When classifying matter, materials are separated into two major categories: pure substances and mixtures. A common point of confusion arises when considering solutions, which appear perfectly uniform to the naked eye. The core question is whether a solution—such as simple salt water or the air we breathe—is correctly categorized as a pure substance or if it belongs to the mixture category.
Defining Pure Substances
A pure substance represents matter that possesses a constant and definite chemical composition throughout the entire sample. This means that every part of the substance is identical in its makeup and properties. Pure substances are further divided into two types: elements and compounds.
Elements, like gold (Au) or oxygen gas (\(O_2\)), consist of only one type of atom and cannot be broken down into simpler substances by chemical means. Compounds, such as water (\(H_2O\)) or table salt (NaCl), are formed when two or more different elements are chemically bonded together in a fixed, specific ratio. A defining characteristic of both elements and compounds is that their components cannot be separated from each other using physical methods like filtration or evaporation.
Understanding Solutions
In contrast to pure substances, a solution is defined as a homogeneous mixture of two or more components. Homogeneous indicates that the mixture has a uniform appearance and composition at the microscopic level, with the components evenly distributed throughout.
A solution is always composed of a solvent and one or more solutes. The solvent is the component present in the largest amount, serving as the medium into which the other substance dissolves. Solutions are not limited to liquids; they can be mixtures of gases, like air, or solids, such as metal alloys like brass.
The Fundamental Distinction
Solutions are not considered pure substances because they fail to meet the strict chemical criteria for purity. The main reason for this distinction lies in their variable composition. Unlike a compound, which maintains a fixed ratio of elements, a solution’s concentration can be altered within the limits of solubility. For example, one can create a salt solution with a low concentration or a high concentration, demonstrating a non-fixed ratio.
Another difference relates to the method of separation. While compound components require a chemical reaction for separation, solution components can be separated by physical means. The solute can be recovered from the solvent through physical processes such as evaporation or distillation. This ability confirms that the substances in a solution are merely physically combined, classifying the solution as a mixture.