Why does a white ppt. reappears on heating (sodium zincate)? (read on)

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Offline abdillah

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Ok so the normal procedure goes like this in QA right? We add excess NaOH to a solution containing zinc ions. White ppt forms and is soluble in excess NaOH (aq).

the equation goes like this.
 Zn(OH)2 + 2OH- → Zn(OH)42-.

We also know that mostly it is also in equilibrium with water ligands like [Zn(OH)3.H2O]-, etc

The thing is on heating the solution, the white ppt reappears? Any explanation for that? Someone told me it is because of the evaporation of water molecules. Since the solubility of the zincate ion is because of the water ligands surrounding it, less water means that the ppt. becomes less soluble? Is this true?

Any takers?

Thanks a million for anyone who can shed some light on this matter.


Offline lightarrow

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I don't know the answer; maybe the reason is the formation of ZnO in a compact form (crystals?) which is less soluble than Zn(OH)2 in OH-.