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Chemistry · Chapter 2: Separation and Purification
Paper Chromatography and Rf Values
Paper chromatography of inks and food dyes, stationary and mobile phases, and calculating Rf values to identify substances.
Key termRememberExampleWatch out
01Concept
Chromatography is a method of separating the components of a mixture by letting a solvent carry them at different speeds through a medium. The simplest kind is paper chromatography, used for coloured substances such as inks and food dyes.
The word comes from Greek chroma (colour) and graphein (to write).
How it works
- The stationary phase is the paper (actually the water held in its fibres). It does not move.
- The mobile phase is the solvent. It moves up the paper by capillary action and carries the dyes with it.
- A dye that is more soluble in the solvent and less attracted to the paper travels further. Different dyes travel different distances, so they separate.
Steps
- Draw a baseline in pencil about 2 cm from the bottom of the chromatography paper.
- Put a small, concentrated spot of each sample on the baseline.
- Stand the paper in a beaker with solvent. The solvent level must be below the baseline.
- Cover the beaker and wait until the solvent has risen near the top.
- Take out the paper, mark the solvent front in pencil at once, and let it dry.
Rf value
Each substance travels a fixed fraction of the solvent's distance, under the same conditions. This fraction is its Rf value (retention factor):
Rf = distance moved by the spot ÷ distance moved by the solvent front
Example: a red dye moves 4.5 cm while the solvent moves 6.0 cm. Rf = 4.5 ÷ 6.0 = 0.75.
- Rf is always between 0 and 1 and has no unit.
- Substances are identified by comparing their Rf values (or spots) with known substances run on the same paper.
- A pure substance gives one spot; a mixture gives two or more.
02Virtual Lab
Try this topic for yourself in the virtual lab. It opens on its own page with steps, tasks and questions.
03Key Formulas
Rf value
Rf has no unit and is always between 0 and 1.
04Real World Examples
Food safety
Food authorities use chromatography to check whether sweets, spices such as chilli powder and drinks contain banned artificial colours.
Forensics
Police forensic labs compare the ink on a cheque or letter with a suspect's pen using chromatography.
Medicines and doping
Pharmaceutical companies check medicines for impurities, and sports bodies test athletes for banned drugs, using advanced forms of chromatography.
05Practice Questions
Try to answer in your own words first, then open the model answer.
Short questions
Q1.What is chromatography?Show answer
Answer
Chromatography is a method of separating the components of a mixture as a solvent carries them at different speeds through a stationary medium such as paper.
Q2.What are the stationary and mobile phases in paper chromatography?Show answer
Answer
- Stationary phase: the paper (water held in its fibres).
- Mobile phase: the solvent that moves up the paper.
Q3.Why must the baseline be drawn in pencil and be above the solvent level?Show answer
Answer
- Pencil: graphite does not dissolve, so it will not run up the paper.
- Above the solvent: otherwise the spots would dissolve into the solvent in the beaker instead of moving up the paper.
Q4.A dye moves 3.6 cm and the solvent front moves 8.0 cm. Calculate the Rf value.Show answer
Answer
- Rf = distance moved by dye ÷ distance moved by solvent
- = 3.6 ÷ 8.0
- = 0.45
Q5.How does chromatography show whether a substance is pure?Show answer
Answer
A pure substance gives only one spot. An impure substance or a mixture gives two or more spots.
Long questions
Q6.Describe how paper chromatography can be used to find out which dyes are present in a black ink.Show answer
Answer
- Draw a pencil baseline 2 cm from the bottom of chromatography paper.
- Put a spot of black ink and spots of known dyes on the baseline.
- Stand the paper in a beaker with a little solvent, below the baseline, and cover it.
- Let the solvent rise; the black ink separates into several coloured spots.
- Mark the solvent front and dry the paper.
- Compare the spots from the ink with the known dyes (same height and colour), or calculate Rf values and compare them. Matching Rf values identify the dyes.