Crystals

Neat Science! My students are making crystals in General Science class using salt (NaCl), Copper II Sulfate (CuSO4), or sugar (C12H22O11).  A couple of students mixed the CuSO4 solution which is blue with the NaCl solution which is clear and the result was not a lighter blue solution but rather a deep green solution.

Figure 1:  Blue Solution - CuSO4, Green Solution - CuSO4 + NaCl 

The answer to this mystery is with what happens to the copper ion (Cu2+).  The copper ion gives off a blue color when dissolved, however, when the sodium chloride was added chloride ions (Cl-) reacted with the Cu2+ to form a complex ion, Copper(II) tetrachloro complex (the copper ion surrounded by four chlorides), which is green.