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I am currently preparing for Junior science competitions and I am coming across many questions where colour of precipitate plays important role... Like

When compound $\ce{Hg2(NO3)2}$ reacts with $\ce{Na2CrO4}$ solution, the colour of the compound formed is--

The question is a bit long and the compound can be determined from a series of experiments given in question. the product is $\ce{Hg2CrO4}$ but guessing the colour is impossible. Is there any way to determine colour of compound i.e. precipitate.??

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  • $\begingroup$ Actually the question is a a bit long and from the question the compound is determined. Let's assume the product can be determined easily. Is there any way to know it's colour?? $\endgroup$
    – An Alien
    Oct 2 '21 at 17:46
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    $\begingroup$ May be the solution is in the details (I am looking for the hints there). Please post the entire question. $\endgroup$
    – M. Farooq
    Oct 3 '21 at 0:34
  • $\begingroup$ @Nilay Ghosh Looks perfect, Brilliant, Thanks $\endgroup$
    – An Alien
    Oct 3 '21 at 3:30
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All chromates are usually yellow, orange or red. The mercury salts are a good example.

  • the mercury(I) chromate $\ce{Hg2CrO4}$ is brown at room temperature, and turns red at $100$°C. It changes crystallization system at high temperature.
  • the mercury(II) chromate $\ce{HgCrO4}$ is yellow at room temperature, and becomes red at $100$°C. This change is due to the formation of a basic salt.
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    $\begingroup$ With 1) the pH dependent equilibrium between chromate and dichromate, and 2) the more dark-red potassium dichromate than the more yellow potassium chromate: could there be dark mercury dichromate? (I don't touch the question if it were Hg(I) or Hg(II).) $\endgroup$
    – Buttonwood
    Oct 2 '21 at 22:56
  • $\begingroup$ The dichromate ion $\ce{Cr2O7^{2-}}$ does not give a precipitate with $\ce{HgCl2}$, but is gives a brownish yellow precipitate with $\ce{Hg(NO3)2}$, according to Treadwell's Qualitative Analysis. $\endgroup$
    – Maurice
    Oct 3 '21 at 9:38

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