GCSE Combined Science Analysis & chromatography revision

In everyday speech "pure" means natural or untouched. In chemistry it is precise: a pure substance is a single element or compound — one kind of particle only. Tap water, sea water, air and orange juice are all mixtures; only distilled water (H₂O molecules alone) is pure.

Purity has a fingerprint you can measure. A pure substance melts and boils at specific temperatures — ice melts at exactly 0 °C at standard pressure. Add an impurity and the particles no longer leave the lattice together: the solid melts over a range of temperatures, starting lower than the pure melting point. Data books list these values, so a quick melting-point measurement is a standard purity check in industry — a pharmaceutical batch that melts too early and too widely is rejected.

A formulation is the opposite ambition: a mixture designed on purpose, with every component present in a measured amount so the product does its job. Sunscreen balances UV filters, oils and emulsifiers; a fizzy drink sets sugar, acid, flavouring and colouring levels; fuels, alloys, fertilisers, medicines and cleaning products are all formulated. Change the recipe and you change the product — which is exactly why the amounts are controlled.

Paper chromatography separates the substances in a mixture using two phases. The stationary phase does not move — the paper. The mobile phase does — the solvent, which creeps up the paper. Each dissolved substance is tugged both ways: it dissolves into the moving solvent but also sticks to the paper. A dye that prefers the solvent races up; a dye that clings to the paper lags behind. Different preferences → different distances → the mixture falls apart into separate spots.

Required practical — analysing food colourings (method): draw a pencil baseline about 1 cm from the bottom of the paper (pencil is insoluble, so the line cannot climb with the solvent). Spot small drops of each known dye and the unknown colouring along the line, drying between touches. Stand the paper in a beaker with just enough solvent to wet the bottom — with the baseline above the liquid surface — and cover with a lid. Wait while the solvent climbs nearly to the top, then remove the paper and mark the solvent front immediately before it dries. Measure distances, calculate Rf values, and identify the unknown by matching its spots to the known dyes run on the same paper.

Typical results: a single pure dye gives one spot; a mixed colouring separates into two or more spots, each with its own Rf. An unknown dye is identified when one of its spots lines up (same height, same Rf) with a spot from a known dye on the same chromatogram. Spots that stay on the baseline never dissolved in the solvent at all — insoluble substances cannot be separated this way.

Every spot gets a number: Rf = distance moved by the substance ÷ distance moved by the solvent. Measure both distances from the baseline — to the centre of the spot and to the solvent front. Because the solvent always moves at least as far as any dye, Rf can never exceed 1, and a dye that never leaves the baseline has Rf = 0. No units — it is one length divided by another.

The number of spots tells its own story. One spot → the sample may be a single pure dye. Several spots → a mixture; the strongest (largest or darkest) spot usually belongs to the most abundant substance. Two samples with identical spots at identical Rf values, run side by side, contain the same dyes — this is how a lab compares suspect inks or checks a colouring is what the label says.

Four colourless gases dominate the tests, and each has a quick, unmistakable check. Hydrogen: hold a lighted splint at the tube mouth and hear a squeaky pop (the explosive burn of H₂ with air). Oxygen: push in a glowing splint — one that is smouldering, not lit — and it relights, because O₂ supports combustion so well. Carbon dioxide: bubble the gas through limewater, which turns cloudy as insoluble calcium carbonate forms. Chlorine: hold damp litmus paper in the gas and it is bleached white — the bleaching happens in solution, which is why the paper must be damp.

Safety matters here too: hydrogen tests use a flame near a flammable gas (small volumes only), and chlorine is toxic — the bleaching test is done in a fume cupboard or with tiny amounts, and never sniff gases directly.