Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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hydrogen peroxide : H2O2
ammonia : NH3
hydroxyl amine sulfate : (NH3OH)2 SO4
hydrochloric acid : HCl
copper sulfate penta hydrate : CuSO4 . 5H2O
Class:
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elem=Cu,N
redox
Summary:
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When copper (II) is present in hydrochloric acid, then it does not react
immediately with hydroxyl ammonium, some heating is required to have a
reaction, resulting in formation of a dark green/brown compound. When the
liquid is made alkaline with excess ammonia, then it quickly becomes
colorless, due to reduction of copper (II) to copper (I), which forms a
colorless complex with ammonia.
A very peculiar reaction occurs on oxidation by oxygen from air. The liquid
is covered by a very thin shiny layer, looking like a strongly coloured oil
on water. It is not clear what it is, more research is needed.
Description:
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Add a large crystal of copper sulfate to some concentrated HCl (appr. 30% by
weight): The liquid becomes bright yellow/green. The crystal is covered by a
green crust/sponge-like solid and appears not to dissolve any further.
Heat liquid until almost boiling: Now the crystal quickly completely dissolves.
The liquid becomes clear and dark green/brown.
Let cool down: Liquid becomes slightly lighter, but it still remains intense
green/yellow/brown. The liquid, sticking to the glass, still looks intense
yellow, in spite of the thin layer.
Add an excess amount of hydroxyl amine sulfate: The solid dissolves, no visible
changes.
Dilute with water, such that the total volume becomes 2.5 times the original
volume: Liquid becomes lighter, no other visible changes besides dilution of
the liquid.
Heat until almost boiling: Liquid becomes darker and the color becomes
brown/green.
Let cool down: Liquid remains brown/green. After a while, the few mm of liquid,
just below the surface, have become much lighter and the color has shifted to
green. This can be explained by assuming that the liquid absorbs oxygen from
the air and that the dark brown compound (a copper (I) or mixed copper (I, II)
compound) is oxidized to copper (II), which gives a green coordination complex
between copper (II) and chloride.
Add excess NH3 (appr. 5% by weight): Liquid becomes blue and quickly after that
it becomes colorless. On contact with air, the colorless liquid forms a brown
compound, which covers the liquid as a thin solid layer and which sticks to
the glass. When the liquid is shaken, then this brown compound dissolves and
the liquid becomes colorless again.
Add 2 drops of H2O2 (appr. 3% by weight): As soon as the drops touch the
surface of the liquid, a dark green/brown compound is formed, which, however,
quickly dissappears again.
Let liquid stand in contact with air: The liquid is covered by a thin layer,
which is dark green/brown. The liquid has no smell of ammonia, but a very
weak odour, resembling that of NCl3 or NH2Cl (a similar, but much stronger,
odour can be obtained by bubbling CL2-gas through a solution of NH4Cl or
dilute NH3).
Pour the liquid on a white smooth surface (e.g. emaille or inert plastic): The
liquid is covered by a very thin solid layer. It is possible to move around
'pieces' of the layer of several square cm without breaking them apart.
In the layer many color-patterns can be observed, similar to the patterns
which can be observed when a very thin layer of light oil or petrol is floating
on a water surface. It looks as if a oil-like compound is formed.
The color of the layer (apart from the reflective color patterns) is yellow/
brown/green.
Add a few drops of HCl (10% by weight) to the liquid on the white surface: The
reflective thin oil-like layer disappears immediately. The white surface is
cleaned completely and the liquid becomes colorless (or has a pale color).