themselves do not expand.
- During a change of state the temperature stays constant even though energy is supplied. That energy overcomes forces between particles rather than raising average kinetic energy.
Pure substances and mixtures
A pure substance melts and boils at a fixed temperature. An impure substance melts over a range, usually lower than the pure melting point. Mixtures are separated using differences in physical properties: filtration removes an insoluble solid from a liquid; evaporation or crystallisation recovers a dissolved solid; simple or fractional distillation separates liquids with different boiling points; chromatography separates dissolved dyes; magnetism removes magnetic material.
Worked example
Ice at –10 °C is heated steadily until it becomes steam above 100 °C. Explain why the temperature stays at 0 °C and at 100 °C. At 0 °C the ice is melting. Energy breaks the forces holding particles in the solid lattice, so the temperature remains at the melting point until all the ice has melted. At 100 °C the water is boiling: energy separates the particles completely to form a gas. Only when each change of state is complete does the temperature rise again.
Traps that cost marks
- Saying particles expand when heated; they move faster and spread out, but the particles stay the same size.
- Confusing melting with dissolving.
- Calling a clear solution pure when it is a mixture.
- Forgetting that in an open container a gas escaping makes the mass appear to decrease, although mass is conserved.