Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

2.2.11Additional lessons

Chromatography — IB Chemistry SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 32 of 98

📚 IB Chemistry SL Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read

Section 1 of 6Principle of Separation

2.2.11 Chromatography

Chromatography separates a mixture by differences in how strongly its components stick to a stationary phase or dissolve in a mobile phase.

Stationary phase
  • This is the solid or liquid that stays fixed, for example the paper or a thin layer of silica on a plate.
Mobile phase
  • This is the solvent that moves up the plate, carrying components with it.
Competition
  • Each component distributes between the phases; those more attracted to the mobile phase travel further.
Result
  • Components with different attractions separate into spots at different heights.

Section 2 of 6Running a Chromatogram

1
Draw a starting line in pencil, because pencil does not dissolve in the solvent and interfere with the results.
2
Place a small spot of each sample on the line and allow it to dry.
3
Stand the plate in a shallow layer of solvent so the starting line is above the liquid level.
4
Allow the solvent to rise until it nearly reaches the top, then mark the solvent front at once.
5
Dry the plate and, for colourless spots, reveal them with a locating agent or ultraviolet light.

Section 3 of 6Rf Values

Rf Values
Formula
  • Rf = distance moved by spot ÷ distance moved by solvent front, both measured from the starting line.
Range
  • Rf has no units and lies between 0 and 1.
Identification
  • A compound is identified when its Rf in the same solvent matches that of a known reference.
Conditions
  • The value changes with solvent and stationary phase, so comparisons must be under identical conditions.
MethodStationary phaseMobile phase
Paperwater held in cellulose fibressolvent
Thin-layersilica or alumina on a platesolvent

Section 4 of 6Interpreting Results

Pure substance
  • A pure compound should give a single spot in several different solvents.
Mixture
  • Two or more spots show at least two components.
Compare with references
  • Spots from unknown samples at the same height as a standard indicate the same substance.
Polarity effects
  • On a polar silica plate, polar compounds stick more strongly and move less far.

Section 5 of 6Worked Example

Question

  • A spot travels 3.6 cm while the solvent front travels 9.0 cm from the starting line.
  • Calculate the Rf value.

  1. Both distances are measured from the starting line to the centre of the spot and to the solvent front.
  2. Rf = 3.6 cm ÷ 9.0 cm = 0.40.

Answer Rf = 0.40

Section 6 of 6Exam Checklist & Tips

2.2.11 – Key Points for the Exam

  • Stationary phase The fixed phase in chromatography, such as paper or silica.
  • Mobile phase The solvent that moves through the stationary phase.
  • Rf value The ratio of the distance moved by a spot to the distance moved by the solvent front.
  • Locating agent A chemical sprayed on a chromatogram to make colourless spots visible.

Common Mistakes

  • Drawing the start line in ink so that it runs with the solvent.
  • Letting the starting line sit below the solvent level.
  • Measuring distances from the bottom of the plate instead of from the starting line.
  • Quoting an Rf above 1, which is impossible.

Last updated:

\n