Answer :

NaCl is a non-volatile substance when it is added to the water it decreases the melting point of the water simultaneously decreasing the vapor pressure of the water.

According to Raoult’s law, the partial vapor pressure of each component in the solution is directly proportional to its mole fraction.

Thus, for water as a solvent in a solution

Where Pw is the partial pressure of water and xW is a mole fraction of water in the solution.

Thus, on adding NaCl the mole fraction of water reduces.

Thus, vapor pressure also reduces.

Thus, both the assertion and reason are true and the reason is the correct explanation of the assertion.

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