- Online Text
- 1. Introduction
- 2. What Is a Solution?
- 3. Solutions and Solubility
- 4. Solution Concentrations
- 5. Analyzing Solutions
- 6. Raoult's Law
- 7. Henry's Law
- 8. Colligative Properties—Vapor Pressure and Osmosis
- 9. Colligative Properties—Freezing and Boiling
- 10. Separation and Purification
- 11. Conclusion
- 12. Further Reading
- Unit Guide (PDF)
Section 10: Separation and Purification
Figure 8-16. Purification by Filtration
When making coffee, we use a filter to separate the coffee solution from the solid particles (the coffee grounds). When camping, we pump water from a stream through a filter to remove dirt particles and microorganisms like Giardia, which leaves us with clean drinking water as the purified product.
© Left: Science Media Group. Right: National Park Service.
Often, we want to remove a solute from a solution, either because the solute is desirable and we want to extract it and use it, or because the solute is undesirable and we want to purify the solvent by getting rid of it. There are a number of ways to do this.
Filtration is a familiar process for anyone who has made coffee. Ground coffee beans are mixed with hot water, and the molecules in the beans dissolve into solution. But the solid residue of grounds is still mixed in with the coffee. To remove it, we pour the coffee through a special piece of paper called a "coffee filter." The liquid and any dissolved molecules pass through the material of the paper, while the larger, solid particles are caught and stay inside the filter. In this way, the coffee is purified from the grounds. (Figure 8-16)
If the molecules of solute can be turned into solid crystals, filtration can be used to remove the solute from a solution. This method of analysis involves precipitating the solute out as a solid. For example, if we had a solution of silver nitrate, but we wanted to collect the silver, we could add sodium chloride to the solution. It would form insoluble silver chloride precipitate, and then we could filter the solution and collect the precious silver that is now separated from the solution it was originally in.
Figure 8-17. Chromatography
The pigments in leaves have been separated by paper chromatography. The pigments that have relatively more affinity for the solvent than the paper have migrated upward faster than those that have relatively more affinity for the paper than the solvent. The result is that the various molecules of different colors are spread out into the pattern seen here.
© Wikimedia Commons, Creative Commons License 2.5. Author: Dominikmatus, 9 January 2013.
Another method of separating and purifying compounds from solutions is called "extraction." Some solutes are more soluble in some solvents as compared to others. For example, if we were to make a strong cup of tea, there would be some caffeine, some flavors, and some brown colorant molecules that could be extracted from the tea leaves into the solution of tea in water. But it turns out that the caffeine is very soluble in organic solvents. If we shook together the tea and an organic solvent, the caffeine would move into the organic solvent and leave behind the flavors and colors in the water. Conveniently, organic solvents and water are immiscible, meaning they won't mix together. We can now say that we have extracted the caffeine from the tea using an organic solvent.
Chromatography is a way of separating the components out of mixtures or solutions. In chromatography, a solution is passed over or through a solid material. Different solute molecules will have differing degrees of molecular attraction for the solid material and so they will move though it at different speeds. The more attracted a molecule is to the solid material, the less quickly it will move. The different rates at which the solutes pass through the solid material are what allow them to separate. In chromatography, the solvent is called the "mobile phase," because it migrates through the solid material, and the unmoving material is called the "stationary phase." (Figure 8-17)