Description of experiment
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Needed compounds:
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hydrogen peroxide : H2O2
sulphuric acid : H2SO4
sodium dichromate dihydrate : Na2Cr2O7.2H2O
Class:
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elem=Cr
redox
Summary:
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Sodium dichromate dissolves in water very well, much better than potassium
dichromate. When it is reduced, then it is hard to crystallize a chrome
(III) salt or a mixed sodium chrome (III) salt.
Description:
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Dissolve some sodium dichromate in a small quantity of water: The liquid
becomes intense orange/red.
Add some concentrated sulfuric acid (98%): Formation of a red precipitate
in the form of very little crystals. The amount of crystals formed is so
large, that the liquid becomes like wet sand and hardly is capable of free
flowing.
Add a little amount of water: All crystals dissolve, the liquid becomes
dark red/orange.
Add hydrogen peroxide (30% by weight), drop wise, until an excess amount
is added: Each drop results in a violent reaction, in which a large amount
of gas is evolved and a dark green (almost black) compound is formed,
which remains in solution. At a certain point, adding further drops does
not yield any reaction anymore and at that point the liquid is dark green
(color of chromium (III) in many cases).
Heat liquid and keep boiling for a while: A fairly large amount of gas is
evolved, the excess amount of H2O2 decomposes. Also when the heat source
is removed, the evolution of gas continues. Shaking the liquid makes the
evolution of the gas more vigorous. At a certain point no more gas is
evolved and all H2O2 is decomposed.
Place liquid in refrigerator (appr. -15 C) after it has cooled down and
let cool down for a few hours: No crystals separate from the liquid.
The liquid, however, becomes very viscous and looks like a thick green
syrup.