A pure substance in chemistry represents a fundamental category of matter with a uniform and unchanging composition. Unlike the everyday definition of “pure,” which may mean clean or natural, the scientific meaning refers to a constant chemical identity throughout the material. This concept is foundational to understanding how matter behaves and interacts in a predictable way. The chemical makeup is fixed, meaning every sample, regardless of its size or origin, will be exactly the same.
Defining Constant Composition
The core attribute that defines a pure substance is its constant, uniform chemical composition. This means the substance consists of only one type of particle, whether that particle is an atom or a molecule. For example, distilled water is composed exclusively of \(\text{H}_2\text{O}\) molecules, and the ratio of two hydrogen atoms to one oxygen atom is fixed. This uniformity ensures that any part of the sample has the same properties as any other part. The fixed ratio is maintained even if the substance changes its physical state, such as when water freezes into ice or boils into steam.
The Two Categories: Elements and Compounds
Pure substances are divided into two distinct categories: elements and compounds. Elements are the simplest form of pure substance and consist of only one type of atom. They cannot be broken down into simpler substances by any ordinary chemical process, making them the fundamental building blocks of all matter. Examples include gold (\(\text{Au}\)), oxygen (\(\text{O}_2\)), and carbon (\(\text{C}\)).
Compounds represent the second type of pure substance, formed when two or more different elements are chemically bonded together in a fixed ratio. The resulting compound has properties entirely different from the elements that formed it, such as how hydrogen and oxygen gases combine to form liquid water. Compounds like carbon dioxide (\(\text{CO}_2\)) or table sugar (\(\text{C}_{12}\text{H}_{22}\text{O}_{11}\)) can only be separated back into their constituent elements through a chemical reaction, such as using electricity to break water down into hydrogen and oxygen.
How Pure Substances Differ From Mixtures
The concept of a pure substance is best understood when contrasted with a mixture, which is a physical blend of two or more substances. The defining difference is that a mixture possesses a variable composition, meaning the amount of each component can be changed. For instance, saltwater is a mixture because the ratio of salt to water can vary, resulting in a different concentration each time.
Another distinction lies in the method of separation required to isolate the components. Mixtures can be separated back into their original pure substances using physical methods, because the components retain their individual properties. Techniques like evaporation, filtration, or magnetism can be used to separate a mixture. In contrast, pure substances, specifically compounds, require a chemical reaction to break the bonds holding the elements together, a far more intensive process.
Key Observable Properties
Pure substances exhibit measurable physical characteristics that are fixed and consistent, which allows scientists to verify their purity in a laboratory setting. For any given pure substance, its melting point—the temperature at which it changes from a solid to a liquid—is a single, specific temperature. Similarly, the boiling point—the temperature at which it changes from a liquid to a gas—is also a constant value.
These fixed points are a direct consequence of the material’s constant composition and uniform particle structure. For example, pure water consistently melts at \(0^\circ\text{C}\) and boils at \(100^\circ\text{C}\) at standard atmospheric pressure. Mixtures, on the other hand, will melt and boil over a range of temperatures, not at a single point, because the presence of impurities disrupts the uniform structure. The addition of even a small amount of impurity will lower the melting point and broaden the temperature range over which the change of state occurs.