A mixture is formed when two or more different substances are physically combined, rather than chemically bonded. Separation relies on exploiting differences in their physical properties, such as size, density, solubility, or boiling point. The appropriate technique is chosen based on which physical property provides the greatest contrast between the substances. The goal is to isolate one or more pure components from the initial combination.
Separation Based on Particle Size and State
Mechanical separation techniques often target mixtures where components have distinct differences in physical dimensions or density. A primary method for separating an insoluble solid from a liquid is filtration, which employs a porous barrier, like filter paper or a membrane. The liquid component, known as the filtrate, passes through the pores while the larger solid particles are retained. This process works because the solid particles are physically larger than the microscopic channels within the filtering medium.
Another technique exploiting density differences is decantation, used for heterogeneous mixtures of a solid and a liquid, or two immiscible liquids. The mixture is allowed to stand, permitting the denser component to settle to the bottom (sedimentation). Once settled, the less dense liquid is carefully poured off, or decanted, leaving the sediment or the denser liquid layer behind.
For mixtures containing extremely fine solid particles that will not settle naturally, centrifugation is employed to accelerate the separation. A centrifuge spins the mixture rapidly, applying a high centrifugal force that pushes the denser components, often the solids, to the bottom. This technique is particularly useful for separating biological materials, such as blood cells from plasma, where particles are too small for simple decantation or quick filtration.
Magnetism is a non-mechanical method used when one component possesses ferromagnetic properties, like iron. By passing a strong magnet over the mixture, the magnetic material is selectively attracted and removed from the non-magnetic bulk. This technique is limited to separating magnetic substances from non-magnetic ones, such as iron filings from sand.
Separation Based on Boiling Point and Volatility
When components are fully dissolved or are miscible liquids, separation must exploit differences in their boiling points and volatility. Evaporation is the simplest thermal technique, used to separate a dissolved, non-volatile solid from a volatile liquid solvent. The solution is heated, causing the solvent to turn into a gas and escape, leaving the solid solute, such as salt from saltwater, behind.
For separating a volatile liquid from a non-volatile solute, or two liquids with vastly different boiling points, simple distillation is used. The mixture is heated until the most volatile component vaporizes. This vapor is then directed into a condenser, where it cools and turns back into a liquid, called the distillate, which is collected.
When the components of a liquid mixture have boiling points that are close together, simple distillation is ineffective for achieving high purity. Fractional distillation is required, using a specialized fractionating column positioned between the mixture and the condenser. As the mixed vapors rise, they undergo multiple cycles of vaporization and condensation on surfaces, referred to as theoretical plates.
This continuous re-distillation allows the vapor to become progressively enriched with the most volatile component (the one with the lowest boiling point) as it moves up the column. The less volatile components condense and fall back into the mixture. The most volatile component eventually reaches the top and passes into the condenser for collection. Fractional distillation is routinely applied in the petrochemical industry to separate crude oil into products like gasoline, kerosene, and diesel.
Advanced Techniques Exploiting Differential Properties
For complex mixtures requiring high-purity separation, specialized techniques exploiting subtle differences in chemical attraction or phase behavior are necessary. Chromatography is a versatile family of techniques that separates components based on their differential partitioning between two phases. The mixture is introduced into a system with a stationary phase (a fixed material) and a mobile phase (a fluid that moves through the stationary phase).
Components separate because they spend different amounts of time in each phase, driven by differences in properties like solubility, adsorption, or molecular size. For instance, in paper chromatography, the stationary phase is the paper, and the mobile phase is a solvent that moves up the paper by capillary action. Components are carried at different rates, depending on how strongly they adhere to the paper versus how readily they dissolve in the solvent, resulting in a physical separation.
Sublimation is used when a solid component can change directly into a gas, bypassing the liquid phase entirely. By gently heating a mixture containing a sublimable solid, such as iodine or camphor, that component vaporizes directly, leaving any non-sublimable solids behind. The pure gaseous component can then be re-solidified on a cooled surface, collecting it as a purified solid.
These advanced methods are valuable when bulk separation techniques, like filtration or simple distillation, are insufficient due to mixture complexity or the need for precise isolation. Techniques like High-Performance Liquid Chromatography (HPLC) or Gas Chromatography (GC) are used in analytical laboratories to separate and analyze otherwise inseparable mixtures. The efficiency of these methods allows for the isolation of compounds that differ by only minor physical characteristics.