The free energy change associated with dissolving a solute in a liter of solvent is called the free energy of a solution, ΔGsolution. The overall ΔGsolution is expressed as the balance of ΔGinteraction against the always-favorable free-energy of mixing, ΔGmixing. Solution formation is favorable if ΔGsolution is less than zero, whereas it is unfavorable if ΔGsolution is greater than zero. In short, for a solution to form and complete dissolution to take place, the Gibbs energy change must be negative. The amount to which a solute dissolves in a solvent is governed by the strength of intermolecular forces between the components. This property of dissolution is termed as solubility.
In addition to the free energy of mixing, non-covalent interactions also contribute to solution formation. Substances that form hydrogen bonds are termed polar substances. In such cases, solute particles' dipole–dipole attractions with solvent particles are as strong as that between molecules in a pure solute or solvent. Hence, the two kinds of molecules mix easily. Likewise, nonpolar liquids are miscible with each other because there is no appreciable difference in the strengths of solute–solute, solvent–solvent, and solute–solvent intermolecular attractions. If a solute and solvent pair have intermolecular attractions similar to that in a pure liquid, ΔGinteraction is negligible. The overall ΔGsolution equals ΔGmixing, and the solute dissolves completely in the solvent. However, if two significantly different substances are mixed, the probability of solution formation depends on the balance between ΔGinteraction and ΔGmixing. They can form a solution even if ΔGinteraction is positive, provided it is not too high.
