3.7 Separation of Solutions and Mixtures Chromatography

Separating mixtures by physical properties

Mixtures can be separated using differences in physical properties like boiling point, particle size, solubility, and polarity. Distillation separates liquids by differences in boiling point (volatility). Filtration separates a solid from a liquid based on particle size. Recrystallization purifies a solid by exploiting differences in solubility at different temperatures.

Chromatography basics

Chromatography separates components of a mixture based on differing affinities for a stationary phase versus a mobile phase. Components that interact more strongly with the mobile phase travel farther/faster; those attracted to the stationary phase move slower. This is widely used to separate and identify components in complex mixtures like inks, pigments, or dissolved chemicals.

Interpreting chromatography results

In paper/thin-layer chromatography (TLC), distance traveled relates to polarity — with a polar stationary phase, less polar substances travel farther (they're less attracted to the stationary phase). The retardation factor (Rf) is calculated as distance traveled by the substance ÷ distance traveled by the solvent front, allowing identification of components by comparing to known standards.

Key terms

Distillation

Separation technique based on differences in boiling point (volatility) between liquids in a mixture.

Filtration

Separation technique that uses particle size differences to separate a solid from a liquid.

Recrystallization

Purification technique that uses solubility differences at different temperatures to isolate a pure solid.

Chromatography

Separation technique based on differing affinities of components for a stationary phase vs. a mobile phase.

Retardation factor (Rf)

Distance traveled by a substance divided by distance traveled by the solvent front; used to identify substances in chromatography.

Try an AP-style question

Which conclusion is best supported?

1. Dye A is less polar than dye B
2. Dye A has more mass
3. Dye A is more polar than dye B
4. The dyes are identical