SEPARATING MIXTURES
Note: click on the words in blue for more information (must have Internet access).
Magnetic Separation
Used to separate magnetic materials from non-magnetic materials e.g iron from sand.
A magnet pulls the magnetic material out from the non-magnetic material. This is typically
used in mining operations to separate metal from its ore.
Filtration
Used to separate insoluble solids from liquids in suspensions, e.g. chalk and water. In
suspensions, an insoluble solid is mixed with a liquid. The solid cannot dissolve in a liquid.
Therefore, when the mixture is passed through a filter, the solid remains in the filter while the
liquid passes through.
Filtration is typically used in water treatment plants to purify water to send to people’s houses.
Some people have water filters on their homes’ water supply to filter water before it gets to
their taps. Other people use portable water filters. Some fridges have built-in water filters as
well.
Water filter installed
on house water supply
Portable water filter
Evaporation
Used to separate a solution of a soluble solid in a solvent, e.g. saltwater. The solution is
heated. The solvent evaporates, leaving the solute.
Note that there are some solutions you can’t separate with evaporation. Have you ever
browned sugar for pelau? What happens when you heat sugar? It melts and burns. So if you
tried to separate sugar-water into sugar and water by evaporation, you’d destroy the sugar.
What to do? You use a special form of evaporation called crystallization. In crystallization, a
hot, saturated solution (refer to the handout on solutions and solubility) is allowed to cool. You
know that as a solvent gets cooler, it cannot hold as much solute. So by allowing the solution
to cool, the solute starts forming crystals at the bottom of the container.
But there is another problem with evaporation. In this separation technique, you just let the
solvent boil off. What if you want to collect both the solvent and the solute? You use the next
technique in our list:
Distillation
Used to separate soluble solids from liquids in solutions without losing the liquid. It can also
be used to separate soluble liquids dissolved into each other e.g. a water-ethanol solution.
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As with evaporation, by heating the solution, the liquid boils away as a vapor, leaving the
solid. But instead of just letting the solvent boil off as vapor, you trap it and use a condenser
to condense it back into a liquid, which you can collect. Or, in the case of a liquid solution, one
liquid boils off and is condensed, leaving the other liquid separate.
Imagine you are distilling a water-alcohol solution. This is the process:
1. The solution is heated.
2. Alcohol boils at 78℃ while water boils at 100℃. When the temperature reaches 78℃, the
alcohol boils out of the solution.
3. The ethanol vapor is trapped and sent though the condenser. Cold water circulates around
the condenser. This cools the alcohol so that it condenses back into a liquid state.
4. The liquid ethanol is collected, while the water remains in the heating flask.
If you’re confused, check the videos for this topic.
Distillation is used to
separate petroleum into
different fuels such as
gasoline, kerosine,
diesel and jet fuel. Each
one of these fuels boils
at a different
temperature. So as the
petroleum is heated,
each fuel boils off, is
condensed and
collected. This is called
refining of petroleum and
is done at a petroleum
refinery.
Reverse Osmosis
Used to separate impurities from water. The dirty water is forced through a semi-permeable
membrane under high pressure. This membrane allows certain molecules to pass through,
but blocks others. The membrane lets water molecules through but blocks molecules of salt,
dirt, poisons and other impurities, along with viruses, bacteria and other microorganisms.
And, finally...
Chromatography (pronounced CROW-mat-TOG-ruff-FEE)
Used to separate mixtures of gases and liquids that are attracted to an adsorbing medium
through which they pass. This may be a bit confusing, so check the topic videos. There are
four types of chromatography:
PAPER CHROMATOGRAPHY
Used to separate liquids. e.g dyes in
inks. This uses a special paper called
chromatography paper. The paper
soaks up the solvent. When the
solvent reaches the ink spots, the
dyes in the ink dissolve and start
moving up the paper. Each dye has a
different attraction to the paper. So the
different dyes move up the paper at
different rates. This separates the
dyes.
A similar process called thin-layer chromatography uses a thin layer of an adsorbent material
like silica gel, aluminum oxide, or cellulose on a flat, inert (non-reactive) substance like glass.
No, there’s no spelling mistake. Pay attention: this is ADSORPTION. This is ABSORPTION.
Absorption is something else entirely. Don’t confuse the two.
COLUMN CHROMATOGRAPHY
Used to separate liquids e.g. dyes in inks.
A long glass tube is packed with an such as aluminum oxide.
The ink is loaded at the top, and the solvent is poured into the tube. The solvent moves down
the tube and dissolves the ink. The dyes move down the tube at different rates because each
has a different rate of adsorption onto the aluminum oxide. So they are separated and can
each be collected at the bottom of the tube.
GAS CHROMATOGRAPHY
This is used to separate mixtures of gases/vapors. The mixture is passed through a device
called a gas chromatograph, which separates them.
SUMMARY
Magnetic separation is used to separate magnetic materials from non-magnetic materials e.g
iron from sand.
Filtration is used to separate insoluble solids from liquids in suspensions, e.g. chalk and
water.
Evaporation used to separate soluble solids from liquids in solutions, e.g. saltwater.
Crystallization is a form of evaporation used when the solute is destroyed by heating e.g
separating a sugar-water solution.
Distillation is used to separate soluble liquids dissolved into each other, or to separate soluble
solids from liquids in solutions without losing the liquid.
Reverse osmosis is used to separate impurities from water.
Chromatography is used to separate mixtures of gases and liquids that are attracted to an
adsorbing medium through which they pass. There are 4 types:
• Paper chromatography
• Thin-layer chromatography (similar to paper chromatography)
• Column chromatography
• Gas chromatography