3.9 Separation

Chromatography

Develop a spinach extract by paper, thin-layer, or column chromatography. Swap the stationary and mobile phases and watch the elution order reverse, then read relative polarities off the chromatogram.

Paper, TLC, ColumnMobile vs StationaryRelative PolarityRf
Topic 3.9

Separation of Solutions and Mixtures

Explain the results of a separation experiment based on intermolecular interactions.

The components of a liquid solution cannot be separated by filtration — dissolved particles pass straight through the filter with the solvent. Filtration only separates a solid suspended in a liquid (a heterogeneous mixture).

Solutions are separated by processes that exploit differences in intermolecular interactions:

Chromatography (paper, thin-layer, and column) separates species by the differential strength of intermolecular interactions between the components of the mobile phase (the moving solvent) and the stationary phase (the paper, silica plate, or column packing). A component that interacts strongly with the stationary phase is held back; one that interacts more strongly with the mobile phase travels far.

The resulting chromatogram lets you infer relative polarities of the components. With a polar stationary phase (paper or silica, rich in –OH groups) and a less polar mobile phase, polar components stick and move little, while nonpolar components move far.

Rf = (distance travelled by the spot) ÷ (distance travelled by the solvent front)

Rf is always between 0 and 1. A larger Rf means weaker attraction to the stationary phase.

Distillation separates by boiling point, which is itself governed by IMF strength: the component with the weaker intermolecular forces has the higher vapor pressure, boils first, and is collected first.

Key points

  • Filtration cannot separate a solution — only a suspension.
  • Chromatography is a competition between attraction to the stationary phase and to the mobile phase.
  • Larger Rf = weaker attraction to the stationary phase = travelled farther.
  • Distillation separates by boiling point, which reflects IMF strength.

Equations

  • not on the sheet
    • distance from origin to the center of the spot
    • distance from origin to the solvent front

Common mistakes

  • Measure from the origin line, not from the bottom edge of the paper.
  • Never start with the origin line below the solvent level — the sample would dissolve straight into the reservoir.
  • A high Rf does not mean "more polar". It depends on which phase is polar; state the phases in your reasoning.
  • Filtration is not a solution-separation technique.

Worked example

On a silica TLC plate (polar stationary phase) developed with hexane (nonpolar mobile phase), compound A has Rf = 0.75 and compound B has Rf = 0.20. Which compound is more polar? Explain.

Compound B is more polar.

The silica stationary phase is polar, rich in Si–OH groups capable of dipole–dipole and hydrogen-bonding interactions. The hexane mobile phase is nonpolar and interacts only by dispersion forces.

Compound B travelled only 20% as far as the solvent front, meaning it spent most of its time adsorbed on the plate rather than moving with the solvent. That requires strong attraction to the polar stationary phase, which a polar compound provides.

Compound A travelled 75% of the way, so it interacts preferentially with the nonpolar mobile phase — the behavior of a less polar compound. ("Like dissolves like" applies to both phases at once.)

Full notes for topic 3.9 →