Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sodium metabisulfite : Na2S2O5
hydrochloric acid : HCl
potassium iodide : KI
potassium chlorate : KClO3
Class:
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elem=Cl,I
redox
Summary:
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Potassium chlorate only reacts slowly with iodide at room temperature.
When heated, the reaction proceeds much faster. Probably KClO3 oxidizes
iodine further, until iodate is formed.
Description:
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Add some KClO3 to water: A (relatively small) part dissolves. On heating all
of the KClO3 dissolves. On standing large transparent crystals appear, shaped
like scales.
Add some solid KI: The solid dissolves, the liquid becomes a little turbid,
slightly milky.
Add some HCl (10% by weight): The liquid becomes pale yellow, the scale-
shaped crystals do not dissolve.
Heat to appr. 60 C: The liquid turns darker slowly, until it becomes dark
brown. The crystals at the bottom dissolve. Above the liquid a clearly
visible purple vapor of elementary iodine can be observed.
Let the liquid cool down again: The liquid turns lighter and little black
crystals of solid iodine are formed at the bottom.
Add some solid Na2S2O5 to the still luke-warm liquid: As soon as the solid
touches the liquid, it starts foaming. The liquid again turns dark brown.
Above the liquid a much thicker vapor of elementary iodine can be observed.
Remark: KClO3 slowly reacts at room temperature. At higher temperatures its
reactions are faster.
The fact that addition of Na2S2O5 results in darkening of the liquid can
be explained by assuming that an excess amount of KClO3 was used and that
a part of the iodide was converted to iodate or iodic acid. On addition
of Na2S2O5 this iodate again is converted to iodide, which in turns reacts
with more iodate to iodine.