Why separate a mixture?
Many of the substances we need for everyday work lie in the earth's crust — metals, mineral oil, salts, sand, clay, coal, minerals, rock. They are rarely there in a pure form. They occur mixed with other substances, so the components we want have to be taken out of those mixtures.
- Removing grit and sand from rice
- Getting salt out of sea water
- Separating various minerals from mineral sands
- Separating the various fuels at a crude oil refinery
- Getting sugar out of cane juice
- Separating gases such as oxygen, nitrogen and argon from the atmosphere
- Obtaining distilled water from ordinary well water or river water
- Making drinking water from sea water
One idea behind all eight techniques
The eight methods in this chapter look very different from one another — one uses a magnet, one uses a still, one uses a strip of paper. But every one of them works the same way underneath. Each finds a physical property in which the components differ, and then exploits that difference.
To choose a separation technique, ask one question: in what physical property do these components differ?
The eight, and the property each uses
Read this table now and come back to it at the end of the skill. It is the whole of section 3.3 in one view.
| Technique | The physical property it exploits |
|---|---|
| Mechanical separation | Density, particle size, shape, magnetic and electrical properties |
| Evaporation | A large difference in boiling point between the components |
| Filtration | Particle size — one component passes the holes and the other does not |
| Crystallisation | Solubility, and how it changes as the solution is concentrated |
| Recrystallisation | The difference in solubility between the wanted solid and its impurities |
| Solvent extraction | A large difference in solubility between two solvents that do not mix |
| Distillation | Differences in boiling point and volatility |
| Chromatography | How strongly each component is attracted to the stationary phase |