What's the the temperature of a cup of water with ice
and one with salt added?
What's causes water tension?
Hydrogen bonds generate water tension, and the adhesion-cohesion
properties of water.
With added salt (eg. Na+ and Cl-), ion-dipole interactions (of
the ions with the polar water molecules) disrupt the
tetrahedral lattice of hydrogen bonds of ice, hence causing
ice to melt at 0 deg C.
This allows water to exist in liquid state below 0 deg C, and
also allows the temperature of the water in the cup to be depressed
slightly below 0 deg C (the extent of this also depends on the
rate of thermal transfer with the surroundings).