Pure substances and chromatography

Chemie GCSE Updated Wed 7 Oct 2026

In chemistry, a pure substance contains only one element or compound. Paper chromatography separates the substances in a mixture, such as the dyes in an ink, so you can tell how many there are and identify them.

Part 1 of 3: Learn it

In short

A pure substance melts and boils at one fixed temperature; a mixture melts over a range.
In chromatography the solvent (mobile phase) carries substances up the paper (stationary phase).
An Rf value is how far a spot travels from the start line, divided by how far the solvent front travels.

Where this is in your specification

Spec points: AQA 4.8.1.1 to 4.8.1.3 and its required practical (Combined Trilogy 5.8.1), Edexcel Topic 2, OCR C2.1

Board	Topic: Chemical analysis
AQA 8462	4.8 (8462); 5.8 (8464)
Edexcel 1CH0	Topics 2 and 9
OCR J248	C2.1 and C4.2
Eduqas C410	1, 11
Cambridge IGCSE 0620	12.3 to 12.5
National 5 C813 75	3(e) Chemical analysis

Pure substances and formulations

In everyday language "pure" means nothing added, such as pure orange juice. In chemistry it means a single substance. You can test purity by melting point: a pure substance has a sharp melting point, while impurities lower it and make it melt over a range.

A formulation is a mixture designed for a purpose, with each part in a measured amount. Paints, medicines, fertilisers and cleaning products are formulations.

The method

Draw a start line in pencil about 1.5 cm from the bottom of the paper.
Put small spots of each sample on the line.
Stand the paper in a beaker with solvent below the line, and cover it.
Take it out before the solvent reaches the top, mark the solvent front, and let it dry.

Why it works

Each substance shares its time between dissolving in the moving solvent and sticking to the paper. One that is more soluble in the solvent travels further. A pure substance gives a single spot in every solvent; a mixture can give several.

Rf values

Rf = spot distance ÷ solvent distance

Measure both distances from the start line. An Rf value is always between 0 and 1, has no units, and is the same for a given substance in the same solvent, so you can compare it with known substances to identify it.

Quick check

An ink gives three spots. What does that tell you?

Show the answer

It is a mixture of at least three substances.

Part 2 of 3: See it worked

Worked examples

Example 1

A dye moves 3.6 cm from the start line, and the solvent front moves 8.0 cm. Calculate the Rf value.

Rf = 3.6 ÷ 8.0

Answer: 0.45.

Example 2

A food colouring has Rf = 0.62 in water. The solvent front is 5.0 cm from the start line. How far has the colouring moved?

Spot distance = Rf × solvent distance
= 0.62 × 5.0

Answer: 3.1 cm.

Common mistakes

Drawing the start line in pen.
Putting the start line below the solvent level, so the spots wash into the solvent.
Measuring distances from the bottom of the paper instead of the start line.
Dividing the solvent distance by the spot distance, giving an Rf above 1.

Quick check

Why must the start line be above the level of the solvent?

Show the answer

Otherwise the samples would dissolve into the solvent in the beaker instead of moving up the paper.

Part 3 of 3: Test yourself

Check yourself

Answer each one in your head or on paper first, then open it to check.

An ink gives three spots. What does that tell you?

It is a mixture of at least three substances.

Why must the start line be above the level of the solvent?

Otherwise the samples would dissolve into the solvent in the beaker instead of moving up the paper.

A spot has moved 2.4 cm and the solvent 6.0 cm. Find Rf.

2.4 ÷ 6.0 = 0.4.

Jobs that use this

Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).

Diese Lernzettel sind auf Englisch, weil sie britischen Prüfungslehrplänen folgen.

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