Question

You have to test a solution with one to five possible representative cations: Pb2+, Fe3+, Al3+, Ca2+, Cu2+. You raise the pH of the solution to 9.5 with ammonia solution, and your unknown solution turns blue with no precipitate. What does this mean about any cations that you should not have in your sample? What cations may still possibly be in your solution? What test could you now try to prove whether there is any Ca2+ in your solution? What would you expect to see if there were no Ca2+ in your solution?

Answer 1

According to the given data:

The addition of ammonia solution (in excess) turns the unknown solution to blue without any precipitate, which indicates that the sample solution contains the cation Cu²⁺.

Note: The above observation indicates the formation of the blue colored [Cu(NH₃)₂]⁺

Test for Ca²⁺: 1. Addition of sodium hydroxide solution to the sample solution containing Ca²⁺ forms a white precipitate {i.e. insoluble Ca(OH)₂}, whereas, 2. the addition of ammonia solution forms a faint-white precipitate {i.e. insoluble Ca(NH₂)₂}