Description of experiment
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Needed compounds:
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hydrochloric acid : HCl
ammonia : NH3
chrome alum : KCr(SO4)2 . 12H2O
Class:
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elem=Cr
precipitation
Summary:
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When Cr3+ is added to an aqueous solution of ammonia, then a precipitate
is created, which does not dissolve on addition of much more ammonia.
Description:
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Add a large excess amount of NH3 (15% solution by weight) to a solution
of chrome alum: Creation of a purplish/grey precipitate. Addition of much
more NH3 does not result in dissolving of the precipitate.
Add an excess amount of HCl (10% by weight) to the liquid with the grey
precipitate: The liquid becomes clear and purple, just like the initial
solution of chrome alum. Above the solution some smoke appears, due to the
reaction between NH3 and HCl.