Matter and Its Nature
Matter and Its Nature opens the ASSEB HSLC General Science paper, and it tests whether you can classify a substance and choose a separation method from evidence rather than from memory. Expect one-mark classification items, method-selection items, and a short-answer item on the interconversion of states.
States and interconversion
Matter exists as solid, liquid and gas because of the balance between intermolecular attraction and thermal energy. Changes of state are physical: melting, vapourisation, condensation, freezing and sublimation. Sublimation - solid to vapour directly - occurs in camphor, naphthalene and ammonium chloride, which is why a mixture containing them is heated gently in a china dish covered by an inverted funnel.
Elements, compounds, mixtures
- Element: one kind of atom; cannot be split chemically (iron, oxygen, graphite).
- Compound: two or more elements chemically combined in a fixed ratio; properties differ from those of the constituents (water, common salt).
- Mixture: components not chemically combined; ratio variable; separated by physical methods (air, salt solution).
Solutions
A solution has a solute dissolved in a solvent; in salt water, water is the solvent. Solubility rises with temperature for most solids. A saturated solution holds the maximum solute at that temperature. A suspension has visible particles that settle; a colloid does not settle and scatters light (Tyndall effect).
Separation techniques
Filtration for an insoluble solid from a liquid; evaporation for a dissolved solid; crystallisation for purer crystals; distillation for a volatile solvent; fractional distillation for miscible liquids with different boiling points (petroleum, ethanol–water); a separating funnel for immiscible liquids; chromatography for pigments; magnetic separation for iron.
Worked example
Separate a mixture of iron filings, sand, common salt and ammonium chloride. Heat gently: ammonium chloride sublimes and is collected on the cool funnel. Remove the iron with a magnet. Add water and stir, then filter: sand stays on the filter paper while salt passes through as filtrate. Evaporate or crystallise the filtrate to recover salt.
Traps that cost marks
Salt water is a mixture, not a compound. When pure crystals are demanded, the answer is crystallisation, not plain evaporation. Do not call sublimation a form of melting. Condensation is gas to liquid, the reverse of vapourisation.