Description of experiment
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Needed compounds:
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perchloric acid : HClO4
potassium chloride : KCl
Class:
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elem=Cl,K
precipitation
Summary:
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Potassium perchlorate is only marginally soluble in cold water, while being
much more soluble in hot water. From perchloric acid solutions, one can easily
precipitate KClO4, almost quantitatively. It is funny to see how a potassium
salt precipitates so quickly and densely.
Description:
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Take 5 drops of 60% perchloric acid (appr. 0.25 ml) and add these to 1 ml of
water. Dissolve a spatula full of potassium chloride in another ml of water in
a separate test tube and then add the two liquids to each other: Immediately, a
compact finely divided crystalline precipitate is formed which quickly settles
at the bottom.
Heat the liquid until the water starts boiling: All of the crystalline material
dissolves and a colorless and clear liquid is obtained.
Let the liquid cool down slowly and when at room temperure put it in a cold
place: Big crystals of potassium perchlorate are formed, some of them sticking
on the surface of the water, others forming at the bottom of the test tube.
When the liquid has cooled down to 10 C or so, decant the liquid from the
crystals and rinse them once with cold distilled water. Then dry them between
filter paper and then add 1.5 ml of water to the crystals and boil again: All
of the crystal mass redissolves.
Let cool down again slowly to 10 C or so: New big crystals of KClO4 are formed.
Decant the water from the crystals and carefully dry the crystals with good
filter paper and then put them aside on a dry and warm place: A perfectly dry,
non-hygroscopic white solid is obtained. This is pure potassium perchlorate.