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Chemical AnalysisHow It Works

How and Why Substances Separate

Part of Chromatography · GCSE GCSE Chemistry revision

This how it works covers How and Why Substances Separate within Chromatography for GCSE Chemistry. Revise Chromatography in Chemical Analysis for GCSE Chemistry with 23 exam-style questions and 15 flashcards. This topic shows up very often in GCSE exams, so students should be able to explain it clearly, not just recognise the term. It is section 4 of 13 in this topic. Use this how it works to connect the idea to the wider topic before moving on to questions and flashcards.

Topic position

Section 4 of 13

Practice

23 questions

Recall

15 flashcards

⚙️ How and Why Substances Separate

Separation happens because different molecules have different relative attractions to the mobile phase and stationary phase. Think of it as a tug-of-war:

  • If a molecule is more attracted to (more soluble in) the solvent, it clings to the solvent and gets carried further up the paper
  • If a molecule is more attracted to (adsorbs more strongly onto) the paper fibres, it resists being carried and moves slowly

In a green food colouring containing yellow and blue dyes: the yellow dye may be more soluble in water (the mobile phase) than the blue dye. As the solvent rises, the yellow dye travels further up because it keeps dissolving into the moving solvent front. The blue dye, being less soluble, sticks more to the paper and lags behind. The two dyes separate into distinct spots.

This balance between solubility in the mobile phase and affinity for the stationary phase is what creates a unique, reproducible position for each substance — giving it a characteristic Rf value.

Keep building this topic

Read this section alongside the surrounding pages in Chromatography. That gives you the full topic sequence instead of a single isolated revision point.

Paper Chromatography MethodKey DefinitionsThe Science Behind Solving CrimesWhat is Chromatography?

Practice Questions for Chromatography

What is the purpose of chromatography?

  • A. To separate the components of a mixture
  • B. To measure the mass of a substance
  • C. To change a substance from a solid to a liquid
  • D. To make a substance more concentrated
1 markfoundation

Explain how a chromatogram can be used to determine whether a substance is pure or a mixture.

3 marksstandard

Quick Recall Flashcards

What does Rf stand for and what does it measure?
Rf = Retention factor. It measures how far a substance travels compared to the solvent (distance moved by substance ÷ distance moved by solvent)
What is chromatography?
A separation technique used to separate mixtures into their individual components based on different affinities for mobile and stationary phases

23 questions on Chromatography — practise free

Instant marking, adaptive difficulty, and 15 spaced repetition flashcards. Free until your GCSEs.

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