Indissolubility

Indissolubility is the property of a substance that does not dissolve in a given solvent under specified conditions. In common usage, it is closely related to insolubility or sparing solubility: a substance with indissolubility has an extremely low solubility, such that the dissolved amount is negligible for practical purposes.

Solubility depends on temperature, pressure, solvent polarity, and the presence of other ions or ligands. For many solids in water, solubility can be described by the solubility product constant (Ksp). Dissolution proceeds until the ion product reaches Ksp, after which the solution is effectively saturated. Substances with very small Ksp values are described as practically insoluble; even with continued contact with the solvent, only a tiny fraction dissolves.

Examples include silver chloride (AgCl), barium sulfate (BaSO4), and calcium sulfate (CaSO4) at room temperature, which are often regarded as insoluble in water. Organic polymers such as polytetrafluoroethylene (PTFE) show indissolubility in most common solvents. In contrast, certain solvents or conditions (acids, bases, complexing agents) can increase apparent solubility, removing indissolubility under those conditions.

In practice, indissolubility is a contextual term. It guides pharmaceutical formulation, environmental fate modeling, and materials design. It does not imply an absolute property; changing the solvent, temperature, or chemical environment can alter solubility. Measurement is typically reported as maximum soluble amount per unit solvent at a defined temperature, or as a Ksp value.