When copper chloride breaks down, the color of the solution changes from blue to green. This is because the copper(II) ions in the solution are reduced to copper(I) ions. The copper(I) ions form a complex with the chloride ions in the solution, which has a green color.

The reaction that takes place when copper chloride breaks down is:

CuCl2 + H2O → Cu(OH)Cl + HCl

The copper(II) ions in the copper chloride solution are reduced to copper(I) ions by the hydrogen gas that is produced in the reaction. The copper(I) ions then form a complex with the chloride ions in the solution, which has a green color.

The change in color from blue to green can be used to identify the presence of copper(II) ions in a solution. This is a useful test for copper, as it is a relatively simple and inexpensive test to perform.