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Copper sulfate solution and potassium iodide solution
![Page 2: Copper sulfate solution and potassium iodide solution](https://reader033.vdocuments.mx/reader033/viewer/2022051216/56649e4d5503460f94b43f0c/html5/thumbnails/2.jpg)
Blue copper sulfate solution is poured into colourless potassium iodide solution.
![Page 3: Copper sulfate solution and potassium iodide solution](https://reader033.vdocuments.mx/reader033/viewer/2022051216/56649e4d5503460f94b43f0c/html5/thumbnails/3.jpg)
The colourless solution changes to a cloudy brown colour – like milky coffee.
![Page 4: Copper sulfate solution and potassium iodide solution](https://reader033.vdocuments.mx/reader033/viewer/2022051216/56649e4d5503460f94b43f0c/html5/thumbnails/4.jpg)
The cloudy substance is poured through filter paper.
The filtrate is a clear orange-brown colour.
![Page 5: Copper sulfate solution and potassium iodide solution](https://reader033.vdocuments.mx/reader033/viewer/2022051216/56649e4d5503460f94b43f0c/html5/thumbnails/5.jpg)
What substance is in the filtrate?
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The filter paper is stained with a very dark substance and a pale precipitate.
Can you identify the dark substance staining the filter paper?
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Rinsing the filter paper with iodide solution and water removes the dark substance, leaving behind a white precipitate.
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A small amount of the white precipitate is collected and added to water. It does not dissolve.
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Silver nitrate solution is added.
A dark precipitate forms.
Can you identify the dark precipitate?
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The liquid above the dark precipitate appears to be a very faint blue.
What do you think is colouring the liquid?
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Upon addition of ammonia solution, this liquid turns royal blue.
Were you right?
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Colourless cyclohexane (a non-polar solvent) is placed in a test tube.
A sample of the clear, orange-brown filtrate is taken.
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When this filtrate is mixed with the cyclohexane, the organic (top) layer turns purple.
What substance turns cyclohexane purple?
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Colourless iodide (I-)
has been oxidised to I2, which forms the orange-brown complex I3- in iodide,
and dissolves in non-polar solvents to form a purple solution.
I2 isn’t very soluble, so in concentrated solution it precipitates out to form the dark solid we saw on the filter paper.
2 I- → I2 + 2e-
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The reaction between silver nitrate solution and the white precipitate produced silver metal (the black precipitate),
and Cu2+, which reacted with ammonia to form the royal blue complex.
Identifying the white precipitate
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Silver metal comes from the reduction of Ag+ to Ag, so the white precipitate must have been oxidised to form Cu2+.
Possible oxidation states for copper are 0, +1 and +2.
Copper metal is brown, not white, so the white precipitate must contain Cu+. It is copper(I) iodide, CuI (sometimes written as Cu2I2).The Cu2+ in copper sulfate is reduced by the I- to Cu+:
Cu2+(aq) + e- → Cu+(aq)
The Cu+ combines with I- to form insoluble copper(I) iodide which is white.
Cu+(aq) + I-(aq) → CuI(s)
So the overall reaction is:
2Cu2+(aq) + 4I- (aq) → 2CuI(s) + I2(s)