The Dissolving Process
  What happens when something dissolves?  

If we focus solely on the solution process, we find an interesting sequence that involves many concepts on which we focused in the past.  We could almost imagine the parts of the solution – the solute (the material being dissolved – often the material in the lesser quantity) and the solvent (the material separating the solute, the substance carrying the solute to different regions of the mixture – usually the solvent is the material in the greater quantity) interacting in three overall steps.  We'll illustrate these three steps using a dissolving ionic crystal as an example.

1.        The bonds holding the solute particles together must be broken.  This bond-breaking step requires energy.

2.        The solvent surrounds the solute particles forming bonds (intermolecular or ionic) between the solute and solvent particles.  This process is sometimes called solvation, or if the solvent is water, it is called hydration.  Since this is a bond-forming step, this process releases energy.

3.        Finally, the cluster of solute and solvent particles are distributed evenly throughout the mixture.  This process requires energy. 

We get an idea as to the measure of whether the process requires energy or absorbs it by measuring the heat of solution, (the heat energy absorbed or released when a solution is formed).  If the heat of solution is exothermic (negative), we can conclude that the energy releasing step - the energy released as the bonds form between the solute and solvent - must be greater than the energy requiring steps (breaking solute and solvent bonds and distributing the solvated solute particles.)  Here are some examples of different heats of solutions