Matter, Mixtures, and Separation Techniques
Classifying matter
Matter is anything with mass and volume. Pure substances are elements (one kind of atom) or compounds (two or more elements chemically bonded in fixed proportions). Mixtures contain two or more substances not chemically combined; they may be homogeneous (uniform, like salt water) or heterogeneous (visible parts, like sand and iron filings). Compounds can only be separated by chemical reactions; mixtures can be separated by physical methods.
Changes of state and particle theory
Solids have closely packed vibrating particles; liquids have particles that can slide past each other; gases have widely spaced fast-moving particles. Melting, freezing, evaporation, condensation, sublimation, and deposition are physical changes - no new substance forms. The particle model explains diffusion, expansion on heating, and why gases compress easily. A trap: mass is conserved during a change of state even though volume may change.
Separation techniques
Match the method to the property difference:
- Filtration - insoluble solid from liquid.
- Evaporation/crystallisation - soluble solid from solution.
- Simple distillation - solvent from solution (uses boiling point).
- Fractional distillation - miscible liquids with different boiling points (e.g. crude oil).
- Chromatography - separates dissolved coloured substances by solubility.
- Magnetic separation - iron from a mixture.
- Decanting - pouring off a liquid from settled solid.
Worked example
Separate a mixture of sand, salt, and iron filings. First use a magnet to remove the iron. Add water and stir to dissolve the salt. Filter to collect the sand as residue and salt solution as filtrate. Evaporate the filtrate to recover salt crystals. Order matters: removing iron first keeps it dry and clean.