Paper chromatography and formulations
Chemistry · GCSE · The School
Chromatography separates a mixture by how each part travels
A spot of the mixture is placed near the bottom of chromatography paper, which then sits in a shallow solvent. The solvent moves up the paper, carrying each substance in the mixture with it — but not at the same speed. A substance that clings more to the paper moves less; one that dissolves more in the solvent moves further. Different substances separate into different spots.
A pure substance gives one spot; a mixture gives several
If a sample produces only one spot at one position, whatever moved is a single pure substance, or at least behaves as one under this solvent. Several spots at several heights means several substances were present. This is the whole point of the technique: it tells you WHETHER something is a mixture, not just what colour it is.
Calculating an Rf value: a worked example
Rf = distance moved by the substance ÷ distance moved by the solvent. A dye spot travels 4.5 cm while the solvent front travels 9 cm. Rf = 4.5 ÷ 9 = 0.5. An Rf value is always between 0 and 1, and the SAME substance gives the same Rf value in the same solvent, which is how an unknown spot is identified — by matching its Rf against a known reference.
Comparing an unknown against known references
To identify an unknown dye, known reference dyes are run on the same paper, in the same solvent, at the same time. Any spot in the unknown that lines up at the same height as a reference spot — and gives the same Rf value — is probably that substance. A spot with no matching reference is something the test did not check for, not proof it is absent from anything.
A formulation is a mixture designed on purpose
Paint, medicine tablets, fuel and cleaning products are all formulations: mixtures where each ingredient is included in a measured amount for a specific purpose. A paint formulation might include a pigment for colour, a solvent to keep it liquid, and a binder to make it stick once the solvent evaporates. Removing or changing one ingredient’s amount changes how the whole product performs.
Formulation is not the same as an accidental mixture
A student answer that says "a formulation is just a mixture" misses the mark, because it misses the word "designed". A puddle of muddy rainwater is a mixture; nobody measured its ingredients for a purpose. A formulation always has a reason behind every ingredient’s presence and amount — that reasoning is what an exam question about formulations is actually asking for.