The separation of substances depends on whether the material is a mixture or a compound. Mixtures are physically combined and separated by physical means, such as filtration or distillation. Chemical means involve a reaction that changes the substance’s identity and are reserved for breaking down compounds, not for separating the physically combined parts of a mixture.
The Fundamental Nature of Mixtures
A mixture is created when two or more substances are combined without undergoing a chemical reaction. The components maintain their original chemical identity and physical characteristics, allowing for physical separation. Mixtures are categorized as homogeneous (uniform composition, like saltwater) or heterogeneous (visibly different components, like sand in water).
Separation Techniques Based on Physical Properties
Separating a mixture relies on exploiting differences in the physical properties of its components. Techniques are selected based on variances in boiling point, particle size, density, or solubility.
Filtration separates an insoluble solid from a liquid based on particle size. The mixture passes through a porous material, like filter paper, which traps the solid particles while allowing the liquid to pass through. For example, separating sand from water leaves the sand as residue and collects the water as filtrate.
Distillation separates liquids with different boiling points or a dissolved solid from its solvent. The mixture is heated until the component with the lower boiling point vaporizes. The vapor is then cooled and condensed back into a purified liquid, separating components based on volatility.
Evaporation is used to recover a dissolved solid from a liquid, such as obtaining salt from seawater, by heating the solvent until it turns into a gas. Chromatography separates mixtures based on the differential rates at which components move through a stationary medium. This exploits subtle differences in solubility and adherence, allowing for the separation of complex mixtures.
Chemical Separation: The Realm of Compounds
Chemical separation, or decomposition, is fundamentally different because it addresses the breakdown of a chemical compound, not a mixture. A compound is a pure substance where two or more elements are chemically bonded together in a fixed ratio, resulting in a new substance with properties distinct from its constituent elements. Separating these elements requires breaking the strong chemical bonds that hold them together, a process that changes the chemical identity of the original substance.
The breakdown of a compound is a chemical reaction that often requires a significant input of energy to break the bonds. For instance, electrolytic decomposition uses electrical energy to split a compound into its simpler components. A classic example is the electrolysis of water (\(\text{H}_2\text{O}\)), which requires passing an electric current through it to break the bonds and yield hydrogen gas (\(\text{H}_2\)) and oxygen gas (\(\text{O}_2\)).
Another method is thermal decomposition, where heat energy is used to break the compound’s bonds. When calcium carbonate (\(\text{CaCO}_3\)) is heated strongly, it decomposes into calcium oxide (\(\text{CaO}\)) and carbon dioxide gas (\(\text{CO}_2\)). These decomposition processes fundamentally alter the starting material, transforming it into new substances with entirely different chemical formulas and properties. This requirement for a chemical reaction to alter the substance’s nature is why chemical means are used for compounds, while physical means are sufficient for separating mixtures.