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1.2Experimental Chemistry

Chromatography — Combined Science 0653 - Chemistry Revision Notes & Topical Questions

5 min read 13 parts Lesson 3 of 10 1 quiz

📚 Combined Science 0653 - Chemistry Hub / Topical Revision
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Lesson summary

1.2 Chromatography — COMBINED SCIENCE (CHEMISTRY) notes: Describe paper chromatography, interpret chromatograms and use Rf values.Chromatography — separates substances in a mixture and gives information to help identify them.Separation — components dissolve in the solvent to diff…

  • Pure substance
  • Black and white chromatograms
  • Locating agent
  • Rf equation
  • Properties
  • Identification

Part 1 of 6Principle of paper chromatography

Chromatography
  • Chromatography separates substances in a mixture and gives information to help identify them.
Separation
  • Separation components dissolve in the solvent to different extents and so travel at different rates.
  • More soluble substances travel further up the paper.
Movement
  • Movement the solvent rises up the paper by capillary action carrying the substances with it.

Part 2 of 6Carrying out paper chromatography

1
Draw a baseline across the chromatography paper in pencil.
2
Place small spots of the samples on the baseline.
3
Lower the paper into the solvent with the baseline above the solvent level.
4
Leave the solvent to rise up the paper.
5
Remove the paper before the solvent reaches the top, mark the solvent front and let it dry.
Pencil
  • Pencil is used because ink would run into the chromatogram.
Solvent
  • Solvent is usually water but can be ethanol.

Part 3 of 6Interpreting a chromatogram

1Pure substance
  • Pure substance gives only one spot.
2Impure substance or mixture
  • Impure substance or mixture gives more than one spot.
3Matching
  • Matching a spot level horizontally with a known spot shows that substance is present.
4Black and white chromatograms
  • Black and white chromatograms spots must be in line with one another to show a match.

Part 4 of 6Locating agents

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 coloured spot.
  • It is applied after the chromatography run.

Part 5 of 6Rf values

Rf equation
  • Rf = distance moved by substance ÷ distance moved by solvent.
    What the symbols mean: Rf has no unit and is always between 0 and 1 · both distances are measured from the baseline
Properties
  • Rf is a ratio, has no units and is always less than 1.
Identification
  • Identification an Rf value is compared with known values measured with the same solvent.
  • Rf changes if the solvent changes.

Part 6 of 6More from the specification

More from the specification
Ninhydrin visualization using locating agents
  • Ninhydrin visualization using locating agents spraying colourless amino acid chromatograms with ninhydrin locating agents produces visible purple spots to calculate Rf values.
Rf value calculation
  • Calculated as the distance travelled by the substance divided by the distance travelled by the solvent front.
Separation based on solubility and adsorption
  • Components separate in paper chromatography based on relative solubility in the mobile phase versus affinity for the stationary phase.
Fractional distillation principles
  • Separates mixtures of miscible liquids exploiting differing boiling points through repetitive vaporization and condensation cycles.
Crystallisation of soluble solids
  • Heating a solution to saturation point followed by slow cooling yields high-purity crystalline solids.
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.
Solubility
  • Mass of solute that dissolves in a given mass of solvent.
Filtration
  • Separating an insoluble solid from a liquid using filter paper.
Chloride
  • Salt of hydrochloric acid.

Key terms

TermMeaning
ChromatographyMethod of separating substances by their different solubilities in a solvent.
ChromatogramPaper showing the separated spots.
BaselinePencil line where samples are placed.
Solvent frontFurthest point reached by the solvent.
Locating agentReagent that makes colourless spots visible.
Rf valueDistance moved by substance ÷ distance moved by solvent.

Avoid these mistakes

1
Drawing the baseline in pen — the ink dissolves and runs.
2
Putting the baseline below the solvent level — the sample washes into the solvent.
3
Inverting the Rf equation — substance distance goes on top.
4
Giving Rf units or a value above 1.

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