A chemical solution is a uniform, homogenous mixture in which one or more substances (the solute) are molecularly dispersed within another substance (the solvent). In practice, the solvent is often a liquid such as water, but solutions can form between any states of matter: gases in gases, gases in liquids, liquids in liquids, solids in liquids and solids in solids. Everyday examples include salt or sugar dissolved in water and carbon dioxide dissolved in soft drinks; other cases are alloys like brass, which are solid solutions.
Classification and common terms
Solutions are described by their concentration (how much solute is present relative to solvent) and by saturation status: unsaturated (more solute can dissolve), saturated (in equilibrium with undissolved solute) or supersaturated (metastable, containing more dissolved solute than at equilibrium). A concentrated solution has a relatively high solute fraction, while a dilute solution has a low one. Specific quantitative measures include molarity, molality and mass percent.
Types of solutions
- Gaseous solutions: the atmosphere is a mixture of gases — a gas in a gas (air is a practical example).
- Liquid solutions: solids or gases dissolved in liquids, e.g., salt in water or carbon dioxide in soda.
- Liquid–liquid solutions: miscible liquids such as ethanol and water form a single phase.
- Solid solutions: alloys and many minerals where atoms substitute or interstitially occupy a crystal lattice.
Factors affecting solubility and behavior
Whether and how much a substance dissolves depends on temperature, pressure (important for gases, governed by Henry's law), and intermolecular forces — commonly summarized as “like dissolves like”: polar solvents dissolve polar solutes, nonpolar solvents dissolve nonpolar solutes. Temperature often increases solubility of solids in liquids, but can decrease solubility of gases. The nature of solute and solvent, and the presence of other dissolved species, also modify solubility.
Properties and important phenomena
True solutions are optically clear and do not scatter light (no Tyndall effect), unlike colloids. They obey laws of ideal mixtures approximately; for volatile components Raoult's law relates partial vapor pressure to composition. Several colligative properties depend only on solute particle number: vapor‑pressure lowering, boiling‑point elevation, freezing‑point depression and osmotic pressure. These effects are widely used — for example, freezing‑point depression explains why salt melts ice on roads and osmotic pressure underlies dialysis and water balance in biology.
Measurement, preparation and applications
Concentration can be measured by volumetric, gravimetric or instrumental methods. Solutions are prepared by dissolving measured solute in solvent and mixing until homogeneous. Applications span chemistry and industry (reaction media, solvents, electroplating), biology and medicine (physiological saline, intravenous solutions), food and beverages (sugar or gas dissolved in drinks), metallurgy (alloys) and environmental science (dissolved gases in natural waters). Practical notes include methods to enhance dissolution (stirring, heating, increasing surface area) and distinctions between solutions and other mixtures such as suspensions or colloids.
For focused guides and data on specific types of solutions, concentration units, or laws that quantify behavior, see introductory textbooks and reference materials on physical chemistry and solution thermodynamics: examples include entries on mineral solid solutions, the formal definition on solvent roles, experimental protocols for preparing solutions, practical summaries for handling gases in solution (CO₂), and specialist resources on solute interactions.
Historically, foundational concepts and quantitative descriptions were developed in the 19th century by researchers such as Raoult and van ’t Hoff, who related solution composition to vapor pressure and osmotic effects; modern physical chemistry extends these ideas to nonideal and complex mixtures. For further reading, consult general chemistry introductions and specialized texts that cover concentration units, equilibrium with undissolved phases, and the thermodynamics of mixing (water as a solvent is often used in examples).
Distinguishing features of solutions — molecular homogeneity, reproducible colligative behavior and dependence on intermolecular forces — make them a central concept in chemistry, bridging laboratory practice, industrial processes and natural systems.