When a solute dissolves, its particles become dispersed among solvent particles. The surrounding interactions between solute and solvent are called solvation; when the solvent is water, the process is often called hydration.

Dissolution rearranges several kinds of interaction:

1. solute particles must separate from one another, which generally requires energy;
2. solvent particles must make room for the solute, which also generally requires energy;
3. new solute-solvent attractions form, which releases energy.

The balance among these contributions determines whether the solution warms or cools as dissolution occurs. A large energy cost for separating the original particles can be offset by strong attractions formed in the mixed state.

Molecular interactions therefore help predict compatibility. Polar molecules tend to interact strongly with other polar molecules, nonpolar molecules can be compatible through dispersion forces, and ions can be stabilized by ion-dipole attractions in polar solvents.

For example, when sodium chloride dissolves in water, water molecules orient their partial charges around the separated ions. Oxygen-rich ends point preferentially toward $\mathrm{Na^+}$, while hydrogen-rich ends point toward $\mathrm{Cl^-}$. These hydration interactions help compensate for the energy required to separate ions from the crystal.

By contrast, nonpolar hexane cannot stabilize separated ions nearly as effectively, so sodium chloride has very low solubility in hexane.

The phrase “like dissolves like” is a useful first heuristic for matching interaction types, not a fundamental law. The heat effect of mixing alone is also not enough to decide whether a solution forms: the number of microscopic arrangements available to the mixed system matters as well. A later thermodynamic treatment makes that competition quantitative.