Why does dissolving a salt in water liberate heat

Why does dissolving a salt in water liberate heat  

Hess s Law Applied to Ions Constructing Born-Haber Cycles 

Dissolving an ionic salt in water often liberates energy. For example, 32.8 kJ mol-1 of energy are released when 1 mol of potassium nitrate dissolves in water. Energy is released, as experienced by the test tube getting warmer. 

Before we dissolved the salt in water, the ions within the crystal were held together by strong electrostatic interactions, which obeyed Coulomb s law (see p. 313). We call the energetic sum of these interactions the lattice enthalpy (see p. 124). We need to overcome the lattice enthalpy if the salt is to dissolve. Stated another way, salts like magnesium sulphate are effectively insoluble in water because water, as a solvent, is unable to overcome the lattice enthalpy. 

But what is the magnitude of the lattice enthalpy We cannot measure it directly experimentally, so we measure it indirectly, with a Hess s law energy cycle. The first scientists to determine lattice enthalpies this way were the German scientists Bom and Haber we construct a Born-Haber cycle, which is a form of Hess s-law cycle. 

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