Spot On

Spot On: the chromatography required practical, hands on

This free game is part of our AQA GCSE Chemistry (8462) course for exams in EXAM YEARS 2027-2029.

Paper chromatography for AQA GCSE Chemistry

Paper chromatography is required practical 6 in AQA GCSE Chemistry (8462): you use it to separate coloured substances, tell them apart and calculate Rf values. The spec (4.8.1.3) says chromatography involves a stationary phase and a mobile phase, and that separation depends on the distribution of substances between the phases. In a textbook that is one sentence and a picture of a finished chromatogram.

Spot On lets you run the practical yourself, on the night shift at a forensic lab. Noor, the lead analyst, talks you through the first run. A "fibre cam" shows a model of the inside of the paper: the paper fibres (the stationary phase) stay still, the solvent (the mobile phase) flows up between them, and each dye molecule keeps switching between dissolved in the solvent (moving) and stuck to the paper (still). A dye that is dissolved 70% of the time keeps up with 70% of the solvent, which is why its Rf value is 0.70.

Every step of the method, and why it matters

You do the method in order, with a checklist on screen: eye protection on, pour about 1 cm of solvent, draw the start line 2 cm up, put a small, concentrated spot of each sample on its cross with a clean capillary tube, hang the paper so the bottom dips into the solvent, and put the lid on. Each step has a real consequence if you get it wrong. A pen start line runs up the paper because pen ink is a mixture of dyes, while pencil (graphite) doesn't dissolve. Pour too much solvent and the spots start under the solvent, so the dyes dissolve into the beaker. Big spots smear into each other, and without a lid the solvent front climbs unevenly.

The big moment is the run. Hold fast forward while the solvent soaks up the paper, then take the paper out just before the solvent reaches the top: the closer you get, the more points you score, but if the solvent reaches the top you can't measure how far it travelled and the run is ruined. Then you mark the solvent front in pencil straight away, before it dries and disappears.

Measuring Rf values

On the light box you line the ruler up so its 0 is on the start line, not the edge of the paper, then tap the solvent front and the centre of each spot. Your lab book fills in as you go: Rf = distance moved by substance ÷ distance moved by solvent. With the lesson's purple sweet colouring you find a pink dye with an Rf of about 0.70 and a blue dye with an Rf of about 0.25, so the colouring is a mixture. Rf values have no units and are always less than 1, and the game asks you to work one out from your own measurements.

The traps are the mistakes examiners see most: measuring from the bottom edge of the paper, measuring to the top of a spot instead of its centre, putting the division upside down, and forgetting to mark the solvent front.

Identify a dye and test for purity

In the food standards case you run a known orange dye next to sweets A, B and C on the same paper with the same solvent, then tap every spot at the same height as the dye (Rf 0.65): sweets A and C contain it and B doesn't. In the next case a supplier claims a green dye is pure. It gives one spot in water, but one spot in one solvent isn't proof. Run it again in ethanol and it splits into a yellow spot and a blue spot, so it is a mixture. A pure substance gives one spot in every solvent, and the Rf value of the check dye changes from 0.40 in water to 0.62 in ethanol, which is why you only compare Rf values from the same solvent.

Finally you inspect five trainees' chromatograms and spot the one mistake on each, then race through a 75-second rush, measuring unknown red dyes and matching their Rf values to a table. A game takes about 8 minutes, works on a computer, tablet or phone, and saves your best score.