A mixture results when two or more substances are physically combined without a chemical reaction. Unlike a compound, where elements are chemically bonded in a fixed ratio, mixtures retain the individual properties of their components. Mixtures can be homogeneous (uniform) or heterogeneous (non-uniform) and are separated using physical processes. The ability to isolate these components is a fundamental practice across science and industry. The choice of separation method depends entirely on the physical or chemical properties that distinguish the components within the mixture.
Separating Components by Physical Size and State
Simplest separation methods exploit physical differences like particle size, density, or physical state, often applied to heterogeneous mixtures.
Filtration uses a porous barrier, such as filter paper, to separate an insoluble solid from a liquid or gas. The liquid portion (filtrate) passes through the pores, while the solid remains trapped on the filter medium.
Decantation relies on density and the force of gravity, where a liquid is carefully poured off a settled solid. This method is effective when there is a significant density difference and the solid particles settle quickly.
For mixtures containing very fine particles, such as blood, centrifugation accelerates the separation process. A centrifuge spins the mixture rapidly, creating a strong centrifugal force that pushes the denser components to the bottom much faster than gravity alone could.
Separating Components by Thermal Properties
Thermal methods exploit differences in how mixture components respond to heat, specifically their boiling points or tendency to vaporize.
Distillation separates miscible liquids with sufficiently different boiling temperatures. The mixture is heated until the most volatile component (the one with the lowest boiling point) turns into a vapor.
The purified vapor is directed into a condenser, cooled by flowing water, causing the gas to return to its liquid state for collection. Simple distillation is effective when boiling points differ by more than 150°C, yielding a relatively pure distillate.
When boiling points are closer together, fractional distillation is necessary. This technique incorporates a fractionating column between the flask and the condenser, providing a large surface area for repeated vaporization and condensation cycles. As the vapor rises, it becomes progressively enriched in the more volatile component, allowing for cleaner separation of liquids with small boiling point differences.
Sublimation is another thermal method, involving a phase change where a solid transitions directly into a gas without first becoming a liquid. This technique separates a solid component that readily sublimes (like iodine) from a non-subliming solid impurity by applying heat.
Separating Components by Solubility and Differential Affinity
Sophisticated separations leverage differences in chemical properties, particularly solubility and component affinity for various surfaces.
Extraction separates a component from a mixture by selectively dissolving it into a new solvent. In liquid-liquid extraction, the mixture is shaken with a second immiscible solvent, causing the desired compound to migrate to the solvent where it is more soluble.
Chromatography is a powerful family of techniques based on differential affinity—the varying degree to which components stick to a surface versus remaining dissolved in a moving fluid.
In all chromatographic methods, the mixture is introduced into a system with a stationary phase (fixed solid) and a mobile phase (moving liquid or gas). Components separate because those with a higher affinity for the stationary phase move slowly, while others prefer the mobile phase and travel quickly.
In column chromatography, the stationary phase is packed into a tube, and components partition between this fixed material and the flowing mobile phase. The components emerge at different times, allowing for their isolation and analysis.
Crystallization, often used for final purification, relies on the principle that a solid’s solubility decreases significantly as temperature drops. An impure solid is dissolved in a minimal amount of hot solvent. As the solution cools, the pure substance precipitates out as organized crystals, leaving impurities dissolved in the cold solvent.