A delicate balance determines whether the dissolving of a salt crystal in water will absorb or give off heat. Energy is required to pull apart the oppositely charged ions in the crystal, and this is called the lattice energy, DH_lat.
In contrast, energy is liberated when the separated ions attract polar water molecules around themselves, and this is the heat of hydration, DH_hyd. The observed heat effect on solution depends on which factor predominates. Typical examples are as follows:

Predictions about heats of solution (DH_sol) become tricky, because 5% errors in theoretical calculations of lattice and hydration energies will completely reverse a prediction. This table makes it look as if the DH_sol values were obtained from DH_lat and DH_hyd.