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Substances are often not pure and exist as mixtures. Several techniques exist to separate the components of a mixture from one another. The technique used is based upon the physical properties of the components within the mixture. Often, several techniques are used together in order to isolate the components from one another.
Solubility: Some substances will dissolve in a solvent (a liquid) while others will not (e.g., salt will dissolve in water yet sand will not). If the solvent changes (e.g., from water to another solvent like hexane), the substances that were orginally soluble may become insoluble. As a result, it is important to know which substances will dissolve and which will not when selecting a solvent.
Separatory funnels are used to separate a mixture of liquids based on solubility. A mixture is added to the separatory funnel along with a second solvent. The funnel is sealed and mixed, allowing the components of the mixture to dissolve into the different solvents. Because the solvents have different densities, the more dense solvent with its dissolved components settles to the bottom while the other solvent settles to the top. Once the layers form, the lower layer is drained from the upper layer.
Above: The separation of components in a mixture using different solvents. Each component is soluble in the different solvents, separating the components from one another. From: https://media.springernature.com/lw685/springer-static/image/art%3A10.1038%2Fs41538-020-00079-8/MediaObjects/41538_2020_79_Fig5_HTML.png
Some soluble substances become insoluble and form a solid if the temperature is changed (often cooling). This process, called recrystallization, allows a solid to reform while the other soluble substances remain dissolved in the solvent.
Above: When the temperature of a mixture is lowered, some soluble components become insoluble and recrystallize (precipitate out of solution). From: https://issr.edu.kh/science/Webpage/Lab_Techniques/Edexcel%20Practical%20Chemistry%20-%20Halesowen/Edexcel2009/recrystallization.gif
Filtration: Soluble components can be separated from solids based on size of the components. When a mixture is poured into a funnel containing a filter paper, the solids will be trapped in the filter while the soluble solids and other small particles pass through. The solids are then washed and rinsed with deionized water to remove any impurities that may be on the solid.
Above: An overview of the process of filtration to separate a solid from a liquid. From: https://keystagewiki.com/images/thumb/9/91/FiltrationDiagram.png/500px-FiltrationDiagram.png
Evaporation: Solids that were separated using filtration often have solvent molecules present. To remove the solvent molecules, the solid is gently heated, often in a drying oven to remove the solvent. Over time, the solvent molecules evaporate and only the solid remains. Bunsen burners can also be used to evaporate the solvent faster but the high temperatures may cause unwanted chemical reactions to occur.
Above: When water evaporates from a mixture, any non-volatile substances will remain in the container. From: https://media.geeksforgeeks.org/wp-content/uploads/20210626010704/WhatsAppImage20210626at10541AM.jpeg
Distillation: Distillation is a technique to separate a mixture based upon the boiling points of the substances. When heat is applied to the mixture, the component with the lowest boiling point vaporizes into a gas first. As the gas moves upward, it is redirected into a condensing unit. The condensing unit lowers the temperature of the gas, causing the gas to condense into a liquid. The liquid is collected in a separate container.
Above: Water has a lower boiling point than salt, causing it to vaporize first. From: https://ichef.bbci.co.uk/images/ic/480x270/p0b51gqv.png
Magnetism: Magnetic solids can be separated from non-magnetic solids using a magnet (e.g., iron filings from sand). This is not a commonly used technique but is useful when it can be applied.
Above: Separating the magnetic components of a mixture using a magnet. From: https://www.minejxsc.com/wp-content/uploads/2025/01/istockphoto-1464562835-612x612-1.webp
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