Q.Why is it essential to wash the precipitate with water before estimating it quantitatively?

Adsorption is the accumulation of a substance (the adsorbate) at the surface of another (the adsorbent), so its concentration is higher at the surface than in the bulk. This is fundamentally different from absorption, where the substance penetrates uniformly throughout the bulk of the material. The general term sorption covers both when they occur together.

Key points:

• Surface phenomenon — it arises because surface particles have unbalanced (residual) attractive forces. A finely divided or powdered solid exposes far more surface per unit mass than a compact crystal, so it adsorbs much more.
• Always exothermic: adsorption lowers the residual surface energy, so enthalpy of adsorption $\Delta H$ is negative. Since molecules lose freedom on the surface, $\Delta S < 0$; for spontaneity $\Delta G = \Delta H - T\Delta S < 0$ requires the negative $\Delta H$ term. …