CRYSTALLIZATION
It is a process of forming crystals. It is
also a method for separating dissolved solids from a solution.
Two common techniques of Crystallization
are:
- By cooling down a hot concentrated
solution.
The solution has to be heated to get rid of some water in
order to obtain crystals from an unsaturated aqueous
solution. The solution becomes more concentrated as the
water boils away. The solvent cannot hold all the
dissolved solids when concentrated solution is cools and
is hot. The reason for this is because a hot solvent
dissolve more solutes than cold solvent. Then the extra
solids will be separated out as crystals.
We can check the solution is concentrated enough by
placing one drop of it on a microscopic slide by using a
glass rod. If the solution is concentrated enough,
crystals should form.
- Slow evaporation of solution at room
temperature.
Crystals can be obtained by evaporating a solution at
room temperature. After the solvent in the solution has
been evaporates, the remaining solution will becomes more
and more concentrated. Then it will becomes saturated.
Further evaporation makes the extra solids separate out
as crystals. It may take several days or maybe even weeks
for crystals to form because evaporation of a solution at
room temperature is a slow process. Note that the beaker
is covered with a piece of filter paper with holes on it
in the below diagram. The used of the filter paper is to
prevent dust and dirt from getting into the solution.
Otherwise the crystals formed might be very small.
Crystals formed by slow cooling or evaporation are large.
For those which formed quickly are usually small. It is
because solute particle need time to arrange themselves
in regular shapes in order to form crystals.After
crystallization, crystals can then be separated from the
solution by using filtration. Use cold distilled water to
wash the crystals two or three times after filtration.
Collect the crystals with a spatula and dry them by
pressing it gently between filter papers.
PURIFYING SOLID BY
CRYSTALLIZATION
Crystallization can be used to purify
solids as well. Assume a sample of cane sugar contains a small
amount of glucose as impurities. They are both soluble in water.
Pure cane sugar can be crystallized and removed from the
solution. In the solution, glucose will remain dissolved.