Why do solutes dissolve in solvents?

1 Answer
Apr 25, 2014

The major factor that determines whether solutes dissolve in solvents is entropy.

To form a solution we must:
1. Separate the particles of the solvent.
2. Separate the particles of the solute.
3. Mix the particles of solvent and solute.

Solutes and solvents with solution from img.sparknotes.com.

#ΔH_("soln") = ΔH_1 + ΔH_2 + ΔH_3#

#ΔH_1# and #ΔH_2# are both positive because it requires energy to pull molecules away from each other. #ΔH_3# is negative because intermolecular attractions are forming.

For the solution process to be favourable, #ΔH_3# should at least equal #ΔH_1 + ΔH_2#.

Nonpolar Solvent – Nonpolar solute

If both solvent and solute are nonpolar, all the #ΔH# values are small. The major factor then is the increase in entropy (disorder) that occurs when a solution forms. This is a favourable process.

Polar Solvent – Polar Solute

If both solvent and solute are polar, all the #ΔH# values are large but similar in size. The major factor again is the increase in entropy.


Polar Solvent – Nonpolar Solute
If a nonpolar solute such as oil mixes with a polar solvent like water, #ΔH_1# is large and positive. This outweighs #ΔH_3#. A solution does not form.