When salts dissolve in water the attractive forces within the ionic
lattice are broken, and are replaced by attractive forces between
individual ions and the polar water molecules that surround them
in a hydration shell.
As we saw in Chapter 12, the heat of solution is the difference
between hydration energy and crystal-lattice energy. In addition,
the salt becomes more disordered when it dissolves, so the entropy
increases. If the combination of entropy increase and heat of hydration
is enough to overcome the crystal-lattice energy, the salt will
dissolve.
We can write the overall process as
Salts such as NaCI are 100% ionized in the crystal and in aqueous
solution. The symbol (aq) indicates that each ion is hydrated, or
surrounded by a shell of polar water molecules in the manner that
we saw first in Chapter 5, and in the drawing at the beginning of
this chapter.
For the sake of brevity, we will not use the (aq) symbol in equations
in this chapter, but you should remember that ions in aqueous solution
always are hydrated, and that hydration energy is largely responsible
for making the salt dissolve. If there were no hydration energy
to balance the loss of energy from crystal attractions, then dissolving
NaCI would be as difficult as vaporizing it, which can be accomplished
only at temperatures above 1400'C.