A solvent is a substance that serves as the medium in which other substances, called solutes, dissolve to form a solution. Solvents are usually liquids in laboratory and industrial practice, although the concept also applies to solid solutions. The distinction between solvent and solute describes their roles in a mixture rather than an intrinsic property of either substance. A solvent is commonly the predominant component, but this is not an absolute requirement; more than one component may be treated as a solvent. (goldbook.iupac.org)
Dissolution and molecular interactions
Dissolution depends on the balance between solute–solute, solvent–solvent, and solute–solvent attractions. Separating particles of the original substances requires energy, while establishing new attractions releases energy. Consequently, dissolution may absorb or release heat. Gases, liquids, and solids can all dissolve in liquid solvents: oxygen, alcohol, and sugar, for example, form solutions in water. (openstax.org)
The stabilizing interaction between a solvent and a dissolved species is called solvation. It can involve electrostatic attraction, van der Waals forces, and more specific interactions such as hydrogen bonding. Solvent molecules may surround an ion or a neutral molecule, producing a local environment that differs from the bulk liquid. Solvation by water is commonly called hydration. (goldbook.iupac.org)
Dissolution is not governed by attraction alone. Within thermodynamics, the tendency toward dispersal of matter also contributes to solution formation. Some substances mix readily even when mixing produces little temperature change, because their intermolecular attractions are similar and dispersal favors the mixed state. Conversely, favorable solute–solvent interactions do not imply unlimited dissolution. (openstax.org)
Classification and solvent properties
Solvents are frequently classified by polarity, broadly understood as their ability to interact with and stabilize dissolved species. Polar solvents generally dissolve many polar substances, whereas nonpolar solvents often dissolve nonpolar substances. The familiar rule “like dissolves like” is useful as an initial approximation, not a universal law: solvent polarity encompasses several distinct interactions and cannot always be represented adequately by a single number. (openstax.org)
A second distinction concerns hydrogen-bond donation. Solvents traditionally called protic, including water and many alcohols, can donate hydrogen bonds. Those commonly called aprotic, such as dimethyl sulfoxide, do not donate strong hydrogen bonds in the same way, although they may accept them. IUPAC terminology emphasizes explicit descriptions of hydrogen-bond donation and acceptance because “aprotic” can misleadingly suggest that a substance cannot lose a proton under any conditions. (old.goldbook.iupac.org)
Organic solvents include hydrocarbons, alcohols, ethers, esters, ketones, amines, and chlorinated compounds. These chemical families are not interchangeable: their members differ in solvent power and hazards. Practical solvent assessment therefore considers molecular interactions alongside physical properties, process safety, health effects, and environmental performance. (archive.cdc.gov)
Solubility and miscibility
Solubility is the maximum concentration of a solute attainable at equilibrium under specified conditions. A saturated solution is at this limit; additional solute does not necessarily dissolve. Solubility depends on the identities of both substances and on conditions such as temperature and pressure. Many solids become more soluble as temperature rises, but exceptions occur. Gas solubility in liquids commonly increases with gas pressure and decreases with increasing temperature. (openstax.org)
Miscibility describes the mutual solubility of liquids. Fully miscible liquids mix in all proportions. Liquids described as immiscible form separate phases, although their mutual solubility is usually small rather than exactly zero. Water and many hydrocarbon liquids illustrate this behavior. Thus, the terms “soluble” and “insoluble” often describe degrees of solubility rather than an absolute boundary. (openstax.org)
Chemical and industrial functions
A solvent can influence a chemical reaction by stabilizing reactants, products, and the transition state to different extents. This changes reaction rates and equilibria; the solvent is therefore not simply an inert container. Its interactions can also affect the absorption of light by dissolved species, making the solvent environment relevant to spectroscopy. (goldbook.iupac.org)
Solvents are widely used in extraction, dry cleaning, paints, and chemical processing. Laboratory purification methods include extraction, recrystallization, and distillation, whose performance depends on factors such as solvent volume and boiling-point differences. In chromatography, solvent systems can also be designed with subsequent separation and recovery in mind. Research on acetone–heptane mixtures, for example, has examined chromatography together with solvent recycling. (acs.org)
Safety and environmental considerations
Solvent hazards vary substantially. Many organic solvents are volatile or flammable, and occupational exposure to some can affect the nervous system. Acute effects may include dizziness, headache, drowsiness, and impaired coordination. These properties cannot be inferred from the label “organic solvent” alone; they depend on the specific substance and exposure conditions. (acs.org)
Green chemistry treats solvent use as a major opportunity to reduce hazardous substances, waste, and energy consumption. Assessment includes both chemical performance and life-cycle impacts, rather than identifying a universally preferable solvent. Alternatives include water-based processes and tunable media such as supercritical carbon dioxide, whose solvent properties change with temperature and pressure. Their suitability remains process-dependent: supercritical carbon dioxide, for instance, requires high-pressure equipment, while solvent recovery itself can carry significant energy costs. (epa.gov)