3. What is the purpose of washing a precipitate? Briefly explain how this is done. 4....

3. The purpose  to wash the precipitate is to remove these adsorbed ions or other such impurities.When a substance gets precipitated, some ions that combine to form the precipitate get adsorbed/occluded/included on/inside the surface of the precipitate.

Three cause of impurities during precipitation of analyte are

1.Adsorption 2.inclusion 3 occlusion

gelateneous precipitate require more washing than crystalline precipitate

The washing of precipitate can be done by two ways

a.A precipitate may be washed directly on the filter by using wash water which will contain one ion common to the precipitate which would en hence the solubility of the impurities but at the same time should decrease the solubility of the precipitate .sometime organic impurities are washed with organic solvent

b)A precipitate may be washed by decantation .In decantation, the precipitate is allowed to settle, and the supernatant liquid is poured off ensuring all impurities are removed in the supernatent liquid.Multiples times decantation is done to ensure complete removal of the impurities.

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