When salt is added in water then endothermic reaction takes place. This means that heat is absorbed from the surrounding and salt is dissolved in water. However, with the addition of the salt in water, boiling point of solvent or water goes up and freezing point of water goes down. Boiling point is the temperature at which the vapor pressure is equal to external pressure. Therefore, when you add salt in water the vapor pressure is lowered and more temperature is required to equalize vapor pressure and external pressure, thereby boiling point increases.
