A pure substance is a material that consists entirely of only one type of particle, whether that particle is an atom or a molecule. Understanding these substances is foundational to all chemical and physical sciences because they provide a constant, predictable reference point for studying how matter behaves. Recognizing the characteristics of a pure substance allows scientists to accurately measure and reproduce experimental results.
Defining Pure Substances
The defining feature of a pure substance is its absolute uniformity; its composition is identical throughout the entire sample. This homogeneity ensures that every part of the substance has the same arrangement of atoms or molecules. For instance, distilled water is exactly $\text{H}_2\text{O}$ everywhere. This uniform composition results in highly predictable and constant physical properties.
A telling characteristic is the presence of fixed points for phase changes, such as melting and boiling. A pure substance will melt or boil at a single, specific temperature under standard atmospheric pressure. For example, pure gold melts consistently at 1,064 degrees Celsius, and pure water boils at exactly 100 degrees Celsius at sea level. Chemists often use these fixed temperatures to determine a substance’s purity.
This constancy is a direct result of the identical nature of all the particles within the sample. Because all the molecules are the same, they require the exact same amount of energy to overcome the attractive forces that hold them together, leading to a sharp, fixed temperature for any phase transition. This consistency provides a clear chemical fingerprint for identification. A pure substance cannot be separated into simpler components using any physical means, such as filtration or evaporation.
Elements and Compounds
Pure substances are organized into two categories: elements and compounds. Elements are the simplest form of pure substance, composed of only one type of atom. They cannot be broken down into anything simpler by ordinary chemical methods. Examples include elements like oxygen ($\text{O}_2$) or metallic elements like gold ($\text{Au}$) and aluminum ($\text{Al}$).
The atomic number, the count of protons in the nucleus, specifically defines each element; there are currently 118 recognized on the periodic table. Even when an element undergoes a physical change, such as melting or boiling, its fundamental atomic structure remains unchanged.
Compounds are also pure substances, but they are made up of two or more different elements chemically combined in a fixed, definite ratio. For example, water ($\text{H}_2\text{O}$) is a compound where two hydrogen atoms are always bonded to one oxygen atom. This fixed ratio maintains the compound’s uniform composition.
The properties of a compound are entirely different from the properties of the individual elements that compose it. For example, sodium chloride (table salt, $\text{NaCl}$) is a stable solid formed from sodium, a highly reactive metal, and chlorine, a poisonous gas. To separate a compound back into its constituent elements requires a chemical reaction, which is a significant difference from the separation methods used for mixtures.
Distinguishing Pure Substances from Mixtures
The fundamental distinction between a pure substance and a mixture lies in the consistency of their composition and the behavior of their physical properties. Unlike the fixed, definite composition of a pure substance, a mixture consists of two or more pure substances that are physically combined in variable amounts. Air, for instance, is a mixture of nitrogen, oxygen, and other gases, but the precise ratio of these components changes depending on location, altitude, or pollution levels.
This variability in composition is directly responsible for the non-constant physical properties seen in mixtures. While a pure substance has a single, fixed boiling point, a mixture will boil or melt over a range of temperatures. Saltwater, for example, will not freeze at the same specific temperature as pure water; instead, its freezing point is depressed, and it transitions to a solid state gradually over a temperature range that is dependent on the salt concentration.
The method required to separate the components also provides a clear chemical distinction. The components of a mixture are not chemically bonded, so they can be separated using relatively simple physical means. These techniques include methods like filtration, evaporation, or distillation, which exploit differences in the physical properties of the components. Separating a compound requires breaking the chemical bonds between the atoms, a process that is significantly more complex and energy-intensive than physical separation.
In summary, a pure substance is defined by its uniformity, fixed chemical formula, and constant physical properties. A mixture is characterized by variable composition, a range of physical properties, and the ease with which its components can be physically separated.