Washing is the process of selectively removing unwanted compounds from a mixture using a solvent. For a washing to be successful the unwanted materials must be more soluble in the solvent than in the mixture. Additionally, the solvent and mixture must be immiscible. Immiscible solvents are not soluble in each other and form two layers when mixed.
The washing of clothes is a good example. Dirty clothes are placed in water. The dirt, the unwanted material, is removed leaving the clothes, what we are interest in, behind. This works because the dirt is soluble in the water and clothes are not.
A Chemical Example of Washing
A student has performed a reaction in the laboratory. When the reaction is complete she has a mixture of her product and some leftover starting material dissolved in water. If the product is polar and the starting material is nonpolar, she could wash the starting material into an organic solvent that is not soluble in water.
An aqueous mixture containing both product (P) and starting material (S).
An immiscible organic solvent is added and the nonpolar starting material is washed into it.