The dissolution of gases, liquids, or solids into a liquid or other solvent is a process by which these original states become solutes (dissolved components), forming a solution of the gas, liquid, or solid in the original solvent. Solid solutions are the result of dissolution of one solid into another, and occur, e.g., in metal alloys, where their formation is governed and described by the relevant phase diagram. In the case of a crystalline solid dissolving in a liquid, the crystalline structure must be disintegrated such that the separate atoms, ions, or molecules are released. For liquids and gases, the molecules must be able to form non-covalent intermolecular interactions with those of the solvent for a solution to form. The free energy of the overall, isolated process of dissolution must be negative for it to occur, where the component free energies contributing include those describing the disintegration of the associations holding the original solute components together, the original associations of the bulk solvent, and the old and new associations between the undissolved and dissolved materials. Dissolution is of fundamental importance in all chemical processes, natural and unnatural, from the decomposition of a dying organism and return of its chemical constituents into the biosphere, to the laboratory testing of new, man-made soluble drugs, catalysts, etc. Dissolution testing is widely used in industry, including in the pharmaceutical industry to prepare and formulate chemical agents of consistent quality that will dissolve, optimally, in their target millieus as they were designed. Some distinctions can be made between solvation, dissolution, and solubility.

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