In order for a liquid to evaporate, the thermal energy needed must be enough to disrupt the attracting forces of the liquid.

Water has hydrogen bonding. If you add salt to water, some of the hydrogen bonding are broke up and become an ion-dipole, which is a stronger bond than hydrogen bond. H₂O molecules of water tend to cling to the Na⁺ and Cl⁻ ions of salt. 

And since the bond on saltwater is stronger than on freshwater, saltwater tends to have a slightly higher boiling point.
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