METHODS OF SEPARATION OF HETEROGENEOUS MIXTURES - CHEMISTRY - 2025

The methods of separation of heterogeneous mixtures are those that seek to separate each of its components or phases without the need for any chemical reaction. They usually consist of mechanical techniques that take advantage of the difference in the physical properties of such components.

A mixture of fruits, cheese, olives, and ham pieces exhibit a variety of physical properties; however, the diner relies on the flavors and colors of these ingredients when separating them using a toothpick. Other mixtures will necessarily and logically require more selective criteria and principles when separating them.

A heterogeneous mixture consisting of more than one component can be separated by several steps or methods. Source: Gabriel Bolívar.

Assume the heterogeneous mixture above. At first glance it can be seen that, although it is the same phase (geometric and solid), it has components of various colors and shapes. The first sieve, orange in color, allows the star to pass through it while retaining the other figures. Similar happens with the second sieve and the turquoise octagon.

The sieves separate based on the shapes and sizes of the figures. Other techniques, however, can be based on the densities, volatilities, molecular masses, in addition to other physical properties of the components to be able to separate them.

Main methods of separation of heterogeneous mixtures

- Magnetic separation

In the example of the geometric mixture, a sieve was applied, for which a strainer (such as in kitchens), a sieve or a sieve can also be used. If all the figures are too small to be retained by the sieve, another separation technique should be used.

Assuming that the orange star had the property of being ferromagnetic, then it could be removed by using a magnet.

This magnetic separation has been taught in schools by mixing sand, sulfur or sawdust with iron shavings. The mix is ​​visually heterogeneous: the dark grayish color of the chips contrasts with their surroundings. As a magnet is approached, however, the iron shavings will move towards it until they migrate out of the sand.

In this way, the two components of the initial mixture are separated. This technique is only useful when one of the components is ferromagnetic at the temperature at which the separation takes place.

- Sublimation

If there is a fairly fragrant figure in the geometric mixture or with a considerably high vapor pressure, then it can be sublimated by applying a vacuum and heating. In this way, for example, the "solid and volatile" turquoise octagon will sublimate; that is, it will go from solid to vapor.

The most common and representative examples are heterogeneous mixtures with iodine. When heated slowly, some of the black-purple crystals will sublimate into purple vapors. Both magnetic separation and sublimation are the least conventionally used methods. In the following image you can see a sublimation process (dry ice):

- Decantation

Settling can be used for two types of heterogeneous mixtures. Source: Gabriel Bolívar.

If in the example of the geometric mixture some of the figures had remained fixed on the container, then those that manage to move would be separated. This is what is known as decantation. In the upper image, two aqueous mixtures are shown: a liquid-solid (A), and another liquid-liquid (B).

Liquid-solid mixture

In the container of A we have a solid at the bottom, strongly adhered to the surface of the glass (in the case of a beaker). If its adhesion is such, then the liquid can be poured or decanted into another container without any problem. The same can be done in the case where said solid is very dense and, carefully, the decantation is carried out in the same way.

Liquid-liquid mixture

In the container of B, however, the black liquid, immiscible and denser than water, moves if the mixture is tilted; therefore, if we try to decant it as before, the black liquid will also drain along with the water. A separatory funnel is then used to solve this problem.

This funnel has the shape of a pear, an elongated top or a backstage, and the mixture B is poured into it. Through the narrow nozzle below, the black liquid is decanted by manipulating a stopcock, so that it drips slowly. Then, through the upper mouth, the water is separated so that it is not contaminated with the residues of the black liquid.

- Filtration

If the liquid-solid mixture cannot be decanted, as it happens in the vast majority of the time and in daily laboratory tasks, then filtration is used: the most common method to separate heterogeneous mixtures. This is the wet version of sieving.

Returning to mixture A from the previous section, suppose that the black solid does not show much affinity for glass, so it does not adhere to it, and also remains suspended with particles of different sizes. No matter how hard you try to decant, some of this pesky solid will always go into the receiving vessel.

Thus, filtration is carried out instead of decantation. The sieve is exchanged for a filter paper with pores of different diameters. The water will pass through this paper while at the same time retaining the black solid.

If you intend to work with the solid later or analyze it, then the filtration will be done with a Buchner funnel and a kitasate, with which a vacuum will be applied inside the receiving container. In this way, the filtration performance is improved while drying (not calcining) the solid on the paper. The following image shows a filtration process:

- Centrifugation

Centrifuge. Source: Matt Janicki via Flickr

There are mixtures that are homogeneous to the naked eye, but are actually heterogeneous. Solid particles are so small that gravity does not pull them to the bottom, and filter paper cannot retain them either.

In these cases, centrifugation is used, with which, thanks to the acceleration, the particles experience a force that pushes them towards the bottom; just as if gravity increased several times. The result is that a two-phase mixture (similar to B) is obtained, from which the supernatant (the top part) can be taken or pipetted.

Centrifugation runs constantly when you want to separate plasma from blood samples, or the fat content of milk.

References

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