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salt15 Coloured copper ions from malachite decomposition
* Photo 1: In the previous experiment you have made a green copper salt solution by the reaction of a piece of malachite with hydrochloric acid.
* Photo 2: Three drops of this solution can be used again:
* Photo 3: Dilute with the drops in the blisters with distilled water .
* Add table salt (NaCl) to blister 2 and a drop of ammonia solution (NH₃) to blister 3.
Observation
Photo 3: Water causes a colour change from green to blue,
Photo 4: NaCl from blue to green and ammonia produces a dark blue colour .
Photo 5: After drying up blue crystals can be seen in blister 1, colourless cubes and a brown solid in blister 2, a light blue solid in blister 3.
Explanation
Right photo: Copper salts contain crystal water like glassy gypsum and cobalt chloride does. In this sketch of a model the copper ion (small grey ball) is in the center of an octahedron (double pyramid with 8 planes).
The Cu ion is surrounded by water molecules on four sides and by 2 sulfate (SO₄) ions on the top and below.
In aqueous solution these sulfate ions are replaced by two more water molecules. These 6 water molecules bonded to the Cu²⁺ ion in the center are called Ligands, the coloured ions are complex ions.
The colour changes to green and to blue can be explaned by exchange of ligands:
Photo 4: Water molecules are repalaced by a chloride ion or by 4 ammonia molecules.
Photo 5: During evaporation of water solid copper chloride Cu[H₂O)4]Cl₂ crystallizes in blister 1 and 3. The cubes in Blister 1 are NaCl crystals, the brown colour can be explained by exchange of water molecules by chloride ions as ligands.

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back............ go on.............first published: 25.10.2001.............. ......last modification: 25.03.2007