A mixture is formed when two or more substances are combined without a chemical reaction, retaining their individual chemical identities and properties. Because the components are only physically intermingled, they can be separated by physical means. Effective isolation relies entirely on the differences in their physical properties.
Defining Mixtures and the Role of Physical Properties
The fundamental distinction between a mixture and a compound is that the components of a mixture are not chemically bonded. A compound, such as water, is a new substance with properties distinct from its constituent elements, making it impossible to separate by simple physical methods. Conversely, substances in a mixture, such as sand and salt, maintain their original properties, allowing them to be separated based on those characteristics.
Scientists exploit differences in specific physical properties to achieve separation. The most commonly targeted properties include:
- Particle size, which dictates how a substance interacts with a filter or sieve.
- Density, which determines how components settle or float in a liquid.
- Boiling point, utilized particularly for separating liquids or dissolved solids.
- Magnetism or differential adhesion, used in more specialized contexts.
Isolation Methods Based on Size and Density
Simple mixtures containing insoluble solids and liquids are often separated by methods that exploit differences in particle size and relative density. A technique like filtration is employed when a solid component is insoluble and significantly larger than the solvent molecules. This process involves pouring the mixture through a porous barrier, such as filter paper, which traps the larger solid particles while allowing the liquid, or filtrate, to pass through.
Another straightforward method is decantation, which relies on a substantial difference in density between components. The mixture is allowed to stand undisturbed, causing the denser component to settle at the bottom through sedimentation. The lighter top layer can then be carefully poured off, or decanted, working well for separating substances like oil and water.
Isolation Methods Based on Boiling Point
Separation methods that involve a change of state, such as evaporation and distillation, are effective for mixtures where one component has a significantly lower boiling point than the other. Evaporation is a straightforward method used to separate a dissolved solid, known as the solute, from a liquid solvent, such as obtaining salt from saltwater. The mixture is heated, causing the liquid solvent to turn into a gas and escape into the atmosphere, leaving the solid residue behind.
While evaporation recovers the solid, the liquid solvent is typically lost. Distillation is a refinement designed to recover both components or to separate two liquids with different boiling points. The mixture is heated in a closed system, causing the lower-boiling component to vaporize, condense in a cooled pathway, and be collected as the purified distillate.
This technique is highly dependent on a measurable difference in the boiling temperatures of the components. For example, separating water (boiling point 100°C) from ethanol (boiling point approximately 78°C) requires heating the mixture above 78°C but below 100°C. This causes the ethanol to vaporize preferentially, and the collected distillate will be highly enriched in the lower-boiling component.
Specialized Techniques Leveraging Unique Traits
Some separation challenges require techniques that target properties other than size, density, or boiling point. When one component of a mixture possesses a magnetic property, such as iron filings mixed with sulfur powder, a simple magnet can be used for separation. This technique, known as magnetic separation, exploits the ferromagnetic nature of certain materials, which are strongly attracted to a magnetic field, leaving the non-magnetic components behind.
For complex mixtures, especially those involving color compounds like inks or plant pigments, chromatography is a powerful tool. This technique separates components based on their differential affinity for two phases: a stationary phase and a mobile phase. The mixture is introduced into the mobile phase, which carries the components across the stationary phase, such as a piece of paper or a column packed with material.
Components that adhere more strongly to the stationary phase move slower, while those more soluble in the mobile phase travel faster. This differential movement causes the mixture to separate into distinct bands or spots, allowing the isolation of substances that are otherwise difficult to separate due to similar physical properties. Chromatography is particularly valuable in analytical chemistry for isolating and identifying trace components in a solution.