Question

How do I know which cations are or are not in a solution based on the color and whether a precipitate forms or not?

Answer 1

Cations are grouped into six different groups depending on their properties. Although cannot be confirmed, the color of the cationic solution is a preliminary indicator of the nature of the cation present in it as many cations give characteristic colors to their salt solutions.

Hydrated Cu²⁺ salt solutions are usually blue in color.

Cr ²⁺, Ni ²⁺, and Fe²⁺ salts are usually green in color

Hydrated Co²⁺ salts are reddish pink in color

Precipitate formation and color are not related. However, from a logical guess one can decide on using the right reagent to induce precipitation having identified the cation from the color of the salt solution. For instance, Copper salt solutions may be precipitated by the addition of HCl followed by passing H₂S to the solution; It is, however, recommened that the student follows a systematic scheme of analysis for the identification of the cation in the solution.