Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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hydrofluoric acid : HF
potassium bromate : KBrO3
potassium chlorate : KClO3
Class:
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elem=F,Cl,Br,K
acid/base
redox
Summary:
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Hydrofluoric acid (48% by weight) dissolves potassium chlorate and potassium
bromate much more easily than water. The bromate also slowly decomposes in a
peculiar way, which is not observed in other acidic solutions (like sulphuric
or nitric acids).
Description:
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Put some solid potassium bromate (appr. 100 mg) in a half-transparent PE test
tube. Add 1 ml of hydrofluoric acid (48% by weight): When the acid touches the
solid, there is a slight hissing/crackling noise. With a little swirling, all
of the solid qiuckly dissolves. The solution turns faintly brown.
Wait for several minutes: Slowly, bubbles of gas are formed on the PE surface
of the test tube. It looks a little bit like soda water, which forms bubbles on
the glass. If the bubbles reach a size of 2 mm or so, then they lose contact
with the PE surface and move upwards. The gas is colorless. In this process,
the liquid slowly becomes more visibly brown. After ten minutes the liquid is
orange/brown and there is a clearly noticeable smell of bromine. The PE also
becomes a little bit colored.
In a solution of HF, 48% by weight, apparently the bromate ion is not stable
and it decomposes. The colorless gas almost certainly must be oxygen.
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Repeat the above experiment with potassium chlorate instead of potassium bromate:
There is no sound when the liquid touches the solid potassium chlorate. The solid
quickly dissolves. The liquid becomes very pale yellow/green. No bubbles are
formed. Also at standing, there are no further changes. This faint yellow liquid
noticeably fumes in contact with air, much more so than plain 48% HF.
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Specific observation: KBrO3 and KClO3 are not very soluble in cold water. You need
a lot of shaking and quite some patience to get these salts dissolved in cold
water and even then, the amount which can be dissolved is limited. In the 48% HF
both solids dissolve very well. Just a few swirls are sufficient to completely
dissolve the solids. No noticeable heat is produced in the process, nor any cooling
down could be observed.
Safety remark: These experiments MUST be conducted in PE test tubes. Do not use
glass. The glass will dissolve and there will be spills of HF. HF is EXTREMELY
dangerous in contact with skin and will cause extreme tisse damage and may lead
to very serious symptoms in different organs in the body.