Question

How do water molecules dissolve a salt?

Answer 1

Any solid, including salts, will dissolve in water when the attractive energy between the solute and the surrounding water molecules is great enough.

Explanation:

Salts dissolve as positive and negative ions. Water binds very strongly to ions because it has a strong dipole moment and because the protons on water are loosely bound and can move from molecule to molecule. This allows water to convert to `OH^-` or `H_3O^+` or even partially charged molecules. These ions can bind strongly to solute ions like `Na^+` or `Cl^-`.

In order to dissolve a salt, two ions must give up the binding energy associated with the crystal lattice. The water must also partially give up the attractive interactions between water molecules. But if the new attractive energy between the salt ions and the water molecules is larger than the energy given up, the dissolution of the ions will be exothermic, and this is usually enough to favor the dissolved state.

Entropy plays a role, too, and if the number of thermally accessible microstates of the dissolved system is larger than the undissolved state, the dissolved state will be favored.

The combination of changes in enthalpy and entropy is called Gibbs Free Energy:
`DeltaG = DeltaH - TDeltaS`

When `DeltaG` is negative, the reaction (dissolving) is favored. Normally, the enthalpy component (`DeltaH`) dominates, so exothermic reactions are almost always favored to form products, but there are rare cases where entropy change (`DeltaS`) plays a bigger role.