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8.3Chapter 8: Pure Substances and Solutions

Chromatography — Alhussan Chemistry - Stage 8 Revision Notes & Topical Questions

5 min read 7 parts Lesson 3 of 10 3 quizzes

📚 Alhussan Chemistry - Stage 8 Hub / Topical Revision
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Lesson summary

8.3 Chromatography — Separating Dissolved Pigments and Chromatograms — Alhussan International Academy — Chemistry (Stages 7–9) notes: Describe how paper chromatography separates and identifies substances in a mixture and interpret a chromatogram.Chromatography = a method used to …

  • Start line above the solvent
  • Pure substance
  • Same substance
  • Green felt-tip ink
  • Uses of paper chromatography
  • Other types

Part 1 of 5What chromatography is

Chromatography
Chromatography a method used to separate and identify the substances in a mixture, and to show whether a substance is pure.
  • It works if all the substances in the mixture dissolve in one solvent.
Principle
  • Principle the solvent (mobile phase) moves through the paper and carries the substances at different speeds.
    • (stationary phase)
  • Substances that dissolve better in the solvent and stick less to the paper move further.
Chromatogram
Chromatogram the paper with the separated spots.

Part 2 of 5Method for paper chromatography

1
Draw a pencil line about 2 cm from the bottom of a strip of chromatography paper.
2
Place a small spot of each ink or dye on the line, keeping the spots small and well spaced, and include known reference substances if available.
3
Stand the paper in a beaker with a shallow layer of solvent such as water, so the solvent level is below the pencil line.
4
Cover the beaker to keep the air saturated with solvent vapour, and let the solvent rise up the paper by capillary action.
5
Remove the paper just before the solvent reaches the top, mark the solvent front with a pencil and let it dry.
1Pencil
  • Pencil pencil lead does not dissolve in the solvent, whereas ink would run.
2Start line above the solvent
  • Start line above the solvent if the line is under the solvent, the spots would dissolve into it.

Part 3 of 5Reading a chromatogram

Pure substance
  • Pure substance it gives one spot, whereas a mixture gives more than one.
Same substance
  • Same substance spots at the same height with the same colour are likely to be the same substance, so unknowns are compared with known substances.
Green felt-tip ink
  • Green felt-tip ink it is a mixture of two dyes, yellow and blue, and the blue dye travels further.
Sweets
  • Sweets a chromatogram of five sweets showed the brown dye is a mixture of red, yellow and blue dyes, while the red dye makes one spot and is pure.
Rf value
Rf value distance moved by the substance ÷ distance moved by the solvent.
  • Rf values are between 0 and 1 and are the same for a given substance in the same solvent and paper.
  • A spot 3.0 cm from the start line with the solvent front at 6.0 cm has Rf = 0.50.

Part 4 of 5Uses and limitations

Uses and limitations
Uses of paper chromatography
  • Uses of paper chromatography separating substances in inks, plants and sweets.
Other types
  • Other types they all separate mixtures and are used to identify substances, measure amounts and check purity.
  • Scientists and pharmaceutical companies use chromatography in making some COVID-19 vaccines.
  • Airport security officers use it to test clothes and luggage for explosives.
  • Scientists use it to identify food nutrients.
  • It is also used to analyse drugs in sport, identify pollutants in water and match inks in crime investigations.
Insoluble substances
  • Insoluble substances they will not move, so another solvent such as ethanol may be needed.
Fair testing
  • Fair testing use the same type of paper, the same solvent and similar spot sizes.
Modern methods
  • Modern methods gas chromatography and mass spectrometry use the same basic principle.

Part 5 of 5Chromatography enquiry: vitamin A in cassava

Problem
  • Problem in Africa and Asia up to half a million children go blind each year, and one cause is a lack of vitamin A.
Scientific question
  • Steve Adewusi (Nigeria) and Howard Bradbury (Australia) asked how much vitamin A is in the roots and leaves of different types of cassava.
Evidence
  • Evidence they used chromatography to measure vitamin A in yellow and white cassava roots and in dark green and light green leaves.
  • They also used secondary data on the vitamin A needed to be healthy from the World Health Organization (WHO),
    • which they trusted because WHO is well respected.
Conclusion
  • Conclusion dark green leaves have more vitamin A than light green leaves.
  • Eating yellow cassava root and cassava leaves can provide enough vitamin A, but white roots alone cannot.
Recommendation
  • Recommendation to prevent blindness, children should eat yellow cassava, not white, and also eat cassava leaves.

Key terms

TermMeaning
Chromatographya method of separating soluble substances using a solvent moving through paper.
Chromatogramthe paper with the separated spots.
Solvent frontthe furthest point reached by the solvent.
Rf valuedistance moved by a substance divided by distance moved by the solvent.
Mobile phasethe moving solvent.
Secondary datadata collected by other people.

Avoid these mistakes

1
Putting the start line under the solvent — the spots would dissolve into the solvent.
2
Using ink to draw the line — the ink would run; use pencil.
3
Measuring Rf to the end of the paper instead of the solvent front — measure to the solvent front.
4
Saying that substances that move furthest are heavier — they are more soluble in the solvent and stick less to the paper.

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