These methods are part of our day to day, whether in open field work, or within the spaces of the kitchen or a business. They are applied for the purpose of purifying or raising the quality of a product. For example, coffee is filtered to recover the precious liquid without the ground beans interfering on the palate.
In this case, it is a liquid-solid mixture, where the liquid is coffee, and the solid is ground beans. In turn, the coffee itself is another mixture: a solution product of an extraction, which is homogeneous.
Thus, there are heterogeneous and homogeneous mixtures. Each of them has its own separation methods, capable of isolating their respective components. How to recognize them from each other? By the presence of more than one material phase. This means that there must be two components that can be perfectly distinguished by the naked eye.
Generally speaking, heterogeneous mixture separation methods require more mechanical work. For homogeneous mixtures, on the other hand, they need the help of temperature. For example, evaporation is the most widely used method to separate solute from solvent in solutions.
Main methods of separating mixtures
Filtration is a separation method that is used to separate, in principle, liquid-solid mixtures. It needs the help of a barrier or filter that allows the liquid to pass through, but retains the solid in its tissue, which cannot pass through the tiny pores of the filter.
Filtration is normally assisted by gravity, and depending on the size of the solid particles, this can be a slow or fast process.
The word ‘filtration’ is also often used when talking about air. For example, air polluted with bacteria is a homogeneous, gaseous mixture. If the air is passed through filters that capture bacteria, the air is said to have been filtered. This ensures that it is clean and sterile.
It is in laboratories where the use of this method is most observed. Almost always after obtaining a precipitate or crystals, they are filtered immediately to get rid of the liquid and preserve the solid.
Decanting bears some resemblance to filtration. It also serves to separate liquid-solid mixtures, with the difference that it does not need any filter, paper, sieve, mesh, etc., to prevent the passage of the solid. For this to be possible, there must be a firm sedimentation; that is, the sediment has to be attached to the internal walls of the container.
For example, decantation is used if you want to separate a mixture of water with sand or sediment. How? Tilting the glass towards another container so that the water, by action of gravity, falls without dragging the sand or sediment.
Another difference that decantation has from filtration is that decantation can also be applied to separate liquid-liquid mixtures. These take place only when the two liquids are immiscible, forming two recognizable phases. Oil-water or gasoline-water mixtures are examples of this type of mixture.
Sublimation is a special separation method used to separate two solids from a heterogeneous mixture. In principle, it needs at least one of the solids to have a very high vapor pressure, so that when heated, and applying a vacuum, it evaporates or sublime, leaving the mixture behind.
The classic example of sublimation is the separation of an iodine-sand mixture. Iodine, being a volatile solid, sublimates as purple vapors that leave the sand. However, sublimation can also be applied to separate a dry ice-ice mixture, since dry ice (solid carbon dioxide) is much more volatile than normal ice (crystallized water).
Also, sublimation can be used to purify samples with fragrant solids, such as camphor and others.
Evaporation is perhaps one of the slowest separation methods out there. It is used, in principle, to separate the solvent from the solute, which ends with the homogeneous appearance of a solution.
The classic example is the evaporation of sugar water to obtain sugar crystals. To speed up the process, use is made of the heat of a flame that heats the solution until the water or solvent boils.
Evaporation is also used to obtain salts from seawater, or to put some gelatinous solids to dry. Likewise, slow and prolonged evaporation is one of the steps that allows excellent crystallization.
– Simple distillation
Unlike decantation, which allows only the separation of liquid-liquid mixtures, distillation allows the separation of homogeneous liquid mixtures that are made up of one or more liquid components. It is based on the difference of the boiling points of the components of the mixture.
For example, water boils around 100ºC, while acetone boils at 56ºC. Then the water-acetone mixture can be distilled to obtain an acetone extract.
The same happens with the ethanol-water mixture, to obtain more concentrated ethanol solutions.
– Fractional distillation
Fractional distillation is a more refined distillation, in the sense that it allows the separation of components whose boiling points are not very different. As there is usually more than one component, each extract is called a fraction. Thus, several fractions with different properties are obtained.
Fractional distillation of crude oil is the best example of the use of this method. From crude oil, various fractions are produced, from which products such as natural gas, gasoline, kerosene, acetone, and diesel, among others, are obtained.
Chromatography is a separation method that also bears some resemblance to filtration. It is much more refined, selective, and less excessive when it comes to the volume of mixture subjected to separation. This mixture can be liquid (liquid chromatography) or gas (gas chromatography).
Instead of using a filter, chromatography requires what is known as a stationary phase. In paper chromatography, using markers, the paper acts as the stationary phase, which is through which the mixture travels as the components (the dyes) separate.
The stationary phase par excellence is a column with a packing that retains the components based on their interactions or affinities. Thus, chromatography can separate ions of different charges, molecules of different polarities or sizes, etc.
Chromatography is essential to analyze complex mixtures, evaluate the action of drugs, detect specific chemical compounds, among other examples of its use.
Centrifugation is “forced sedimentation”. This is because, thanks to centripetal acceleration, suspended solid particles coalesce to define a phase. In this way, the supernatant can then be removed, or an aliquot taken for analysis.
Centrifugation is widely used in the separation of plasma from blood cells. It also serves to separate butterfat from milk, or to speed up the sedimentation of some precipitates.
Along with chromatography, centrifugation is one of the most sophisticated methods for separating mixtures.
– Magnetic separation
As its name indicates, it makes use of the phenomenon of magnetism to achieve the separation of the components of different types of heterogeneous mixtures.
An example would be a mixture of powdered limestone and iron filings. Manually you could proceed to separate the pieces of filing, but it would take a lot of time and patience. Instead, a powerful magnet is used to attract iron filings and leave limestone dust behind.
Similar mixtures are iron-sulfur, iron-sand, or iron-mud. Other metals, such as nickel, cobalt, dysprosium, and gadolinium, are also attracted to magnets.
Another example of magnetic separation occurs in certain garbage dumps, where scrap metal is processed and magnets separate metal objects; such as nuts, golf clubs, keys, etc.
Of all the separation methods, this is perhaps the least used within the laboratory or in everyday life. It is mostly restricted to industrial processes.