Solubility

A solution is a homogeneous mixture of two main substances: the solute and the solvent. The solvent is present in greater quantity than the solute.

In a dilute solution, the proportion of solute relative to the solvent is small, whereas for a concentrated solution, the proportion is large.

When forming a solution, the solute and the solvent do not react chemically. Instead, the molecules redistribute, with individual solvent molecules surrounding the solute molecules and interacting through attractive forces. Such a dissolution process is called solvation.

To favor solvation, the solute–solvent interactions should be stronger than the solvent–solvent and solute–solute interactions.

In a solution, the solute and solvent can be any state of matter. The solution itself is a single “phase”.

The opposite of solvation is precipitation, triggered by strong solute–solute interactions. When the rate of solvation equals the rate of precipitation, a solubility equilibrium is established.

A solute’s solubility is its maximum possible concentration at solubility equilibrium in a given amount of solvent. Solubility is affected by temperature and other physical conditions like molecular polarity.

Nonpolar solutes dissolve in nonpolar solvents through intermolecular London dispersion forces, whereas polar or ionic solutes dissolve in polar solvents through ion–dipole or dipole–dipole interactions.

Water is a polar solvent. Hydrophilic, or water‑loving, compounds are water‑soluble due to the ionic charges or polar groups that enable sufficiently strong water–solute interactions. They are less soluble in nonpolar solvents because the solute–solute interactions are much stronger than any possible interaction between nonpolar solvent and polar solute molecules.

Water‑insoluble, fat‑soluble nonpolar compounds like oils are hydrophobic, or water-fearing, as they cannot form hydrogen bonds to compete with solvent–solvent interactions.

Amphiphilic or amphipathic compounds have both polar and nonpolar groups. For example, soaps are metal salts of fatty acids. In water, the soap molecules aggregate in spheres called micelles. Their nonpolar hydrophobic fatty acid tails point inwards while their polar hydrophilic heads are outside.

When removing oil stains using soap, the tails surround the nonpolar oil to form water-soluble micelles, allowing the oil to be washed away.