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Separation Techniques




I. Hand Separation

An example which could be separated by hand might be a dry mixture of salt and sand. Manually picking out the sand does not change the chemical identity of the salt or the sand.

Even though this technique is crude, it does show up in making an important discovery. It was the discovery of optical isomers by Louis Pasteur in 1844, the first major discovery of his scientific career. He was able to separate what had been thought to be one compound into two. He carefully crystallized the compound and, the key insight, saw that it crystallized into right-handed and left-handed crystals.

He able to pick out the opposite handed crystals and demonstrate that solutions of each rotated polarized light in opposite directions. It turns out, for a number of reasons, that Pasteur was very lucky. However, notice that he DID see that luck had presented something important to him and he was able to follow it through.

II. Filtration

Filtration is a bit more sophistication than manually picking out stuff. Mix the sand/salt with water. The salt dissolves, the sand does not. Pour through a filter to separate the sand, then heat the salt water to drive off the water. All physical changes.

Often, in chemistry, a reaction will be carried out and a solid material formed where there was none before. Filtration is the most common technique to remove the solid material.

Filters range widely in sophistication. Common ordinary filter paper (as might be used to make coffee) is inexpensive. The filter used in an oil filter for a car costs a bit more and so on. There are special application filters in various areas, both chemistry and other, where the filters are quite expensive.

Sometimes, the solid portions is what you want and sometimes it is the material you discard, keeping what passed through the filter. In 1943, Glenn Seaborg discovered element 94, later named plutonium. He and his co-workers carried out a series of steps, both chemical and physical, to purify the plutonium, one of which was a filtration. They kept the solid and discarded what passed through the filter.

Sometime later (2-3 years? I'd have to check), Seaborg discovered that elements 95 and 96 were also in the samples prepared. However, they were soluble, stayed in solution, passed through the filter and remained undiscovered due to the then single-minded focus on plutonium. (Plutonium was used to build the first atomic bomb and was also used in the bomb dropped on Nagasaki, three days after a uranium bomb was dropped on Hiroshima.)


III. Distillation-Imagine a solution of alcohol and water. Heat it. The lower boiling component (alcohol) will come off first, so you hold the temperature until all the alcohol is gone. You've separated the water and the alcohol with only physical changes involved. (The actual technique is a bit more sophisticated, but you get the idea.)


IV. Chromatography-Chromatography is a family of analytical chemistry techniques for the separation of mixtures. It involves passing the sample, a mixture which contains the analyte, in the "mobile phase", often in a stream of solvent, through the "stationary phase." The stationary phase retards the passage of the components of the sample. When components pass through the system at different rates they become separated in time, like runners in a marathon. Ideally, each component has a characteristic time of passage through the system. This is called it's "retention time."

A chromatograph takes a chemical mixture carried by liquid or gas and separates it into its component parts as a result of differential distributions of the solutes as they flow around or over a stationary liquid or solid phase. Various techniques for the separation of complex mixtures rely on the differential affinities of substances for a gas or liquid mobile medium and for a stationary adsorbing medium through which they pass; such as paper, gelatin, or magnesium silicate gel.




Chromatography #2

V. Centrifugation-In Centrifuges the centrifugal force is mechanically generated by turning the equipment containing the fluid in a circular path causing the fluids to separate. This method has been used in the laboratories and primitive industries for over a century. It has mainly been used to separate fluids in static state, i.e. ,specific volumes which needed to be separated. When the volume was large or it was in a dynamic state , i.e., flowing, centrifuges were not capable to deal with the situation. Relatively recently , however, new equipment were designed to deal with such conditions. Please refer to manufacturer’s sites for more information on these equipment


Inside the Centrifuge Movie

VI. Crystallization

The process of forming crystals from a solution.  Usually cooling a hot solution and reducing the solubility therefore producing a pure solid.


VII. There are More

There are a number of other separation techniques which exploit physical properties.


Matter Terminology    Classifying Matter  Phases of Matter  Physical and Chemical Changes  Separation Techniques  Vapor Pressure   Phase Changes  Heating Curve  Phase Diagrams

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