A solute is a substance that is dissolved or dispersed at the molecular, ionic or atomic level within a solvent to form a solution. In a conventional solution, the solute is often the component present in the smaller amount, while the solvent is present in the larger amount, although this is a useful convention rather than an absolute definition for every mixture. In salt water, sodium chloride is the solute and water is the solvent.
In a sugar solution, sugar molecules are the solute and water is the solvent. Solutes can be solids, liquids or gases, and they can remain molecularly intact or dissociate, ionise or react after entering the solvent. The chemical identity and physical behaviour of the solute influence solubility, concentration, conductivity, osmotic pressure, boiling point elevation, freezing point depression and many other properties of the solution.
When a solute dissolves, its particles interact with the solvent. For molecular solutes, these interactions may include hydrogen bonding, dipole dipole attraction, dipole induced dipole attraction and dispersion forces. For ionic solutes, solvation stabilises separated ions. Sodium chloride provides a useful example: the crystal contains a lattice of sodium and chloride ions, but when it dissolves in water, individual hydrated ions become dispersed throughout the aqueous phase.
The term solute therefore describes the substance being incorporated into the solution, not necessarily the physical form it had before dissolving. A solid solute can become individual molecules or ions in solution, and a gaseous solute such as oxygen can become dissolved molecular O2. The microscopic state of the solute in the solution must therefore be distinguished from its original physical state.
Solutes can be electrolytes or nonelectrolytes. Electrolyte solutes produce mobile ions in solution and therefore allow the solution to conduct electricity. Soluble ionic compounds are generally strong electrolytes when they dissociate extensively, while weak acids and weak bases are weak electrolytes because they establish ionisation equilibria. Nonelectrolyte solutes, such as many sugars, dissolve as electrically neutral molecules and do not produce large concentrations of mobile ions.
This distinction is important because two substances can have similar concentrations but produce very different electrical conductivities. The behaviour of a solute therefore depends not only on how much dissolves but also on what chemical species are actually present after dissolution. Chemical reactions with the solvent can further complicate the picture, as occurs when acids donate protons to water or bases accept them.
The concentration of a solute can be expressed in several ways. Molarity is the number of moles of solute per litre of solution and is widely used in laboratory calculations. Molality is the number of moles per kilogram of solvent and is particularly useful when temperature changes could alter solution volume. Mole fraction expresses the amount of a component relative to the total number of moles of all components.
Mass concentration can be expressed in grams per litre, while percentage concentrations can be based on mass, volume or mass per volume depending on the convention. These quantities are not interchangeable without considering density, molecular mass and the precise definition being used. A statement that a solution contains a certain percentage of solute is therefore incomplete unless the type of percentage is specified.
The amount of solute that can dissolve is limited by solubility under specified conditions. An unsaturated solution contains less solute than the equilibrium solubility limit, while a saturated solution is at equilibrium with undissolved solute under the stated conditions. A supersaturated solution contains more dissolved solute than the stable equilibrium amount and can crystallise when disturbed. Solubility depends on the identity of both solute and solvent, temperature, pressure and the presence of other dissolved species. For gases, pressure can have a particularly strong influence. For ionic compounds, common ion effects can decrease solubility, while complex formation can increase apparent solubility by consuming free ions. Consequently, solute concentration and solubility are related but distinct concepts.
Solutes can profoundly alter the physical properties of solutions. Colligative properties depend primarily on the number of dissolved particles rather than their chemical identity, although electrolyte dissociation affects the effective particle count. Adding a nonvolatile solute can lower the solvent's vapour pressure and raise its boiling point while lowering its freezing point. Dissolved particles also create osmotic pressure, which is particularly important in biological systems. Electrolyte solutions can additionally affect electrical conductivity and ionic strength. The extent to which a solute changes these properties depends on concentration, dissociation, association and interactions with the solvent. These effects form the basis of antifreeze formulations, desalination principles, osmometry, biological fluid regulation and many analytical methods.
Solutes are important in nearly every branch of applied chemistry. Nutrients such as nitrate, phosphate and potassium ions are dissolved in soil water and supplied to plants. Oxygen and carbon dioxide dissolve in water and participate in aquatic ecosystems. Pharmaceuticals are formulated as solutions or dissolved during processing. Industrial reactions often occur with reactants dissolved in a solvent because molecular contact and transport can be improved.
Dyes, fragrances, salts, acids, bases and cleaning agents can all function as solutes in appropriate formulations. Environmental chemistry frequently measures contaminants as dissolved solutes in water. In medicine, the concentration and identity of dissolved solutes are critical because changes in electrolyte concentrations can alter osmotic balance, membrane potentials and cellular function.
A solute should not be confused with a substance that merely appears to disappear. Dissolution means the substance becomes dispersed within the solvent, but chemical reactions can transform it into new species. Nor is a solute necessarily always a solid. Oxygen dissolved in water is a gaseous substance acting as a solute, while ethanol can act as a solute in an aqueous mixture despite itself being a liquid.
In concentrated mixtures, identifying one component as the solvent and another as the solute can become a matter of convention. The scientifically important ideas are composition, phase, interactions and chemical speciation. Understanding solutes therefore requires considering how substances enter solutions, what species they form, how much can dissolve, how concentration is measured and how the dissolved particles affect the physical and chemical behaviour of the entire system.
