After heating the solution, you decant the liquid and wash the remaining solid with distilled water. What you are removing by this washing?

In order to remove the remaining liquid present the solid material.

We are washing the solid in order to remove the liquid particles that is attached to the solid material with the help of water. Water is a universal solvent that dissolve large number of solutes in it.

Water removes the liquid from the solid by attaching with that liquid or dissolving in it so that's why we use water for washing a solid in order to remove the impurities from the solid material in which it is placed.

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The solvent

Explanation:

After the solid is recoverd by decanting the liquid, some solvent particles may still adhere to the solid.

In order to remove the solvent that may have adhered to the solid completely , the solid is washed before it is now dried.