When salt is added to ice it becomes a new material with
a lower freezing point. Lets call this new material "salty ice"
This means that if we
have some ice at 0o C and we add some salt it will become a different material
("salty ice"). This new material may well have a freezing point of say –5 oC. If the surrounding air temperature is also 0 degrees C then our "salty ice" will start
melting, at 0o C it is 5 degrees above its freezing point.
As you know
when you melt something you have to add heat energy to break the bonds between the
molecules and turn it into a liquid. To do this it must take in heat from its immediate
surroundings (in this case the original pure ice and any liquid water that may be present) and
so the surroundings cool.
This may seem like a contradiction but a similar thing
happens when you put a drop of meths on your hand. The meths evaporates because it take
in heat energy from your hand and so your hand cools. (Here the molecules of meths
actually gain energy and move faster)
However when the salty ice takes in heat
energy from its surroundings it does not rise in temperature. This only happens when
ALL the solid has melted. At this point the liquid will have dropped in temperature to
our example of -5 oC. Up to this point the energy goes to breaking the
intermolecular bonds.