HETEROGENEOUS SYSTEM: CHARACTERISTICS, CLASSIFICATION AND METHODS - CHEMISTRY - 2025

A heterogeneous system is that portion of the universe occupied by atoms, molecules or ions, in such a way that they form two or more distinguishable phases. By "portion of the universe" is understood to be a drop, a ball, the reactor, rocks; and by phase, to a state or mode of aggregation, whether solid, liquid or gaseous.

The heterogeneity of a system varies from its definition from one field of knowledge to another. However, this concept shares many similarities within cooking and chemistry.

Source: Pexels

For example, a pizza with its surface packed with ingredients, like the one in the image above, is a heterogeneous system. Similarly, salad, a mix of nuts and grains, or a fizzy drink also count as heterogeneous systems.

Note that its elements are visible at first glance and can be separated manually. What about mayonnaise? Or milk? At first glance they are homogeneous, but microscopically they are heterogeneous systems; more specifically, they are emulsions.

In chemistry, ingredients consist of reagents, particles, or a substance under study. The phases are nothing more than physical aggregates of said particles, which provide all the qualities that characterize the phases. Thus, the liquid phase of alcohol "behaves" differently from that of water, and even more so, from that of liquid mercury.

In certain systems, the phases are as recognizable as a saturated sugar solution, with crystals in the background. Each one by itself can be classified as homogeneous: above a phase formed by water, and below, a solid phase composed of sugar crystals.

In the case of the water-sugar system, we do not speak of a reaction, but of saturation. In other systems, the transformation of matter is present. A simple example is the mixture of an alkali metal, such as sodium, and water; It is explosive, but at first, the piece of metallic sodium is surrounded by water.

As with mayonnaise, there are heterogeneous systems within chemistry that macroscopically pass for homogeneous, but under the light of a powerful microscope, their true heterogeneous phases shine.

Characteristics of the heterogeneous system

What are the characteristics of a heterogeneous chemical system? In general terms they can be listed as follows:

-They are composed of two or more phases; in other words, it is not uniform.

-It can consist, in general, of any of the following pairs of phases: solid-solid, solid-liquid, solid-gas, liquid-liquid, liquid-gas; Furthermore, all three can be present in the same solid-liquid-gas system.

-Its components and phases are distinguishable, in the first instance, at a glance. Therefore, it is enough to observe the system to draw conclusions from its characteristics; such as color, viscosity, size and shape of crystals, odor, etc.

-It usually involves a thermodynamic equilibrium, or a high or low affinity between the particles within a phase or between two different phases.

-The physicochemical properties vary according to the region or direction of the system. Thus, the values ​​for, for example, the melting point, may range from one region of a heterogeneous solid to another. Also (the most common case) the colors or hues change throughout the solid (liquid or gas) as they are compared.

-They are mixtures of substances; that is, it does not apply to pure substances.

Degree of observation

Any homogeneous system can be considered heterogeneous if the scales or degrees of observation are modified. For example, a carafe filled with pure water is a homogeneous system, but as its molecules are observed, there are millions of them with their own velocities.

From the molecular point of view, the system continues to be homogeneous because it is only H ₂ O molecules. But, by further reducing the scale of observation to atomic levels, the water becomes heterogeneous, since it does not consist of a single type of atom but hydrogen and oxygen.

Therefore, the characteristics of heterogeneous chemical systems depend on the degree of observation. If you consider the microscopic scale, you may come across multifaceted systems.

A solid A, apparently homogeneous and silver in color, could consist of multiple layers of different metals (ABCDAB…) and therefore be heterogeneous. Therefore, A is macroscopically homogeneous, but heterogeneous at the micro (or nano) levels.

Likewise, the same atoms are heterogeneous systems, since they are made of vacuum, electrons, protons, neutrons, and other subatomic particles (such as quarks).

Classification

Considering then a macroscopic degree of observation, which defines the visible characteristics or a measurable property, heterogeneous chemical systems can be classified in the following ways:

Saturated solutions (liquid-liquid, liquid-solid, liquid-gas)

Saturated solutions are a type of chemical heterogeneous system in which the solute cannot continue to dissolve and forms a phase separate from that of the solvent. The example of water and sugar crystals falls into this classification.

Solvent molecules reach a point where they cannot accommodate or solvate the solute. Then the additional solute, solid or gaseous, will quickly regroup to form a solid or bubbles; that is, a liquid-solid or liquid-gas system.

The solute can also be a liquid, which is miscible with the solvent up to a certain concentration; otherwise they would be miscible at all concentrations and would not form a saturated solution. By miscible, it is understood that the mixture of the two liquids forms a single uniform phase.

If, on the other hand, the liquid solute is immiscible with the solvent, as is the case with the oil and water mixture, the least amount added the solution becomes saturated. As a result, two phases are formed: one aqueous and the other oily.

Solutions with precipitated salts

Some salts establish a balance of solubility, due to the fact that the interactions between their ions are very strong and they regroup into crystals that water cannot dissociate.

This type of heterogeneous system also consists of a liquid phase and a solid phase; But, unlike saturated solutions, the solute is a salt that does not require large amounts to precipitate.

For example, mixing two aqueous solutions of unsaturated salts, one of NaCl and the other of AgNO ₃, precipitates the insoluble salt AgCl. Silver chloride establishes a solubility equilibrium in the solvent, with an off-white solid being observed in the aqueous container.

Thus, the characteristics of these solutions depend on the type of precipitate formed. In general, the chromium salts are very colorful, as well as those of manganese, iron, or some metallic complex. This precipitate can be a crystalline, amorphous or gelatinous solid.

Phase transitions

A block of ice can constitute a homogeneous system, but when it melts, it forms an additional phase of liquid water. Therefore, the phase transitions of a substance are also heterogeneous systems.

Additionally, some molecules can escape from the ice's surface into the vapor phase. This is due to the fact that not only liquid water has vapor pressure, but also ice, although to a lesser extent.

Heterogeneous systems of phase transitions apply to any substance (pure or impure). Thus, all the solids that melt, or the liquid that evaporates, belong to this type of system.

Solids and gases

A very common class of heterogeneous systems in chemistry are solids or gases with various components. For example, the pizza in the image falls into this classification. And if instead of cheese, paprika, anchovies, ham, onions, etc., it had sulfur, coal, phosphorus and copper, then there would be another heterogeneous solid.

Sulfur stands out for its yellow color; coal for being a black solid; the phosphor is red; and shiny, metallic copper. All are solid, therefore, the system consists of a phase but with several components. In everyday life the examples of this type of system are incalculable.

Also, gases can form heterogeneous mixtures, especially if they have different colors or densities. They can carry very small particles, as occurs with water inside clouds. As they grow in size, they absorb visible light and the clouds turn grayish as a result.

An example of a heterogeneous solid-gas system is smoke, which is made up of very small carbon particles. For this reason the smoke from incomplete combustion is blackish in color.

Fractionation methods

The phases or components of a heterogeneous system can be separated taking advantage of differences in their physical or chemical properties. In this way, the original system is fractionated until only homogeneous phases remain. Some of the more common methods are as follows.

Filtration

Filtration is used to separate a solid or precipitate from a liquid. Thus, the two phases manage to separate, although with a certain level of impurity. For this reason, the solid is generally washed and subsequently dried in an oven. This procedure can be done either by applying vacuum, or simply by gravity.

Decantation

This method is also useful for separating a solid from a liquid. It differs somewhat from the previous one, in that the solid is generally firm in consistency and is completely deposited at the bottom of the container. To do this, simply tilt the mouth of the container at a suitable angle so that the liquid flows out of it.

Similarly, decantation allows two liquids to be separated, that is, a liquid-liquid system. In this case, a separatory funnel is used.

The biphasic mixture (two immiscible liquids) is transferred to the funnel, and the liquid with lower density will be located at the top; while the one with the highest density, in the lower part, in contact with the outlet opening.

Source: Pixabay

The upper image represents a separating or separating funnel. This glassware is also used for liquid-liquid extractions; that is, to extract a solute from the initial liquid by adding another liquid in which it is even more soluble.

Sifting

Sieving is used to separate solid components of different sizes. It is very common to find in the kitchen a sieve or sieve to clean the grains, purify the wheat flour, or remove solid residues from thick juices. In chemistry, it can be used to separate small crystals from larger ones.

Magnetization

This method is used for solid-solid systems where one or more of the components are attracted by a magnet. Thus, the initial heterogeneous phase is purified as the magnet removes the ferromagnetic elements. For example, magnetization is used to separate tinplate from trash.

Centrifugation

Centrifugation separates a suspended solid from a liquid. It cannot be filtered because the particles swim uniformly occupying the entire volume of the liquid. To separate the two phases, a quantity of the heterogeneous mixture is subjected to a centrifugal force, which sediments the solid at the bottom of the centrifuge tube.

Sublimation

The sublimation separation method is applied only for volatile solids; that is, for those with high vapor pressure at low temperatures.

On heating the heterogeneous mixture, the volatile solid escapes into the gas phase. An example of its application is the purification of a sample contaminated with iodine or ammonium chloride.

Examples

So far, several examples of heterogeneous chemical systems have been mentioned. To complement them, additional ones are listed below and others outside the chemical context:

-The granite, the stones of a river, the mountains, or any rock with veins of many colors.

-Minerals also count as heterogeneous systems, since they are made up of various types of solid structures made up of ions. Its qualities are the product of the interaction between ions of a crystalline structure and impurities.

-The soft drinks. In them there is a liquid-gas equilibrium, which by decreasing the external pressure, decreases the solubility of the dissolved gas; for this reason, many bubbles (gaseous solute) are observed rising to the surface of the liquid when uncovered.

-Any reaction medium that involves reagents in different phases, and that also require a magnetic stirrer to guarantee a higher reaction speed.

-Heterogeneous catalysts. These solids provide sites on their surface or pores where the contact between the reactants is accelerated, and they do not intervene or undergo an irreversible transformation in the reaction.

-A frieze wall, a mosaic wall, or the architectural design of a building.

-Multi-layered gelatins of many flavors.

-A Rubik's cube.

References

1. Equilibrium in Heterogeneous Systems. Recovered from: science.uwaterloo.ca
2. Fernández G. (November 7, 2010). Homogeneous and heterogeneous systems. Recovered from: quimicafisica.com
3. Jill. (June 7, 2006). Homogeneous and Heterogeneous Systems. Recovered from: chemistryforstudents.blogspot.com
4. LoveToKnow. (2018). Examples of Heterogeneous Mixture. Recovered from: examples.yourdictionary.com
5. Shiver & Atkins. (2008). Inorganic chemistry. In The elements of group 15. (fourth edition). Mc Graw Hill.
6. Wikipedia. (2018). Homogeneity and heterogeneity. Recovered from: en.wikipedia.org
7. F. Holleman, Egon Wiberg, Nils Wiberg. (2001). Inorganic Chemistry. Recovered from: books.google.com