when we add a non-volatile solute to the solution it's boiling point increases , thus when we add the same non-volatile solute to the solution why does it's melting point decease instead of increasing ? Pls explain in terms of vapour pressure.

Dear Student,

When non-volatile solute is added to solvent, it decreases the vapour pressure of the solvent. Now, at freezing point, vapour pressure of solid and liquid must be same, or else the system will not be in equilibrium. Lowering of  the vapour pressure leads to lowering of the temperature at which the vapour pressure of liquid and frozen forms of the solution will be equal. As a result, depression in freezing point of solution is observed. 

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