A solution is definitively not a pure substance, but rather a specific type of mixture. The distinction lies in the concept of fixed composition, which is the defining characteristic of a chemically pure material. While a solution may appear uniform, its makeup differs fundamentally from the single, unvarying composition found in a pure substance.
What Defines a Pure Substance
A pure substance in chemistry is any form of matter that has a fixed and uniform chemical composition throughout. This definition is limited to two categories: elements and compounds. Elements, like gold (Au) or oxygen gas (\(\text{O}_2\)), consist of only one type of atom. Compounds, such as distilled water (\(\text{H}_2\text{O}\)) or table salt (NaCl), are also pure substances because they consist of two or more elements chemically bonded together in a fixed, definite proportion. A pure substance displays constant and distinct physical properties, such as a sharp, unchanging melting point and boiling point; for example, pure water always boils at \(100^{\circ}\text{C}\) at standard pressure.
Understanding Solutions as Homogeneous Mixtures
A solution is classified as a homogeneous mixture, a combination of two or more substances with a uniform appearance and composition at the molecular level, which often causes them to be confused with pure substances. Solutions consist of at least two components: a solute (the substance being dissolved) and a solvent (the substance doing the dissolving). The primary characteristic that disqualifies a solution from being a pure substance is its variable composition. The amount of solute dissolved in the solvent can be changed continuously up to the limit of solubility, demonstrating a non-fixed ratio between the components. Furthermore, a solution can be separated back into its original components by physical processes, such as evaporating the solvent to recover the solute.
Key Differences Between Solutions and Chemical Compounds
Compounds are formed when atoms chemically react and bond together in a specific, fixed ratio, resulting in a new substance whose properties are entirely different from the elements that formed it. For example, sodium and chlorine combine chemically to form the compound sodium chloride (table salt). Conversely, the components in a solution are merely physically intermingled and do not undergo a chemical reaction to form new chemical bonds. The solute and the solvent retain their individual chemical properties, even when uniformly dispersed. This lack of fixed proportions and the retention of individual identity is the basis for classifying solutions as mixtures and compounds as pure substances.