Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sulphuric acid : H2SO4
sodium hydroxide : NaOH
sodium sulfite : Na2SO3
ammonium dichromate : (NH4)2Cr2O7
potassium dichromate : K2Cr2O7
Class:
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elem=Cr
precipitation
redox
Summary:
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Dichromate is reduced by sulfite in neutral/slightly alkaline enviromments.
Under these conditions a jelly-like precipitate is formed, which is fairly
stable and does not dissolve immediately in dilute sulphuric acid. This
jelly-like precipitate is a chromium (III) compound.
The compound, however, does dissolve easily in strongly alkaline liquids.
The formation of this compound is not affected by the type of the cation
in the chromium-compound.
Description:
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Sequence 1:
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Add a fairly concentrated solution of sodium sulfite (appr. 1.3 mol/l) to
a solution of potassium dichromate: Liquid initially becomes yellow, but
it quickly turns green. After a while of standing it has become dark green
with a yellow hue and it remains clear.
Heat the liquid for a while: The liquid becomes even darker green.
Let cool down: Large pieces of a jelly-like solid are formed, which float
around in the liquid.
Add a small amount of dilute H2SO4 (appr. 2 mol/l): The liquid becomes blue/
green and large solid particles remain floating around in the liquid, it does
not become clear.
Add more H2SO4 (2 mol/l): Strong smell of SO2. Liquid does not become clear.
Add even more H2SO4 (2 mol/l): The liquid slowly becomes clear. Apparently
the precipitate is fairly stable and a very acidic environment is needed to
dissolve the precipitate.
Dilute the liquid with a lot of water: The liquid becomes slightly turbid
again.
Sequence 2:
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Add a fairly concentrated solution of sodium sulfite (appr. 1.3 mol/l) to
a solution of ammonium dichromate: Liquid initially becomes yellow, but
it quickly turns green. After a while of standing it has become dark green
with a yellow hue and it remains clear.
Heat the liquid for a while: The liquid becomes darker green, the last trace
of yellow hue disappears.
Let cool down: The liquid completely solidifies and looks like a green,
opaque jelly. The test tube must be shaken strongly before the jelly-like
mass is broken into smaller parts.
Add some solid NaOH: The solid dissolves and the jelly liquefies. The final
result is a clear moss-green liquid, with a fairly strong smell of ammonia
(due to the decomposition of ammonium in alkaline environment).
Dilute with a large quantity of water: Liquid becomes opaque again.
Mix the final liquid of sequence (1) with the final liquid of sequence (2):
The resulting liquid becomes even more opaque.