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What was the purpose of washing your product with either ice-cold water or alcohol? NOTE: This...

What was the purpose of washing your product with either ice-cold water or alcohol? NOTE: This lab was about the preparation of common alum.

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Answer #1

KAl(OH)4(aq)    + 2H2SO4(aq) + 8H2O(l) ----> KAl(SO4)2.12H2O(s)

Under the experimental conditions, the alum has a limited solubility in water, and this solubility further decreases at ice-cold water (solubility of any solute decreases at lower temperature). Further excess sulfuric acid in reaction mixture has to be removed which is done by washing with water or ethanol. The dilution of sulfuric acid in water is highly exothermic (evolves heat) which will increase the temperature of solution which in turn cause the solubility of alum and loss of compound. Hence, washing is done  with either ice-cold water or alcohol.

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