Boiling Point Elevation
The boiling point of a solution is higher than that of the pure
solvent. Accordingly, the use of a solution, rather than a pure
liquid, in antifreeze serves to keep the mixture from boiling in a hot
automobile engine. As with freezing point depression, the effect
depends on the number of solute particles present in a given amount of
solvent, but not the identity of those particles. If 10 grams (0.35
ounces) of sodium chloride are dissolved in 100 grams (3.5 ounces) of
water, the boiling point of the solution is 101.7°C (215.1°F; which is
1.7°C (3.1°F) higher than the boiling point of pure water). The
formula used to calculate the change in boiling point ( Tb) relative
to the pure solvent is similar to that used for freezing point
depression:

Tb = i Kb m,

where Kb is the boiling point elevation constant for the solvent
(0.52°C·kg/mol for water), and m and i have the same meanings as in
the freezing point depression formula. Note that Tb represents an
increase in the boiling point, whereas Tf represents a decrease in
the freezing point. As with the freezing point depression formula,
this one is most accurate at low solute concentrations.

From:
http://www.chemistryexplained.com/Ce...roperties.html