chemistry problem

melting point or boilin point depend on how much energy, U
has to be imparted into the system
to separate the species from a distance, r
by breaking the intermolecular force, F

A relation is tuned out: dU=F*dr (dU means change of U)
where both the strength of intermolecular force and the separation are both variables
it is necessary to determine which factor overwhlem another.

In solid state, water molecules in a crystal are packed in a regular arrangement
and hence only have a small separation.
Once some ionic species (salt) participate into the crystal,
the lattice of the crystal is destroyed
and the separation between water molecules within a solid salt solution
is greater then that within pure water.
The effect of the increasing separation is significant.
The water molecules in solid salt solution are more easily to be separated.

In liquid state, the separation of water molecules
is much greater than that in solid
there is much empty space for ionic species to participate into the system
without much changing the separation of the water molecules.
the effect of the increasing separation is negligible.

Compared to another factor,
the strength of intermolecular force, which is an significant factor.
The stronger the intermolecular force, the harder the ease of separating the species
The interaction between water molecules and ionic species is greater then
that between only water molecules.
Therefore, the boiling point of salt solution is higher.




Hope this may help you.