States of matter, particle theory and separation techniques
The States of matter, particle theory and separation techniques domain tests how particle arrangement and motion explain properties, changes of state and methods of separating mixtures.
Particle model
- Solids: particles closely packed, regular arrangement, vibrate about fixed positions; fixed shape and volume.
- Liquids: particles close but irregular, move past each other; fixed volume, take shape of container.
- Gases: particles far apart, random fast motion; fill container, compressible.
- Heating increases kinetic energy. Melting and boiling occur at fixed temperatures for pure substances; impurities lower melting point and raise boiling point.
- Diffusion is net movement of particles from high to low concentration; faster at higher temperature and for lighter particles. Brownian motion is random motion of visible particles due to collisions with smaller particles.
Separation
Filtration separates insoluble solid from liquid. Evaporation removes solvent to leave dissolved solid; crystallisation gives larger purer crystals. Simple distillation separates solvent from solution; fractional distillation separates liquids with different boiling points. Chromatography separates dissolved coloured substances; Rf = distance moved by spot / distance moved by solvent front. A separating funnel separates immiscible liquids; magnetism removes magnetic solids.
Worked example
Separate sand, salt and water. Filter: sand remains as residue, salt solution passes as filtrate. Evaporate the filtrate to crystallisation point, then cool to form salt crystals. To identify a dye, measure Rf: spot moves 3.0 cm, solvent front 6.0 cm, so Rf = 0.50.
Traps
Saying particles expand on heating rather than move faster; Rf greater than 1; using evaporation when crystals are required; confusing distillation with filtration; adding units to Rf, which has none.