Chromatography
Chromatography is a method used to separate mixtures and help identify the substances in them. It is especially useful for analysing coloured substances like inks or food colourings.
What is chromatography?
- A technique used to separate and identify the components in a mixture
- In paper chromatography, we use:
- Stationary phase = the paper
- Mobile phase = the solvent (e.g. water or ethanol)
💡 Substances move at different speeds, depending on how strongly they are attracted to the paper and the solvent.
How to do paper chromatography
- Draw a pencil line near the bottom of the chromatography paper
- Add a small spot of ink or dye mixture onto the line
- Place the paper into a beaker with a small amount of solvent – below the line
- The solvent moves up the paper, carrying the dyes with it
- Different dyes move at different rates, creating a pattern of spots
Interpreting chromatography results
- Each spot represents a different substance
- A pure substance will produce one spot only
- A mixture will produce multiple spots
- If two samples produce spots at the same level, they may be the same substance
Rf values (not required for foundation tier)
Rf = distance moved by substance ÷ distance moved by solvent
This can be used to identify substances by comparing with known values.
Questions
- What is the mobile phase in paper chromatography?
- What does a pure substance look like on a chromatogram?
- Why must the baseline be drawn in pencil?
- What does each spot on the paper represent?
- What two things affect how far a substance travels?
Summary
- Chromatography separates mixtures using a mobile phase (solvent) and a stationary phase (paper)
- It is useful for identifying substances in mixtures
- A pure substance shows one spot; a mixture shows several
- Substances move at different rates based on their attraction to the paper and solvent
- Commonly used in forensics, food testing, and chemical analysis