Question

Please help! Thanks :D4. How could you tell a Cu(NO3)2 solution from a Ni(NO3)2 solution?

Answer 1

Cu(NO₃)₂ solutions have a deep blue color due to the Cu²⁺ ions because of the electronic transitions within the copper ion that absorb certain wavelengths of light while Ni(NO₃)₂ solutions are usually greenish, because of the Ni²⁺ ions.

How to tell the compounds apart

As indicated above, color is one of the ways to tell the compounds apart. Another way to check is through precipitation. Adding a few drops of KOH solution to a Cu(NO₃)₂ solution will produce a light blue precipitate of Cu(OH)₂ because Cu²⁺ ions readily react with OH⁻ ions to form the insoluble hydroxide.

In contrast, adding KOH to a Ni(NO₃)₂ solution will result in a pale green precipitate of Ni(OH)₂, but it is much less noticeable and may require more concentrated solutions or standing for a longer time.

Answer 2

To tell a Cu(NO3)2 solution from a Ni(NO3)2 solution, you can add KI to the solution. If the solution contains Cu(NO3)2, the brown precipitate will be formed, but if the solution contains Ni(NO3)2, no precipitate will be formed.

This is the best way to distinguish between these two solutions, since adding KOH or Na2C2O4 will result in blueish precipitates, being harder to identify if it is Cu(NO3)2 or Ni(NO3)2 and with Na2SO4 no reaction will happen.

It means that the answer is: By adding KI and seeing if a brown precipitate is formed.