[Zorya (OP)]

Hi!
I've been doing a science school project about inorganic salts and the growth of single giant crystals ("giant" being around 2 cm in diameter). One of the salts I worked with was Copper chloride, CuCl₂. It did not turn out great as the crystal clusters (producing single crystals of CuCl₂ was near impossible) looked more like a porcupine than a classic crystal, but as I handled the supersaturated solution with CuCl₂ I discovered something interesting.

CuCl₂ in its solid state (in the bin and as spiky clusters) had a turquoise colour, more towards blue than green, while the supersaturated solution were very deep green, poured into a large bottle almost black. So far so good, many salts change colour when solved in water. But as I was making the supersaturated solution, I saw that the solution went from cyan blue (almost like the tone of the sea around paradise islands) to more and more green as the concentration of CuCl₂ increased. The effect was reversed when I washed the containers I had used, the deep green solution turned blue as I flushed with water.

One time I was trying to investigate this phenomenon further. My idea was to fill several test tubes with solutions of different concentration to compare the colours. I filled the test tube to about 1/3d with saturated CuCl₂-solution and added water straight from the tap. What I got was one test tube with layers of different concentrations and tones. In the the bottom 2/3ds there were three different tones of green, the bottom one deep green, the top one lighter and going towards blue, and the middle one somewhere in between. They were quite clearly separated from each other. The top third of the solution was light blue.

Then the strange thing. Between the light blue top layer and the bluish green layer beneath, there was a layer, about 3 mm thick. It was white, well separated from the other layers and looked almost like some kind of foam. I did not have a camera at the moment, so I couldn't take a picture at the moment. Maybe some other time.

My questions are:
a) How come a solution of CuCl₂ changes colour with the concentration?
b) What was the foam-like layer I observed in the test tube?

Happy for answers, as me and my chemistry teacher only has theories and hasn't been able to find anything clear

[RD]

Copper Chloride changes colour with concentration and with temperature...

The color of the dilute solution depends on temperature, being green around 100 °C and blue at room temperature.

http://en.wikipedia.org/wiki/Copper(II)_chloride

[Bored chemist]

The pretty pictures helped. They say that the crystal is hung from a copper thread. Concentrated Copper (II) chloride solutions will dissolve metalic copper to give a mixed Cu(I) + Cu(II) complex that's brown/ black. On dilution some CuCl (which is white) is precipitated.

[lightarrow]

...
My questions are:
a) How come a solution of CuCl₂ changes colour with the concentration?

About this, I think the reason is the fact at higher concentrations it forms complexes with chloride, like CuCl₄^2-, which are green and mask the blue colour of Cu(H₂O)₆²⁺, which then remains as the colour you see at low conc. A similar effect with many others metal compounds, as FeCl₃, which is intense yellow-green when concentrated, because of  FeCl₄^-, and colourless when diluted, because that complex is dissociated.