Question

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

Answer 1

Hint: Most of the impurities are water soluble and make hydrogen bonds with most of the elements present in the precipitate. They can be easily removed by washing the precipitate with water before estimating it quantitatively.

Complete step by step answer:
Precipitation is defined as formation of a solid from a solution when the reaction occurs in a liquid solution. The chemical that is responsible for the formation of this solid is called the precipitant.
Some amount of the electrolytes is mixed to form the precipitate which remain adsorbed on the surface of the particles of the precipitate. In order to remove these impurities, precipitate is washed with water because water drives off the remaining soluble impurities. The washed liquid must be a substance which does not interfere with the analysis and cause further precipitation. This washed liquid is generally discarded by washing the precipitate twice for the best result.
Washing the precipitate means to pour the liquid or water over the precipitate while continuing to filter it, thus allowing the liquid to wash over the precipitate and then pass the filter. This procedure is supposed to eliminate impurities that may be clinging to the surface of the precipitated substance. Other than water, ethanol free ether is also used to wash off the impurities.

Hence, it is essential to wash the precipitate with water before estimating it quantitatively.

Note:
If we don’t wash the precipitate, it will affect the yield of the product on drying. Precipitate must be free from soluble impurities but must be insoluble in solution. For ideal washing, liquid should not have any solvent action on the precipitates and do not form any volatile product by interfering with precipitates.