This how it works covers How and Why Substances Separate within Chromatography for GCSE Chemistry. Revise Chromatography in Chemical Analysis for GCSE Chemistry with 20 exam-style questions and 15 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. 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
20 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.