Applies to: AQA, OCR Gateway, Edexcel (combined and separate), Edexcel International GCSE, and within Cambridge IGCSE's listed separation techniques.

Draw a dot with a black felt tip pen on some paper, then get that dot wet. It doesn't stay black. It splits into blues, purples, sometimes even a bit of yellow or pink. That's chromatography, and it's one of the easiest required practicals to do properly at home.

What it tests: Whether a substance is a pure single dye or a mixture of several, by how far each part travels through paper.

What you need: A strip of filter paper or a coffee filter cut into strips, a few washable felt tip pens (not permanent marker), a shallow cup, water, and a pencil.

How to do it: Draw a line in pencil about two centimetres from the bottom of your paper strip. Pencil matters here, ink from a pen would run and interfere with the results. Add a small dot of felt tip ink on the pencil line. Pour a shallow layer of water into the cup, no more than a centimetre deep, and stand the paper in it so the bottom touches the water but the ink dot stays above the waterline. Watch as the water travels up the paper and carries the ink with it, separating into different colours at different speeds.

Safety note: No real hazards here, just use washable pens rather than permanent marker to avoid staining surfaces.

Why it matters for the exam: Chromatography questions usually ask you to interpret a finished result: how many substances were in a mixture, based on how many spots appear, or to calculate an Rf value (the distance a spot travelled divided by the distance the water travelled). Having actually watched a black pen split into three or four colours makes that idea far easier to picture on the exam paper.

On motivation: If your first attempt gives a smudgy mess rather than neat separated bands, that's genuinely fine. The concept, that different substances travel through paper at different rates, is what matters, not whether your paper looks like a textbook diagram. Try a different pen colour and watch it again if the first one wasn't clear.