For making salt water, you have to first dissolve NaCl (Sodium Chloride or Salt) in water (H2O ) . The ions in sodium chloride (Na+ and Cl− ) get dispersed in water. As hydrogen bonding is present in H2O or water and it is hard to turn water into gas as it is a polar molecule. The ions in water, disrupts the hydrogen bonding and hence, this makes water boil at low temperature ( salt water ) .
See, pure water has high boiling point than tap water and tap water has high boiling point than salt water. Do you know why?
The pure water has no impurity to disrupt its hydrogen bonding while tap water has little salts also . But, salt water has the ions Na+ and Cl - which disrupts the hydrogen bonding.