--- title: "Paper Chromatography and the Rf Value" description: "Separate a suitable mixture by paper chromatography and calculate Rf from distances measured from the baseline. Interpret separated spots and compare unknowns with known samples under matching conditi" 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: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Paper Chromatography and the Rf Value Separate a suitable mixture by paper chromatography and calculate Rf from distances measured from the baseline. Interpret separated spots and compare unknowns with known samples under matching conditions, without treating one spot or an equal Rf as proof of purity or identity. 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 - Describes a supervised paper-chromatography method using a pencil baseline above the solvent, and explains why both details matter - Calculates an Rf value from a chromatogram and states what must be held constant for it to be meaningful - Uses separated spots as evidence of a mixture and explains why one visible spot does not prove purity ## Lesson: Split the ink, read the spots You have used differences in properties to separate mixtures. Paper chromatography uses a moving solvent and a stationary phase held on the paper. The solvent is the mobile phase. Dissolved substances move with it, but interact differently with the solvent and stationary phase. They may travel different distances and separate. An ink can contain several dyes, but different inks need not give the same colours. **Example.** Work with a teacher and follow the solvent's safety instructions. Draw a pencil baseline, add small sample spots and let them dry to limit spreading. Put the paper's lower edge in solvent, keeping the spots above the liquid. Pen ink may dissolve and add unwanted spots. Submerged sample spots may dissolve into the container. Remove the paper before the front reaches the top. Mark the solvent front at once, while it is wet. *(drawing: The starting spots stay above the liquid in the container. Mark the solvent front before it dries.)* Rf = baseline-to-spot-centre distance / baseline-to-solvent-front distance. Use the same units for both distances; the ratio has no units. For a spot that travels 6 cm while the front travels 12 cm, Rf = 6 / 12 = 0.5. A valid value lies from 0 to 1: a spot cannot travel beyond the solvent front. A value above 1 suggests a measurement or calculation error. Two or more separated sample spots show a mixture of detectable components. One spot is consistent with a pure substance, but is not proof: different components may overlap, fail to dissolve or remain undetected. The number of visible spots need not equal the total number of substances. Compare Rf values using the same type of paper, solvent and temperature, with comparable development conditions. Running known and unknown samples side by side helps keep their conditions the same. Matching spot colours and Rf values supports a possible match, but different substances can share an Rf. A match does not uniquely identify a dye or pen. Changing the solvent can change how a substance divides between the moving solvent and stationary phase. The same unchanged dye can then have a different Rf. Solubility matters, but interactions with the stationary phase matter too. A solvent change does not guarantee a greater Rf or clearer separation. **Recap.** Use a pencil baseline above the solvent. Rf = spot distance / front distance, both from the baseline. Match conditions when comparing. One spot does not prove purity; equal Rf does not prove identity. *Lesson, questions and flashcards written for this dataset.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Choosing How to Separate Mixtures](https://lightmysky.com/learn/science/choosing-how-to-separate-mixtures-mt_U_tJvy3cbB) ## Optional connections - [Pure Substances & Mixtures](https://lightmysky.com/learn/science/pure-substances-and-mixtures-mt_MBTVB-E-S7)