Why do we need to use very hot water in washing the black precipitate? When the recovered copper was washed with methanol and acetone, what was/were taken out from the solids?
You haven't really given enough information, but I will try to work out what the experiment was that you were doing. If you were asked to wash a solid in very hot water, it was presumably copper oxide. The impurities that you wanted to remove (copper sulphate?) are more soluble in hot water than cold, so hot water will purify the black solid more efficiently. Methanol and propanone (acetone) will help to remove water, and make sure that the final product is completely dry.
