The common belief that the term “solution” refers only to a substance dissolved in water or another liquid often stems from everyday experiences with sugar or salt. This limited view fails to capture the true breadth of a chemical solution, which is not confined by the liquid state of matter. The chemical definition is far more expansive, encompassing mixtures where the components can be gases, liquids, or solids. This article explores the precise chemical meaning of a solution and demonstrates how solutions can exist entirely without a liquid component.
Defining the Chemical Solution
A solution is formally defined as a homogeneous mixture of two or more substances. The term “homogeneous” is key, meaning the composition and properties of the mixture are uniform throughout, even when viewed at a molecular level. Unlike simple mixtures, the individual components of a solution do not settle out or separate over time. This uniform dispersion is what distinguishes a true solution from a suspension or a colloid.
Solutions are composed of two primary parts: the solute and the solvent. The solute is the substance that is dissolved, typically present in the smaller amount within the mixture. Conversely, the solvent is the substance that does the dissolving, and it is usually the component present in the greatest quantity. For example, in a mixture of table salt and water, the salt is the solute and the water is the solvent.
The Role of Phase in Solution Formation
The physical state, or phase, of the solvent determines the phase of the resulting solution. Solvents are not restricted to the liquid phase, as they can also be solids or gases, which greatly expands the possibilities for solution formation. The resulting solution will always take on the physical state of the solvent. For instance, a solid acting as the solvent will yield a solid solution, regardless of whether the solute was a gas, liquid, or solid.
Considering the three phases of matter—gas, liquid, and solid—there are nine possible combinations of solute and solvent phases. Since all gases are completely miscible, a gas dissolved in a gas always forms a solution. Liquid solvents, such as water, can dissolve solutes that are gas (carbon dioxide in soda), liquid (alcohol in water), or solid (salt in water).
Solutions Without Liquids: Examples
Gaseous solutions are the most common non-liquid examples, with air being the most familiar. Air is a homogeneous mixture where nitrogen gas acts as the solvent, dissolving other gaseous components like oxygen, argon, and carbon dioxide. The molecules of these different gases are completely and uniformly dispersed throughout each other, satisfying the definition of a solution.
Another major category is solid solutions, which are formed when a solid acts as the solvent. Alloys, such as brass and bronze, are prime examples of solid-in-solid solutions. Brass is a solution of zinc (the solute) dissolved within a crystalline matrix of copper (the solvent). Bronze is similarly a solution of tin dissolved in copper. In these alloys, the atoms of the solute are uniformly dispersed within the atomic structure of the solvent metal.
Other solid solutions include a gas dissolved in a solid, such as hydrogen gas absorbed into the metal palladium. This process, where hydrogen atoms are interspersed within the palladium’s crystal lattice, demonstrates the solvent capacity of a solid.