Rinsing the solid with a pure liquid (like distilled water)
helps remove unwanted soluble species. Thus, the solid is more likely to
be free of contaminants when dry. NOTE: The solid must not be soluble in
the liquid that will be used for the rinsing procedure.
To rinse the solid, add a few milliliters of solvent to the solid and thoroughly mix the solution until the solid is re-suspended in the liquid. Centrifuge the solution, decant the liquid, and repeat the cleaning proces
until the supernatant shows no sign of impurity.
Re-suspending the solid will make sure that the solution that has been trapped in the solid cake at the bottom of the test tube is mixed with the rinse solution.
The solid should be rinsed more than once. Successive washings help remove more impurities.
(a)
(b)
(c)
IMAGE. The same sample washed three times. (a) Initially, the supernatant is very blue due to the impurities in the sample. (b),(c) With successive washings, the supernatant becomes less colored as the impurities are removed.
Why re-suspend the solid? 
(a)
(b)
(c)