Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sulphuric acid : H2SO4
hydrogen peroxide : H2O2
hydrochloric acid : HCl
sodium sulfide : Na2S . 3H2O
cobalt sulfate : CoSO4.7H2O
Class:
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elem=Co,S
redox
precipitation
Summary:
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Cobalt sulfide does not dissolve in dilute acids and it does not dissolve
in concentrated hydrochloric acid (at least not easily). With the help of
hydrogen peroxide it is possible, however, to dissolve the compound.
Description:
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Add a solution of sodium sulfide to an excess amount of a solution of cobalt
sulfate: A black precipitate is formed, after a while, when the precipitate
has settled, a clear pale pink liquid is above the black precipitate.
Decant the liquid and take the precipitate and add some dilute H2SO4 (appr.
1 mol/l): The solid does not dissolve noticeably, although there is a faint
smell of H2S.
Again decant the liquid and add some 30% HCl: The solid still does not
dissolve. The liquid becomes pale blue/cyan. This is probably due to remnants
of remaining traces of cobalt in solution.
Add some H2O2 (appr. 30% by weight): The liquid becomes colorless, the solid
slowly dissolves, without evolution of a gas. After a while, the liquid is
clear and light pink. The speed of dissolving the sulfide is low, it takes
several minutes to dissolve a small quantity of the solid.
During this experiment, there is a fairly strong smell of chlorine.