Description of experiment
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Needed compounds:
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ammonia : NH3
sodium chloride : NaCl
hydrochloric acid : HCl
sodium sulfite : Na2SO3
copper sulfate penta hydrate : CuSO4 . 5H2O
Class:
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elem=Cu
coordination
redox
Summary:
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Copper (II) is reduced by sulfite. With chloride the resulting copper (I)
can be kept in solution. The copper (I) compound is very susceptible
to oxidation by oxygen from the air.
Description:
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Add an excess amount of a concentrated solution of sodium sulfite to a hot
solution of copper sulfate: Formation of a yellow/brown precipitate.
Add a few drops of hydrochloric acid (appr. 10% by weight) to the cooled
down liquid (with the precipitate): Formation of a white solid, which quickly
disappears again, when the liquid is shaken. The white solid probably is CuCl.
The yellow/brown precipitate does not dissolve. A faint odour of SO2 can be
observed.
Add much NaCl: The solid dissolves now. The liquid becomes almost clear, it
looks like a very concentrated solution of table salt.
Add a lot of water: The liquid remains colorless and clear.
Add a small amount of ammonia (appr. 5% by weight): The liquid becomes light
blue. This blue color is not like the sky-blue color of aquous copper (II)
ions, but it looks like the color of the copper-ammonia (cuprammine) complex,
but strongly diluted.
At the surface of the liquid, the blue color is much more intense than deeper
inside the liquid. Oxygen is taken from the air and this oxygen converts the
colorless copper (I) compound in the liquid to the intense blue copper (II) -
ammonia complex.