A solution is a homogeneous mixture in which one or more substances are uniformly dispersed at the molecular, ionic or atomic scale within another substance or combination of substances. The substance present in the larger proportion is commonly called the solvent, while the substance dissolved within it is called the solute, although this terminology can be less straightforward for mixtures containing several components in comparable amounts. Solutions can exist as gases, liquids or solids.
Air is a gaseous solution containing nitrogen, oxygen, argon, carbon dioxide and other gases. Salt dissolved in water is an aqueous solution containing hydrated ions. Alloys such as brass can be regarded as solid solutions in which atoms of different elements occupy positions within a metallic structure. The defining feature is homogeneity at the relevant scale, meaning that the composition is uniform throughout a single phase.
In an aqueous solution, water molecules interact with dissolved particles and can separate and stabilise them. When sodium chloride dissolves, the crystal lattice breaks apart sufficiently for sodium and chloride ions to become surrounded by oriented water molecules. This process is called hydration and is a particular form of solvation. Polar water molecules have partially positive hydrogen regions and a partially negative oxygen region, allowing them to interact electrostatically with ions.
Molecular solutes can also dissolve through interactions such as hydrogen bonding, dipole dipole interactions and dispersion forces. Whether dissolution is favourable depends on the balance among solute solute, solvent solvent and solute solvent interactions, as well as entropy and temperature. A solution is therefore not simply a substance disappearing into another substance. The solute remains present as individual molecules, ions or particles dispersed throughout the solvent phase, although chemical reactions can occur simultaneously.
Solutions differ from suspensions and colloids. In a true molecular or ionic solution, the dissolved components are sufficiently dispersed that the mixture is homogeneous and does not normally separate under gravity. Suspensions contain much larger particles that can settle over time and can often be removed by filtration. Colloids occupy an intermediate range and can remain dispersed for long periods while scattering light through the Tyndall effect.
Milk, for example, is a colloidal dispersion rather than a simple molecular solution because its fat droplets and other components form dispersed phases. A sugar solution, by contrast, contains individual dissolved sugar molecules and is transparent under ordinary conditions. These distinctions matter in chemistry because the size and state of dispersed particles determine diffusion, filtration, light scattering, sedimentation and other physical behaviours.
Concentration describes how much solute is present relative to a specified amount of solution or solvent. Molar concentration, commonly written as molarity, is the number of moles of solute per litre of solution. Molality is the number of moles of solute per kilogram of solvent and is particularly useful in calculations involving temperature changes because mass does not expand with temperature in the same way volume does. Other measures include mass concentration, mole fraction, percentage concentration and parts per million.
A solution can be dilute or concentrated, but these terms are relative unless a numerical concentration is provided. Concentration should also not be confused with solubility. Solubility is the maximum equilibrium amount that can dissolve under specified conditions, whereas concentration describes the actual amount present. A solution can therefore be unsaturated, saturated or supersaturated depending on its relationship to the solubility limit.
A saturated solution contains the maximum equilibrium amount of a solute that can remain dissolved under specified conditions. If additional solute is added to a saturated solution, some may remain undissolved. An unsaturated solution contains less dissolved solute than the equilibrium maximum, while a supersaturated solution contains more dissolved solute than is normally stable under the same conditions and is therefore metastable. Supersaturated solutions can crystallise when disturbed or when a suitable nucleation site is introduced.
Solubility equilibria are dynamic because dissolution and crystallisation can occur simultaneously. At saturation, the rates of the opposing processes can become equal even though individual molecules or ions continue to move between solid and solution. This is a general example of dynamic equilibrium and illustrates why equilibrium does not mean that microscopic processes have stopped.
Solutions can undergo chemical reactions as well as physical dissolution. When an acid is dissolved in water, proton transfer to water may occur. When an ionic salt dissolves, ions become solvated and may subsequently participate in precipitation, complexation or acid base reactions. The solvent can therefore be chemically active rather than merely an inert container. Water undergoes self ionisation, and many solutes alter the equilibrium.
Electrolytes produce ions and allow solutions to conduct electricity, whereas nonelectrolytes such as many molecular organic compounds dissolve without producing significant concentrations of ions. Strong electrolytes are extensively ionised or dissociated in solution, while weak electrolytes establish equilibria between ionised and un-ionised forms. These distinctions influence conductivity, colligative properties, reaction rates and acid base behaviour.
Solutions are central to laboratory chemistry, biology, medicine, environmental science and industry. Blood plasma is a complex aqueous solution containing ions, small molecules, proteins and other dissolved substances, although its full structure includes colloidal and cellular components as well. Pharmaceutical solutions provide dissolved active ingredients in suitable solvents. Industrial processes use aqueous solutions for extraction, electrochemistry, cleaning, synthesis and separation. Environmental chemistry studies dissolved oxygen, nutrients, metals and pollutants in natural waters.
Solution chemistry is also fundamental to electrochemical cells because mobile ions participate in charge transport and redox reactions. The ability of a solvent to dissolve selected substances is exploited in recrystallisation, extraction and purification. Consequently, the concept of a solution connects microscopic molecular interactions with large scale chemical processing.
A solution should not be thought of as necessarily liquid. Gaseous and solid solutions are important, and even liquid mixtures can contain multiple solutes and solvents. Nor does dissolution always mean that the solute remains chemically unchanged. Some substances dissociate into ions, while others react with the solvent. The word homogeneous refers to uniform composition at the relevant scale, not to the absence of microscopic structure. Solvated molecules and ions continue moving, colliding and interacting constantly. A solution is therefore a thermodynamic and molecular system with measurable composition, activity, chemical potential and equilibrium properties. Understanding solutions requires distinguishing solvent from solute, concentration from solubility, dissolution from reaction, and homogeneous mixtures from colloids and suspensions.
