---
title: "Paper Chromatography and the Rf Value"
description: "A mixture spotted on paper separates because each component divides differently between the paper and the moving solvent. The ratio of the distances travelled is a number that identifies a substance u"
canonical: https://lightmysky.com/learn/science/paper-chromatography-and-the-rf-value-mt_BAcCcz11ls
source: https://lightmysky.com/learn/science/paper-chromatography-and-the-rf-value-mt_BAcCcz11ls.md
retrieved: 2026-09-12
---

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# Paper Chromatography and the Rf Value

A mixture spotted on paper separates because each component divides differently between the paper and the moving solvent. The ratio of the distances travelled is a number that identifies a substance under fixed conditions, which is what makes the method analytical rather than merely pretty.

Subject: Science · Area: Matter & Materials · Ages 14 to 15
Page: https://lightmysky.com/learn/science/paper-chromatography-and-the-rf-value-mt_BAcCcz11ls

## Ready when they can

- Run a chromatogram with a baseline drawn in pencil above the solvent, and say why each of those details matters
- Calculate an Rf value from a chromatogram and state what must be held constant for it to mean anything
- Decide from a chromatogram whether a sample is a pure substance or a mixture

## Lesson: Split the ink, read the spots

Black ink looks single until paper pulls it apart into bands of blue, red and yellow. As solvent climbs, each dye splits its time between paper and liquid differently, so each climbs at its own speed. That split is what separates them.

**Example.** Draw the baseline in pencil, above the solvent level. Pen ink would dissolve and join the race, and spots dunked under the solvent wash off in one blob. Mark the solvent front before it dries, then circle each spot.

The Rf value names each spot: distance from baseline to spot centre divided by distance to the solvent front, a number between 0 and 1. One spot means a pure substance, and several mean a mixture. Identify unknowns by running them beside known samples on the same strip.

**Tip.** Rf numbers only compare under shared conditions: same paper, same solvent, same run. A new solvent retunes the race, so record your conditions like an analyst.

**Recap.** Pencil line, patient solvent, measured spots: one spot is pure, several are a mixture, and equal Rf values under equal conditions name the dye.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Pure Substances & Mixtures](https://lightmysky.com/learn/science/pure-substances-and-mixtures-mt_MBTVB-E-S7)
- [Choosing How to Separate Mixtures](https://lightmysky.com/learn/science/choosing-how-to-separate-mixtures-mt_U_tJvy3cbB)