A pure compound represents a foundational concept in chemistry. Understanding its nature is the first step toward grasping how different substances interact and behave. This classification of matter is based on strict rules concerning composition and physical behavior.
Defining a Pure Compound
A pure compound is a substance made up of only one type of molecule or formula unit. This singular composition gives the compound a uniform structure. For a substance to be classified as a pure compound, it must obey the Law of Definite Proportions, meaning its constituent elements are always present in the exact same fixed ratio by mass.
This fixed relationship is described by stoichiometry, which dictates the precise whole-number ratio of atoms combining to form the compound. For example, a molecule of water is always composed of two hydrogen atoms and one oxygen atom (\(\text{H}_2\text{O}\)), and this 2:1 ratio never changes in a pure sample. The physical and chemical properties of the resulting compound are unique and entirely different from the properties of the individual elements that formed it.
A pure compound is distinguished from a pure element, which is the simplest form of matter made up of only one type of atom, such as gold or oxygen. A compound must contain two or more different elements that have been chemically bonded together. The elements within the compound cannot be separated by simple physical processes.
Physical Properties as Indicators of Purity
The purity of a compound is determined by observing its physical properties, which must be consistent and fixed. The uniformity of the molecules in a pure compound results in predictable and specific physical characteristics, particularly concerning phase changes. These properties, such as density, melting point, and boiling point, are specific to that substance and act as a unique chemical fingerprint.
A hallmark of a pure compound is its fixed and sharp melting point, which is the precise temperature at which the solid changes to a liquid. Similarly, a pure liquid will have a fixed boiling point, the exact temperature at which it converts to a gas under standard atmospheric conditions. The consistency of the intermolecular forces holding the identical molecules together permits this precise phase transition at a single temperature.
When a substance is impure, the presence of foreign particles disrupts the uniform intermolecular forces, causing the phase change to occur over a range of temperatures instead of at a single point. For example, while pure water boils sharply at \(100^\circ \text{C}\) and freezes at \(0^\circ \text{C}\), the addition of an impurity typically lowers the freezing point and elevates the boiling point. Scientists use the sharpness and accuracy of these phase change temperatures as a standard laboratory technique to assess a sample’s purity.
Pure Compounds Versus Mixtures
The most significant distinction between a pure compound and a mixture lies in the nature of the combination and the methods required for separation. A mixture is formed when two or more substances are physically combined, without any chemical bonds forming between the components. Unlike a pure compound, the components of a mixture retain their individual properties.
Mixtures do not adhere to a fixed chemical ratio, meaning the proportions of their components can be varied arbitrarily. For example, one can mix any amount of salt with any amount of sand, and the result is still a mixture. Because the components are only physically combined, mixtures can be separated back into their original substances using physical techniques like filtration, evaporation, or distillation.
In contrast, the components of a compound are chemically bonded, requiring a chemical reaction to break them down into their elements. Furthermore, mixtures do not possess fixed physical properties. They typically melt and boil over a range of temperatures, directly contrasting with the precise, sharp melting and boiling points of pure compounds.