Separation & Purification Techniques
Section: 12. Experimental Techniques & Chemical Analysis | Syllabus: Cambridge IGCSE Chemistry (0620)
Choosing a Method
Which separation or purification technique is suitable depends on the properties of the substances in the mixture - whether they are soluble, their boiling points, and whether they are solids or liquids.
| Method | Property Used | Separates |
|---|---|---|
| Using a suitable solvent | Solubility | A wanted substance from a mixture, by dissolving it while leaving other substances undissolved |
| Filtration | Solubility (particle size) | An insoluble solid from a liquid |
| Crystallisation | Solubility | A soluble solid from a solution, as pure solid crystals |
| Simple distillation | Boiling point | A solvent from a solution, or liquids with very different boiling points |
| Fractional distillation | Boiling point | Liquids with similar boiling points |
Using a Suitable Solvent
Choosing the right solvent lets one substance in a mixture be dissolved and separated from the rest, which remain undissolved. For example, in a mixture of salt and sand, water is a suitable solvent because it dissolves the salt but not the sand - the sand can then be removed by filtration, and the salt recovered from the filtrate by crystallisation.
Filtration and Crystallisation
Filtration separates an insoluble solid from a liquid by passing the mixture through filter paper: the solid (the residue) is trapped, and the liquid (the filtrate) passes through.
Crystallisation is used to obtain a soluble solid from its solution as pure crystals:
- Heat the solution gently to evaporate some of the solvent, until crystals just begin to form at the surface
- Leave the solution to cool slowly so that crystals form as solubility decreases
- Filter off the crystals, wash with a little cold distilled water, and dry
Worked Example
- Question (based on a real exam question): a blue solid, Q, is soluble in water. Which method should be used to obtain pure solid Q from its aqueous solution?
- Answer: crystallisation - Q is soluble, so filtration (which only removes insoluble solids) will not work. Evaporating some of the solvent and cooling allows pure crystals of Q to form, which are then filtered off and dried.
Simple and Fractional Distillation
| Simple Distillation | Fractional Distillation | |
|---|---|---|
| Apparatus | Flask, thermometer, condenser | Same, plus a fractionating column between the flask and condenser |
| Best for | A solvent from a solution, or liquids with a large boiling-point difference | Liquids with boiling points close together |
| Example | Obtaining pure water from salt solution | Separating ethanol from water, or crude oil into fractions |
In both methods, the mixture is heated until the substance with the lower boiling point evaporates; the vapour then passes into a condenser, where it cools and condenses back into a liquid (the distillate), which is collected separately from the original mixture. A fractionating column gives repeated evaporation and condensation as vapour rises through it, so it can separate liquids whose boiling points are too close together for simple distillation to separate cleanly.
Assessing Purity Using Melting and Boiling Point
The melting point and boiling point of a substance can be used to identify it and to check how pure it is.
- A pure substance melts and boils at one exact, sharply-defined temperature, matching a known data-book value
- An impure substance melts over a range of temperatures below the pure melting point, and boils over a range of temperatures above the pure boiling point
- The presence of a dissolved impurity therefore lowers the melting point and raises the boiling point of a substance, and widens the temperature range over which melting or boiling occurs
Worked Example
- Question (based on a real exam question): one test-tube contains pure water; another contains water with some sodium chloride dissolved in it. Explain how measuring the boiling point of each sample would identify which is pure.
- Answer: pure water boils at exactly 100°C; the salt solution, being impure, boils above 100°C. The sample that boils at exactly 100°C is the pure water.
Common Mistakes
- ❌ Using filtration to try to obtain a soluble solid from solution
- ✓ filtration only removes insoluble solids - a soluble solid passes straight through the filter paper with the liquid, so crystallisation must be used instead
- ❌ Assuming an impure substance still has one single, sharp melting or boiling point
- ✓ impurities widen the melting/boiling range and shift it (melting point lower, boiling point higher) compared with the pure substance
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