1.1.2Additional lessons
Separating Mixtures — IB Chemistry SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 1 of 98
Section 1 of 5Separating Solids and Liquids
Choose the method according to what is dissolved and what is not.
Filtration
- An insoluble solid is trapped by filter paper while the liquid and any dissolved substances pass through as the filtrate.
Evaporation
- Heating a solution drives off the solvent as vapour, which leaves the dissolved solid behind.
Crystallisation
- A hot saturated solution is cooled slowly so that crystals form, then filtered off and dried.
Decanting
- A solid that has settled at the bottom of a liquid is separated by carefully pouring off the liquid above it.
Section 2 of 5Separating Liquids and Dissolved Substances
Differences in boiling point or solubility are the basis of these methods.
Simple distillation
- Separates a solvent from a solution, or liquids with widely different boiling points, by evaporating then condensing the vapour.
Fractional distillation
- Separates miscible liquids with closer boiling points, using a column in which the more volatile liquid rises further.
Paper chromatography
- Different substances travel different distances because they spend different amounts of time on the stationary and the mobile phase.
Separating funnel
- Immiscible liquids settle into two layers, and the denser lower layer is drained off through the tap.
| Mixture | Method | Property used |
|---|---|---|
| Sand and water | Filtration | Particle size / solubility |
| Salt from seawater | Evaporation | Volatility |
| Ethanol and water | Fractional distillation | Boiling point |
| Oil and water | Separating funnel | Immiscibility and density |
| Coloured inks | Chromatography | Affinity for the phases |
Section 3 of 5Chromatography Results
The retardation factor, Rf, helps identify each substance on a chromatogram.
1
Draw a pencil baseline near the bottom of the paper and place a small spot of each sample on it.
2
Stand the paper in a solvent with the level below the baseline, and let the solvent rise until it nearly reaches the top.
3
Mark the solvent front immediately, then measure how far each spot has moved from the baseline.
4
Calculate Rf = distance moved by the spot ÷ distance moved by the solvent front, which is always between 0 and 1.
Section 4 of 5Worked Example
Question
- On a chromatogram the solvent front has moved 8.0 cm and a spot has moved 3.2 cm from the baseline.
- Calculate its Rf value.
- Rf = distance moved by spot ÷ distance moved by solvent front.
- Rf = 3.2 ÷ 8.0.
Answer Rf = 0.40
Section 5 of 5Exam Checklist & Tips
1.1.2 – Key Points for the Exam
- Filtrate The liquid that passes through the filter paper in filtration.
- Distillation Separation of liquids by evaporation followed by condensation, using differences in boiling point.
- Chromatography A technique that separates substances by their different distribution between a stationary phase and a mobile phase.
- Rf value The ratio of the distance moved by a spot to the distance moved by the solvent front.
Common Mistakes
- Using filtration to separate a dissolved solid from a solvent, as dissolved particles pass through filter paper.
- Drawing the chromatography baseline in ink, which can itself run with the solvent.
- Setting the solvent level above the baseline so the sample dissolves into the solvent.
- Quoting an Rf value greater than 1; the spot cannot move farther than the solvent front.
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