CuSO4 is soluble in water but ferric oxide is not soluble in water.
Ferric oxide is an insoluble oxide hence it does not dissolve in water. Since the substance is insoluble in water, it exists as solid particles when added to water. When this mixture is filtered, ferric oxide is separated from water.
On the other hand, copper (II) sulfate is quite soluble in water hence it can not be separated from water by filtration since it dissolves completely in water. Hence, copper (II) sulfate can not be separated from water by filtration but ferric oxide can be separated from water by filtration.
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