A pure substance is a material composed of only one type of element or compound, possessing a definite, unchanging chemical formula, such as pure gold or water. Its chemical makeup is consistent throughout. Confirming purity is foundational in chemistry, as any deviation means the material is a mixture. A mixture involves two or more substances combined physically, allowing their ratio to be varied. Verifying purity relies on observing the substance’s fixed physical characteristics, which are disrupted by contaminants.
Observation of Uniformity and Fixed Ratios
The most basic observation for identifying a pure substance is confirming its homogeneity. A pure substance must appear uniform at every point, having the same appearance and composition throughout the sample. This contrasts with a heterogeneous mixture, like sand and water, where components are visibly distinct. Even a homogeneous mixture, such as saltwater, looks uniform but lacks the defining feature of a pure compound: a fixed chemical ratio.
A compound is a pure substance because its constituent elements are chemically bonded in a precise, non-variable ratio, such as water (\(\text{H}_2\text{O}\)). This fixed proportion is an intrinsic property of the compound. Conversely, a homogeneous mixture, like bronze, can have a slightly different proportion of components from one batch to the next. The ability to change the relative amounts of the components while still maintaining the mixture confirms that a material is not a pure substance.
Quantitative Testing Using Physical Markers
The most definitive method for confirming purity involves measuring specific physical properties unique to the substance. Pure substances exhibit precise, established physical properties, regardless of the sample size. These properties include density, refractive index, and the temperature at which a phase change occurs.
Measuring a substance’s melting point (MP) or boiling point (BP) provides a measurable indicator of purity. A pure solid will melt sharply at one specific temperature, which is its known melting point. For example, pure ice melts at \(0^\circ \text{C}\). The presence of impurities interrupts the solid’s orderly structure, resulting in melting point depression.
An impure solid will melt over a range of temperatures, and the melting point will be lower than that of the pure substance. A pure liquid will boil at a single, fixed temperature, such as water’s boiling point of \(100^\circ \text{C}\). Contaminants cause boiling point elevation, meaning the liquid boils at a higher temperature and over a temperature range. Comparing the measured temperature against the known value provides a quantitative assessment of purity.
Verification through Inability to Separate Physically
A practical way to verify purity is by testing resistance to common physical separation techniques. Mixtures are defined because their components are only physically combined and can be separated by physical means. This includes techniques such such as:
- Filtration
- Distillation
- Evaporation
- Magnetic separation
If a material is a pure substance, it cannot be broken down into other materials using these physical processes. For instance, boiling pure water converts the entire sample to steam, which can be condensed back into water, leaving no residue. If the water contained salt (a mixture), boiling would separate the water from the salt, which would remain in the vessel.
A compound can only be broken down into its constituent elements through a chemical reaction, such as using electricity to decompose water. The inability to use non-chemical, mechanical, or thermal methods to isolate different components confirms the material consists of only one pure element or compound. This confirms the substance has a single, uniform identity.