METHODS FOR SEPARATING HOMOGENEOUS MIXTURES - CHEMISTRY - 2024

The methods of separation of homogeneous mixtures are all those that, without making use of chemical reactions, allow to obtain the components or solutes that integrate the same phase; that is, of a liquid, solid or gas.

Such homogeneous mixtures consist of solutions, in which the solute particles are too small to be distinguished with the naked eye. They are so small that there are no filters narrow or selective enough to retain them while the solution passes through them. Neither help for their separation techniques such as centrifugation or magnetization.

Illustrative example of how homogeneous mixtures can be separated in stages. Source: Gabriel Bolívar.

Above is an example of how solutions are separating into their components. The initial mixture (brown), is separated into two components, equally homogeneous (orange and purple). Finally, from the two resulting mixtures, the solvent (white) and the four respective pairs of solutes (red-yellow and red-blue) are obtained.

Among the methods or techniques for separating solutions we have evaporation, distillation, chromatography and fractional crystallization. Depending on the complexity of the mix, more than one of these methods may have to be used until homogeneity is broken.

The main methods of separation of mixtures

- Evaporation

Evaporation is the simplest method to separate homogeneous mixtures of a single solute.

The simplest homogeneous mixtures are solutions where a single solute has dissolved. For example, in the image above there is a colored solution due to the absorption and reflection of visible light with the particles of its solute.

If it has been shaken well during its preparation, there will be no lighter or darker regions than others; they are all equal, uniform. These colorful particles cannot be separated from the solvent by any mechanical method, so you will need energy in the form of heat (red triangle) to achieve this.

Thus, the colored solution is heated under the open sky to speed up and allow the solvent to evaporate out of its container. As this happens, the volume that separates the solute particles decreases and therefore their interactions increase and slowly end up settling.

The end result is that the colored solute remains at the bottom of the container and the solvent has completely evaporated.

The drawback with evaporation is that, rather than separating solutes, its objective is to eliminate the solvent by heating it to its boiling point. The remaining solid can be composed of more than one solute and therefore other separation methods are required to define it in its isolated components.

- Distillation

Distillation

Distillation is perhaps the most widely used method of separating homogeneous solutions or mixtures. Its use extends to salts or molten metals, condensed gases, solvent mixtures, or organic extracts. The solute is most of the time a liquid, the boiling point of which differs by several degrees from that of the solvent.

When the difference between such boiling points is high (greater than 70 ºC), simple distillation is used; and if not, then a fractional distillation is done. Both distillations have multiple setups or designs, as well as a different methodology for mixtures of different chemical nature (volatile, reactive, polar, apolar, etc.).

In distillation, both the solvent and the solutes are conserved, and this is one of their main differences with respect to evaporation.

However, rotary evaporation combines these two aspects: a liquid-solid or liquid-liquid mixture, such as that of a dissolved and miscible oil, is heated until the solvent is eliminated, but this is collected in another container while the solid or oil remains. in the initial container.

Air distillation

The condensed air is subjected to cryogenic fractional distillation to remove oxygen, nitrogen, argon, neon, etc. Air, a homogeneous gaseous mixture, turns into a liquid where nitrogen, being the major component, theoretically acts as a solvent; and the other gases, also condensed, as liquid solutes.

- Chromatography

Chromatography, unlike other techniques, cannot provide even remotely similar yields; that is, it is not useful for processing an entire mixture, but only an insignificant fraction of it. However, the information it provides is analytically extremely valuable, as it identifies and classifies mixtures based on their composition.

Paper or thin layer chromatography. Source: Gabriel Bolívar.

There are different types of chromatography, but the simplest, the one explained in colleges or pre-university courses, is that of paper, whose principle is the same as that developed on a thin layer of an absorbent material (commonly silica gel).

The image above shows that a beaker, filled with water or a certain solvent, is placed on a paper that has been marked with a reference line with drops or dots of three selected pigments (orange, purple and green). The beaker is kept closed so that the pressure is constant and it is saturated with the solvent vapors.

Then, the liquid begins to rise up the paper and carries the pigments. Pigment-paper interactions are not all the same: some are stronger, some are weaker. The more affinity the pigment has for the paper, the less it will ascend through the paper relative to the line that was initially marked.

For example: the red pigment is the one that feels less affinity for the solvent, while the yellow hardly rises due to the fact that the paper retains it more. The solvent is then said to be the mobile phase, and the paper the stationary phase.

- Fractional crystallization

Illustrative example of fractional crystallization. Source: Gabriel Bolívar.

And to finish there is the fractional crystallization. This method could perhaps be classified as a hybrid, since it starts from a homogeneous mixture to end up with a heterogeneous one. For example, suppose you have a solution in which a green solid has dissolved (top image).

The green particles are too small to separate manually or mechanically. It is also found that the green solid is a mixture of two components and not a single compound of this color.

Then, a solution thereof is heated and left to rest while it cools. It turns out that the two components, although closely related to each other, their solubilities in a certain solvent are slightly different; therefore, one of the two will begin to crystallize first and then the other.

The blue-green component (in the middle of the image) is the first to crystallize, while the yellow component remains dissolved. As there are bluish-green crystals, they are filtered hot before the yellow crystals appear. Then, as the solvent cools a bit more, the yellow component crystallizes and another filtration is done.

References

1. Whitten, Davis, Peck & Stanley. (2008). Chemistry (8th ed.). CENGAGE Learning.
2. Chelsea Schuyler. (2019). Chromatography, Distillation and Filtration: Methods of Separating Mixtures. Study. Recovered from: study.com
3. CK-12 Foundation. (October 16, 2019). Methods for Separating Mixtures. Chemistry LibreTexts. Recovered from: chem.libretexts.org
4. Good Science. (2019). Separation of Mixtures. Recovered from: goodscience.com.au
5. Clark Jim. (2007). Thin layer chromatography. Recovered from: chemguide.co.uk