Topic 3.9 Separation of Solutions and Mixtures (Chromatography): explain how chromatography, distillation and filtration separate the components of a mixture by exploiting differences in their interactions and properties.

What this topic is asking

The College Board (Topic 3.9) wants you to explain how common separation techniques, especially chromatography, but also distillation and filtration, separate a mixture by exploiting differences in the physical properties and intermolecular interactions of its components. The recurring idea is that components which interact differently with their surroundings move or change state at different rates, and so can be pulled apart.

Chromatography

In paper chromatography the stationary phase is the polar paper (cellulose) and the mobile phase is a solvent that travels up the paper. A component that binds tightly to the paper moves slowly and stays near the start; one that prefers the solvent moves quickly and ends up farther along. The distance each component travels relative to the solvent front is a fingerprint of its intermolecular interactions, and is how the components are identified and separated.

Distillation

Because boiling point is set by intermolecular force strength (Topic 3.1), distillation is really a separation by intermolecular forces. A mixture of ethanol (boils at $78\ ^\circ\text{C}$) and water (boils at $100\ ^\circ\text{C}$) can be partly separated this way, with the more volatile ethanol collected first.

Filtration and the unifying idea

Filtration is the simplest case: it separates an undissolved solid from a liquid by passing the mixture through a porous barrier that holds back particles larger than its pores. It separates by particle size, not by intermolecular forces.

The unifying theme of the topic is that separation always exploits a difference in a physical property between components. Chromatography exploits differing affinities for two phases, distillation exploits differing boiling points (intermolecular forces), and filtration exploits differing particle sizes. If two substances had identical properties they could not be separated by these physical means; the bigger the difference, the easier the separation.

Try this

Q1. State the property exploited by (a) distillation and (b) filtration. [2 points]

• Cue. (a) differences in boiling point (intermolecular forces); (b) differences in particle size.

Q2. In paper chromatography, explain why a component that interacts strongly with the paper travels only a short distance. [1 point]

• Cue. It is held on the stationary phase rather than carried by the mobile phase, so it moves slowly and a short way.