Revision notes written to the specification, with examiner tips and the required practicals. Every point here has flashcards in the Stickwise app.
Purity & chromatography
In chemistry, pure means a single substance only, either one element or one compound. A pure substance melts and boils at a sharp, exact temperature, whereas a mixture melts over a range of temperatures. This is different from everyday language, where 'pure orange juice' is not chemically pure at all. A formulation is a mixture that has been designed as a useful product, with each component measured out for a particular job, such as a paint, medicine, fuel, alloy or fertiliser.
Chromatography
The mobile phase, the solvent, climbs up the stationary phase, the paper, carrying substances at different speeds. The more a substance dissolves in the solvent rather than sticking to the paper, the further it travels. A pure substance produces one spot, while a mixture separates into several spots.
Rf = distance moved by the spot ÷ distance moved by the solvent (always ≤ 1)
🧪 Required practical: In this required practical, draw the start line in pencil, since ink would run in the solvent. Spot the samples onto the line, then stand the paper in a shallow layer of solvent with the line above the solvent level, or the samples would dissolve away. Put the lid on, wait for the solvent to rise, and then mark the solvent front. Matching Rf values can then be used to identify a substance.
Gas tests
HydrogenA lit splint held near the gas produces a squeaky pop.
OxygenA glowing splint held in the gas relights.
Carbon dioxideBubbling the gas through limewater turns it milky (cloudy).
ChlorineDamp litmus paper is bleached white; this test should be done in a fume cupboard.
🎯 Examiner tip: Always give the full observation, such as 'the glowing splint relights', rather than just describing the method as 'use a splint'. The observation itself is what earns the mark.
Ion tests & spectroscopy (Triple) Separate science only
Metal ions — flame tests
Lithium Li⁺Lithium ions produce a crimson flame.
Sodium Na⁺Sodium ions produce a yellow flame.
Potassium K⁺Potassium ions produce a lilac flame.
Calcium Ca²⁺Calcium ions produce an orange-red flame.
Copper Cu²⁺Copper ions produce a green flame.
Metal ions — hydroxide precipitates
Adding sodium hydroxide solution produces a blue precipitate with copper(II) ions, a green precipitate with iron(II) ions, and a brown precipitate with iron(III) ions. Calcium, magnesium and aluminium ions all give a white precipitate, but only the aluminium precipitate redissolves when excess sodium hydroxide is added.
Non-metal ions
CarbonatesAdding dilute acid to a carbonate produces fizzing, and the gas released turns limewater milky, showing it is carbon dioxide.
HalidesAdding nitric acid and silver nitrate produces a precipitate whose colour identifies the halide: chloride gives a white precipitate, bromide gives a cream precipitate, and iodide gives a yellow precipitate.
SulfatesAdding hydrochloric acid and barium chloride to a sulfate produces a white precipitate.
In flame emission spectroscopy, light from the sample is split into a line spectrum: each metal ion produces its own characteristic pattern of lines, and the intensity of the lines shows the concentration. Instrumental methods like this are more sensitive than the eye, working with tiny samples, and they are more accurate and faster. They can also identify the ions in a mixture, where overlapping flame colours would otherwise mask each other.
🎯 Examiner tip: The choice of acid matters: use nitric acid for the silver nitrate halide test, and hydrochloric acid for the barium chloride sulfate test. Using the wrong acid produces a false precipitate, which is a common mistake examiners test for.
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