Why do solutes dissolve in solvents?

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.

LIKE DISSOLVES LIKE.

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.