[Zamion]

I need to know why some chemical reactions (solubility lab) consist of a change in colour but do not form a precipitate. Specifically why mixing a solution of Cr⁺³ and a solution of CrO₄^-2 causes a colour change but no precipitate forms. Also, why mixing a solution of Cr⁺³ and a solution of CrO₄^-2 causes a colour change but no precipitate forms. Thanks.

[DevaDevil]

Specifically why mixing a solution of Cr+3 and a solution of CrO4-2 causes a colour change but no precipitate forms. Also, why mixing a solution of Cr+3 and a solution of CrO4-2 causes a colour change but no precipitate forms. Thanks.

2 times the same question?

Anyway, chromate has a yellow color; dichomate (in equilibrium with chromate) an orange color, and trivalent chromium a blue-greenish color.

Mixing these will keep the ions intact, there is nothing they will precipitate with, but of course the mixture of those colors will lead to a different color. (eg. Blue and yellow mixed will lead to green of course)

[Zamion]

Sorry, meant to say Cr⁺² and CrO₄^-2 mixed together, as well as Ni⁺² and CrO₄^-2 mixed together. 

This makes sense, the colours were just hard to recognize/distinguish in this particular lab. I guess the solutions were just really diluted to throw us off. Thanks.

[organoman]

Precipitation occurs if compounds forming in reactions are insoluble in the solvent. while colour changes if the product formed emit in different wavelength than the reactant.