Capstone: Identify the Unknown T
The whole of this Triple-only run of tests builds to one skill: read a result and name the ion that made it. Each observation below is the unmistakable signature of exactly one species — drag or tap each one into the box for the ion it identifies.
Drag or tap each observation into the ion it identifies, then press Check.
- Pure substances & mixtures — a pure substance (single element or compound) melts and boils at one sharp temperature; a mixture melts/boils over a range, and impurities lower the melting point.
- Formulations — a mixture designed as a useful product, each component in a measured amount for a purpose (fuels, paints, alloys, medicines, fertilisers, foods).
- Chromatography & Rf — mobile phase (solvent) + stationary phase (paper); the more soluble a substance, the further it travels. Rf = distance moved by substance ÷ distance moved by solvent (no units, < 1). A pure substance gives one spot in every solvent. (Required practical 6)
- Gas tests — hydrogen: lit splint → squeaky pop; oxygen: glowing splint relights; carbon dioxide: limewater turns milky; chlorine: damp litmus bleached white.
- Flame tests T — lithium crimson, sodium yellow, potassium lilac, calcium orange-red, copper green; a strong colour can mask a weaker one.
- Cations with NaOH T — Cu2+ blue, Fe2+ green, Fe3+ brown; Al3+/Ca2+/Mg2+ white (only aluminium redissolves in excess). A 2+ ion takes two OH–, a 3+ ion takes three.
- Anion tests T — carbonate + dilute acid → CO2 (limewater milky); halide + nitric acid + silver nitrate → AgCl white / AgBr cream / AgI yellow; sulfate + hydrochloric acid + barium chloride → white BaSO4. (Required practical 7)
- Instrumental methods T — accurate, sensitive and rapid; flame emission spectroscopy gives a line spectrum that identifies metal ions (even several in a mixture at once) and measures their concentration.
That completes the analyst’s toolkit: melting point for purity, chromatography for the components of a mixture, a splint or a drop of limewater for a gas, and a flame or a precipitate for an ion — with instrumental methods waiting in the modern lab when speed, sensitivity and tiny samples matter. For Paper 2, pair these notes with C7 Organic Chemistry (crude oil is the classic mixture, separated by physical means) and carry the same gas-testing skills into C9 Chemistry of the Atmosphere (where carbon dioxide and the products of combustion take centre stage). The ionic equations and charge-balanced formulae behind every precipitate are set out in C4 Chemical Changes.