Description of experiment
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Needed compounds:
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hydrochloric acid : HCl
potassium hydroxide : KOH
copper sulfate penta hydrate : CuSO4 . 5H2O
Class:
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elem=Cu
precipitation
Summary:
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Copper hydroxide is decomposed upon heating and loses water, even if
it is surrounded by water.
Description:
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Add a 0.5M solution of copper sulfate to a concentrated solution of KOH:
Formation of a bright blue precipitate, which has a strong structure. In
the test tube the bottom part is filled with the KOH-solution, encapsulated
in a solid layer of the blue precipitate (copper hydroxide) and the glass.
Above the solid layer is a large layer of 0.5M solution of copper sulfate.
Even moderate shaking of the test tube does not break the solid layer.
Strong shaking of the test tube: The solid layer breaks and a large piece
of the blue precipitate comes in the liquid. Still some parts of the KOH
solution cannot be mixed with the copper sulfate solution.
Heat the tube to approximately 65 C: The bright blue precipitate becomes
black. At certain places it becomes dark red/brown. After a sufficiently long
time of heating no blue precipitate can be observed anymore. All of the
precipitate has turned dark brown / black. After shaking, the liquid becomes
dark grey, due to many black particles floating through the liquid.
Add an excess amount of hydrochloric acid (10% by weight): The liquid
immediately becomes clear and its color is green/blue. Some large chunks
of black material remain. These larger chunks slowly dissolve and after
approximately 1 minute of shaking these black chunks are dissolved
completely. The final liquid is clear and its color is green/blue.