Some manufacturers add iron filings to cereal to increase its iron content! The bits of iron will stick to a magnet, but the cereal won't. So you can easily separate the mixture by stirring a bar magnet through a slurry of water and finely crushed cereal.
the more similar the properties are, the more difficult it is to separate them
Many elements occur in forms with slightly different masses. For example, uranium occurs as uranium-235, which can be used to construct atomic bombs, and uranium-238, which can't. The two are very difficult to separate because they are nearly identical otherwise. The technical difficulties in separating this mixture is one of the factors that has limited the proliferation of nuclear weapons.
| technique | basis for separation | apply this technique to: |
| adsorption / desorption | phase transfer to a solid surface | liquid or gaseous mixtures that contain at least one component that adsorbs |
| chromatography | phase transfer from a mobile mixture to a stationary phase | liquid or gaseous solutions that contain several components with differing affinities for the stationary phase |
| condensation | phase separation by condensing gases in the mixture to liquids | gaseous mixtures containing at least one gas with a much higher boiling point than the others |
| dialysis | phase transfer through a porous membrane that allows some molecules to pass through, but not others | solutions containing small molecules mixed with very large molecules |
| effusion | gases with faster molecules flow through tiny pinholes faster than gases with slow molecules | gaseous mixtures containing gases with different molecular weights |
| dissolution (washing, solvent extraction) | soluble components can be washed away, leaving behind insoluble components (phase transfer to a washing solvent) | mixtures of solids with different solubilities |
| electrorefining | separate a metal from impurities by dissolving it and then plating it onto an electrode | solid mixtures with a metal as one component |
| filtration | collect solid particles on a filter | heterogeneous mixture containing a solid phase |
| floatation | dense components sink, and lighter ones float | heterogeneous mixture with phases with different densities |
| ion exchange | ions in the mixture bind to surfaces with oppositely charged sites (phase transfer to an ion exchange resin) | solutions containing ions |
| precipitation | convert solutes to an easily separated solid form | solutions containing a solute that can be precipitated |
| scrubbing | bubble mixture through a solution that selectively absorbs a component (phase transfer from gas to solution) | gaseous mixtures containing a solute that can be selectively absorbed by a scrubbing solution |
| stripping | a gas bubbled through the mixture carries off
the most volatile components
(phase transfer from solution to gas) |
a liquid mixture containing at least one volatile component |
| volatilization | components with widely differing volatility can be driven out of the mixture by heating (phase change from solid or liquid to gas) | a mixture containing components with differing volatility |
Blowing air through a straw in a glass of soda will cause it to go flat, because the air carries off the volatile carbon dioxide.
- distillation is collecting and condensing volatilized components
Alcohol can be separated from fermented corn mash by heating the mash to vaporize the alcohol. The vapor is collected and passed through coils of copper tubing, where it cools and condenses as a liquid once again. Moonshiners sometimes used old car radiators for the condensation step; the soldered joints added a toxic quantity of lead to the shine!
- drying is complete volatilization of some components in the mixture
Separation of water from clothes on a clothesline is one obvious example. The separation of salt from seawater using evaporating pools is another.
- sublimation is volatilization of a solid (without melting!)
Dry ice (solid carbon dioxide) is probably the most familiar example of a solid that sublimes. But water ice can also be converted directly into water vapor without melting, at low pressure. Snow on mountain peaks disappears without moistening the soil.
Separation by sublimation is sometimes called 'freeze drying'. Instant coffee is manufactured by freeze drying. (Boiling the coffee destroys the delicate molecules that give coffee its flavor, and so does exposure to air after a certain time, so distillation or simple drying isn't used). Fresh coffee is frozen to form a mixture of ice and coffee crystals. The pressure over the mixture is lowered so that the ice sublimates, leaving the coffee crystals behind.
Copyright © 1997-2005 by Fred Senese
Comments & questions to fsenese@frostburg.edu
Last Revised 02/23/18.URL: http://antoine.frostburg.edu/chem/senese/101/matter/print-separation.shtml