A pure substance is matter that possesses a constant composition and properties that remain uniform throughout the sample. It is either a single element (like gold) or a compound (like distilled water), but never a blend of different materials. Identifying material purity is a foundational practice in science and industry, ensuring reliability in manufacturing and engineering. Pure substances exhibit fixed, reproducible characteristics under controlled conditions, which determines their effectiveness and predictable behavior.
Defining Purity Through Fixed Characteristics
A substance is defined as pure if it contains only one type of particle, such as an atom or a molecule. Pure substances are categorized as elements (one kind of atom) or compounds (molecules chemically bonded in fixed proportions). The most reliable indicator of purity is the invariance of physical properties, meaning these characteristics do not change regardless of where the substance is sampled.
These fixed characteristics are known as intensive properties, meaning they do not depend on the sample size. For instance, a pure substance always has a specific, measurable density, regardless of the sample volume. This predictable behavior provides a reliable standard against which any sample can be measured. Their definite and uniform composition results in fixed values for their melting and boiling points.
Key Physical Tests for Identification
The most direct way to assess purity is by precisely measuring fixed physical properties and comparing them to established reference values. For solids, Melting Point Determination records the temperature at which the solid transforms into a liquid. A perfectly pure solid melts sharply over a very narrow temperature range, often less than one degree Celsius. Impurities disrupt the crystal lattice structure, causing the melting point to be lowered and occur over a broader temperature range.
For liquids, Boiling Point Determination is the corresponding method used to gauge purity. A pure liquid boils at a constant, specific temperature at a given atmospheric pressure as it changes phase to a gas. Impurities cause boiling point elevation, meaning the substance begins boiling at a temperature higher than the reference value. This occurs because impurities decrease the liquid’s vapor pressure, requiring more energy for vaporization.
Another technique involves measuring the sample’s Density, defined as mass per unit volume. Since a pure substance has a constant composition, its density is a fixed, reproducible value at a specified temperature and pressure. Any deviation from the known density suggests the presence of contaminants or a variation in composition. These measurements provide quantifiable data points to confirm alignment with the standardized properties of a pure material.
How Mixtures Differ from Pure Substances
Mixtures are physical combinations of two or more elements or compounds that are not chemically bonded together. Unlike pure substances, mixtures do not possess a fixed chemical structure and can be formed in any ratio of components. This variable composition causes their physical properties to be inconsistent and unpredictable.
While a pure substance has a sharp, single melting point, a mixture melts and boils over a range of temperatures. The physical properties of a mixture, such as density, change depending on the concentration of its constituent parts. The presence of multiple components means the sample’s behavior is not uniform throughout.
A definitive way to distinguish a mixture from a pure substance is by examining whether it can be separated by physical methods. Mixtures can be broken down into their individual components using processes like distillation, filtration, or chromatography. In contrast, a pure substance cannot be separated into simpler components by any physical means; if it is a compound, breaking its bonds requires a chemical reaction.