Chemistry - matter and particles (states, mixtures, separation)
What the exam tests
Particle model, states of matter, pure substances and mixtures, and separation techniques. Expect questions on diffusion, changes of state and choosing a separation method.
Core rules
- Particles are always moving; higher temperature means greater average kinetic energy and faster diffusion.
- Solids have fixed shape and volume; liquids have fixed volume but flow; gases fill their container and are compressible.
- Melting and boiling are changes of state, not chemical reactions. During a change of state, temperature stays constant while energy breaks or forms attractions.
- A pure substance has a fixed melting and boiling point; an impure substance melts over a range and boils higher.
- Filtration separates an insoluble solid from a liquid. Evaporation or crystallisation recovers a dissolved solid. Distillation separates liquids with different boiling points or recovers the solvent. Chromatography separates dissolved coloured substances.
Worked example
To separate a mixture of sand and salt: add water and stir, so the salt dissolves. Filter: sand remains as residue, salt solution passes through as filtrate. Evaporate the filtrate gently; water leaves and salt crystals remain. If you need pure water instead, use distillation and collect the condensate.
Traps
- Filtering does not separate dissolved salt from water.
- Smoke and fog are tiny particles suspended in gas or air, not true solutions.
- Boiling point is a property of the substance, but it changes with pressure.
- Do not describe dissolving as melting.