So the heat of solution is the sum of energy required to break the intermolecular forces of compound one. Plus the sum of the energy is required to break the intermolecular forces of compound two plus the energy release when you form the new solution. And that gives you the enthalpy of solution or heat of solution. Now, when this guy is negative, when he's exothermic the bonds form are stronger than the bonds broken, so they're more stable. Now, when it's endothermic, when it's positive, the bonds form are weaker and less stable than the bonds that were broken. Now let's compare ideal and non ideal situations.