Page 16 - Chemistry - XI

Characterisation and Purification
2 Characterisation and Purification
of Chemical Substances
of Chemical Substances

EXPERIMENT - 5
Objective
To obtain pure crystals of potash alum from its impure solution.

Principle/Theory
Crystallisation is a very common processes of purifi cation of compounds present in very impure conditions.
If the crude is in solution, it is well and good, otherwise a suitable solvent which dissolves the crude is
selected and a saturated solution of impure crude is prepared. Solid impurities are removed by fi ltering the
solution. Saturated solution is boiled and cooled slowly when the solute crystallises out leaving the greater
part of impurities in the solution. Crystals thus formed are collected by fi ltration. The process is repeated

until we get the crystals of desired purity. Sometimes a small crystal of pure substance is added into the
cooling solution just to start the process of crystallisation. This is known as seeding and the tiny crystal
which is added works as nucleus and new crystals grows near them. Purity of crystals is checked by colour
of the crystal. Impure crystals have diff erent colours.

Materials Required
Beaker (250 mL), glass funnel, tripod stand, china dish, glass rod, burner, sand bath, fi lter paper, and

impure potash alum

Procedure
1. Take 50 mL of distilled water in Glass Rod
a beaker and prepare a saturated
solution of potash alum at room
temperature by adding solid samples

in parts and stirring the solutions. Beaker
2. Stop adding solids when it does Fig. 5.2: Crystals pressed
not dissolve further. gently between
3. Filter the saturated solution so as pads of fi lter papers
to remove suspended impurities Residue

using fi lter paper.
4. Transfer the fi ltrate to a china dish Funnel
and heat it on a wire gauze till China Dish
20 mL of solution is left.
5. Keep it undisturbed for cooling till Filtrate
crystals of pure potash alum are
crystals of pure potash alum are
formed. Fig. 5.1: Removing Insoluble Impurities Fig. 5.3: Octahedral
formed.
by Filtration
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