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12.3Experimental Techniques and Chemical Analysis

Chromatography — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 8 parts Lesson 3 of 5 1 quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Lesson summary

12.3 Chromatography — IGCSE CHEMISTRY notes: Separate and identify substances using paper chromatography, locating agents and Rf values.Principle — the solvent moves up the paper by capillary action carrying the substances, which separate because their solubilities differSubstanc…

  • Pure substance
  • Known compound
  • Fixed value
  • Spot left on the baseline
  • Substance travelling furthest
  • Covering the beaker

Section 1 of 8Paper chromatography method

Principle
  • Principle the solvent moves up the paper by capillary action carrying the substances, which separate because their solubilities differ
  • Substances with higher solubility travel further.
Solvent
  • Solvent usually water, or ethanol for substances insoluble in water
1
Draw a start line in pencil, because ink would run into the chromatogram.
2
Place small spots of the sample, and known reference substances, on the line.
3
Lower the paper into the solvent with the line above the solvent level so the spots do not wash away.
4
Remove the paper when the solvent is near the top and mark the solvent front in pencil.
5
Dry the paper and compare the spots.

Section 2 of 8Interpreting chromatograms

Pure substance
  • Pure substance gives one spot
Mixture
  • Mixture separates into several spots
Matching
  • Matching spots in line horizontally with a reference spot are the same substance, e.g. brown ink containing red, yellow and blue
  • In black-and-white exam chromatograms the spots must line up.
Known compound
  • Known compound a reference spot is included to help identify unknown spots

Section 3 of 8Locating agents (Supplement)

Problem
  • Problem colourless substances such as amino acids and sugars cannot be seen on the paper
Locating agent
Locating agent a substance that reacts with the sample to give a visible coloured spot
Use
  • Use the chromatogram is treated with the agent after the run has finished

Section 4 of 8Retention factor, Rf (Supplement)

Rf
Rf distance moved by substance ÷ distance moved by solvent front
  • It is a ratio, has no units and is always less than 1.
Fixed value
  • Fixed value a compound always has the same Rf in the same solvent, but a different solvent changes it
Use
  • Use compare the calculated Rf with known values under the same conditions to identify substances
Measuring
  • Measuring use a ruler carefully from the start line to the centre of each spot and to the solvent front

Section 5 of 8Exam-style details

Exam-style details
Spot left on the baseline
  • Spot left on the baseline the substance is insoluble in that solvent
Substance travelling furthest
  • Substance travelling furthest is the most soluble and has the largest Rf
Covering the beaker
  • Covering the beaker a lid reduces evaporation of the solvent

Section 6 of 8More from the specification

More from the specification
Solution
  • A homogeneous liquid mixture formed when one or more solutes dissolve completely in a solvent.
Simple distillation
  • A separation method used to recover a pure liquid solvent from a solution containing dissolved solids by boiling and condensation.
Fractional distillation
  • A technique that separates miscible liquids with different boiling points using a fractionating column.
Melting and boiling points
  • Specific characteristic temperatures at which pure substances change phase;
    • impurities cause melting over a range and raise the boiling point.
Crystallisation
  • The formation of solid crystalline solute from a hot saturated liquid solution as it cools slowly.
Components
  • The individual constituent substances that make up a mixture, separated based on physical property variations.
Sulfate
  • Salt of sulfuric acid.
Fractional distillation
  • Separating a mixture of liquids by their boiling points.
Simple distillation
  • Separating a solvent from a solution by evaporating and condensing it.
Saturated solution
  • A solution that cannot dissolve any more solute at that temperature.
Distillation
  • Separating a liquid from a solution by boiling and condensing the vapour.
Boiling
  • Liquid to gas throughout the liquid at the boiling point.
Melting
  • Solid to liquid at the melting point, absorbing energy.
Crystallisation
  • Obtaining a dissolved solid from a solution by evaporating some of the solvent and cooling.
Liquid
  • A state with a fixed volume that flows, particles sliding past each other.
Filtration
  • Separating an insoluble solid from a liquid using filter paper.
Chloride
  • Salt of hydrochloric acid.

Section 7 of 8Key terms

TermMeaning
ChromatographyMethod separating substances by their different solubilities in a solvent.
Solvent frontThe furthest point the solvent reaches on the paper.
Locating agentChemical that makes colourless spots visible.
Rf valueDistance moved by substance ÷ distance moved by solvent front.
ChromatogramThe paper showing the separated spots.
Capillary actionMovement of solvent up the paper.

Section 8 of 8Exam technique

1
Drawing the baseline in pen — pencil does not dissolve in the solvent.
2
Letting the solvent cover the spots — the solvent level must be below the baseline.
3
Believing the biggest spot is the most soluble — the substance travelling furthest has the largest Rf.
4
Writing Rf with the solvent distance on top.

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