Conc. HCl is corrosive and toxic. Use care.
Chemicals and Solutions:
1 M CuSO4 solution
2 Test tubes
Add some copper solution to one test tube
Add some water to the solution - nothing happens except the solution gets more dilute.
Add some copper solution to the other test tube and add some concentrated HCl to the solution - it will turn green.
Add some water to the now green solution and it will turn blue again.
CuSO4 + 4Cl- ---> [CuCl4]2- (green)
[CuCl4]2- + 6H2O ---> [Cu(H2O)6]2+ (blue)