Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sodium hydroxide : NaOH
chrome alum : KCr(SO4)2 . 12H2O
hydroxyl amine sulfate : (NH3OH)2 SO4
Class:
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elem=Cr
precipitation
coordination
Summary:
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Chromium (III) forms a lavender precipitate, when combined with hydroxyl
amine in alkaline environments. This compound does not dissolve in
strongly alkaline environments.
Description:
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Sequence 1: Formation of lavender compound from neutral environment
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Add a mixture of solid hydroxyl amine sulfate and solid chrome alum to some
water: Both solids dissolve, formation of a purple liquid. The chrome alum
dissolves more slowly than the hydroxyl amine sulfate.
Heat the liquid, while still some solid chrome alum exists: The last few
crystals of chrome alum dissolve quickly. The liquid becomes green. The
presence of hydroxyl amine sulfate apparently does not affect the formation
of a green compound (this is described in another experiment, where chrome
alum is heated with water).
Add an excess amount of a solution of NaOH: Formation of a lavender/greyish
precipitate (especially, when viewed at daylight it looks more grey than
lavender).
Add much more solution of NaOH: The precipitate does not dissolve.
Sequence 2: Formation of lavender compound from alkaline environment:
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Add an excess amount of a solution of NaOH to a hot solution of chrome alum
(which is green instead of violet): A green/brown precipitate can be observed
for a moment, which, however, quickly dissolves. The liquid becomes clear
and green again, but the hue of the green is different. The original liquid
was more like blue/green, the alkaline liquid is moss-green.
Add a solution of hydroxyl amine sulfate: The liquid becomes much darker and
the color shifts slowly from green to grey/purple. At a certain point in
time, a purple/lavender solid precipitates. When more hydroxyl amine sulfate
is added, then the liquid becomes lighter and more of the precipitate is
formed.