1.1.2Additional lessons
Separating Mixtures — IB Chemistry HL Revision Notes & Topical Questions
5 min read 4 parts Lesson 1 of 172
Part 1 of 4Methods Using Solubility and State
The right method depends on whether the components are solids, liquids or dissolved, and on how their properties differ.
Filtration
- Separates an insoluble solid from a liquid because the solid particles are too large to pass through the filter paper.
Evaporation
- Removes a liquid solvent by heating to leave behind a dissolved solid, such as salt from salt water.
Crystallisation
- A hot saturated solution is cooled slowly so that pure crystals form and impurities stay in solution.
Distillation
- Separates a solvent from a solute by boiling the solution and condensing the vapour in a condenser.
Part 2 of 4Liquid Mixtures and Chromatography
Liquid Mixtures and Chromatography
Fractional distillation
- Separates miscible liquids with different boiling points, because the one with the lower boiling point vaporises first.
Paper chromatography
- Separates substances by their different attraction to the paper and solubility in the moving solvent.
Rf value
- The distance moved by a spot divided by the distance moved by the solvent front, which has no units.
Choosing
- Match the method to the property that differs: size, solubility, boiling point or polarity.
| Mixture | Method | Property used |
|---|---|---|
| Sand and water | Filtration | Particle size |
| Salt and water | Evaporation or distillation | Boiling point |
| Ethanol and water | Fractional distillation | Boiling point |
| Inks | Paper chromatography | Solubility and attraction to paper |
Part 3 of 4Planning a Separation
A mixture of sand, salt and water shows how methods are combined.
1
Filter the mixture so the sand stays on the filter paper while salt solution passes through as the filtrate.
2
Wash the sand with a little distilled water to remove any remaining salt solution, then dry it.
3
Heat the filtrate to evaporate some water until the solution is saturated.
4
Allow the concentrated solution to cool slowly so that salt crystals form, then filter them off.
5
Dry the crystals between filter papers, giving separate samples of sand and salt.
Part 4 of 4Exam Checklist & Tips
1.1.2 – Key Points for the Exam
- Filtrate The liquid that passes through the filter paper.
- Residue The solid left on the filter paper.
- Miscible Describes liquids that mix completely to form one phase.
- Rf value Ratio of spot distance to solvent front distance in chromatography.
Common Mistakes
- Using filtration to separate a dissolved solid, as it passes straight through the paper.
- Marking the chromatography start line with ink instead of pencil.
- Using simple distillation for two liquids with similar boiling points.
- Heating a solution to dryness when pure crystals are wanted, which gives small impure crystals.
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