GCSE Chemistry Analysis revision

In everyday speech "pure" means natural; in chemistry it means one single element or compound — nothing else mixed in. The evidence is thermal: a pure substance melts (and boils) at a sharp, exact temperature, while impurities lower the melting point and smear it across a range. That is how regulators check pharmaceuticals and how you prove a colourless liquid really is water.

A formulation sits at the other extreme: a mixture designed for a purpose, with each component in a carefully measured amount. Fuels, alloys, fertilisers, paints, medicines and cleaning products are all formulations — the recipe is the product. Change the proportions and you change the properties, which is why formulations are made to precise formulas and batch-checked.

Paper chromatography separates dissolved substances. The stationary phase is the paper; the mobile phase is the solvent creeping up it. Each substance races between the two: it moves only when dissolved in the solvent, and pauses when stuck to the paper. Substances more attracted to the solvent travel further; those more attracted to the paper lag behind. Draw the baseline in pencil (ink would dissolve and chromatograph itself), keep the solvent below the baseline, and watch the spots split apart.

Distances become identities through the Rf value: Rf = distance moved by the substance ÷ distance moved by the solvent. Rf is always between 0 and 1 and has no units. A single spot means the substance is pure (or only one component has moved); multiple spots reveal a mixture. Comparing Rf values — ideally with reference dyes run on the same sheet — identifies each component.

The limewater result is chemistry you already know: CO₂ + Ca(OH)₂ → CaCO₃ (white, insoluble) + H₂O — the cloudiness is fine calcium carbonate. Safety note for the chlorine test: chlorine is toxic, so it is made in tiny quantities in a fume cupboard, and the litmus must be damp so the gas dissolves and bleaches it.

Flame tests identify metal ions by the colour they give a clean blue Bunsen flame (wire dipped in concentrated HCl first, then into the sample): lithium crimson, sodium yellow, potassium lilac, calcium orange-red, copper green. For mixtures or faint colours, chemistry turns to instruments (see below).

The anion tests complete the set. Carbonates: add dilute acid and the gas released turns limewater cloudy. Sulfates: add dilute hydrochloric acid, then barium chloride solution — a white precipitate of barium sulfate confirms SO₄²⁻. Halides: add dilute nitric acid, then silver nitrate — chloride white, bromide cream, iodide yellow precipitates. The acidification step removes carbonate or hydroxide ions that would otherwise give false precipitates.

Instrumental methods — flame emission spectroscopy (FES) is the GCSE example — identify metal ions from the light they emit in a hot flame. Each metal emits a characteristic pattern of wavelengths; a spectrometer measures them, and the intensity of light tells you how much of the ion is present. Samples are compared against a calibration curve built from known concentrations.

Why labs invest in instruments: they are fast (seconds per sample), sensitive (tiny traces detected), accurate (objective readings, no colour-judgement error) and they handle mixtures — FES can identify several metals in one sample where a manual flame test would be hopeless. The trade-off: instruments cost a great deal to buy and maintain, so simple tests still have their place in teaching labs and quick spot-checks.