HOMOGENEOUS SYSTEM: CHARACTERISTICS, CLASSIFICATION, METHODS - CHEMISTRY - 2024

A homogeneous system is that portion of the universe that is made up of a single phase of matter. It can be a totally uniform phase, or consist of an ordered and symmetric mixture of elements, which in the case of homogeneous chemical systems, are particles (molecules, atoms, ions, etc.).

Nature tends, through uncertain or well-known mechanisms, to homogenize some property or the entire system itself. On Earth there is an orchestra of balance between homogeneous and heterogeneous systems, considered as such by visual explorations.

Source: Maxpixel

That is, in the first instance, the eyes qualify whether a system (any object or space) is homogeneous or not. If it is superficially, the next step is to ask yourself what its composition is like and in what way its elements are arranged. With this in mind, it can be affirmed or not (with some certainty) if the system presents homogeneity in its properties.

For example, in the image above you have the image of a coffee cup, a plate and a sugar wrapper with a happy face. If these three elements were considered for a study, then the system would be heterogeneous, but if only the black coffee inside the cup were studied, in this case we would speak of a homogeneous system.

Why? Because, at first glance, black coffee looks smooth and you might think that so is the interior. If sugar were added without stirring, it would settle to the bottom of the cup and the initial homogeneous system would become heterogeneous.

However, if the coffee were stirred until the sugar was completely dissolved, its homogeneity would return, although with the new organoleptic property that it is now sweeter than before. To be homogeneous, each drop of coffee extracted from any corner of the cup must taste exactly the same.

On the other hand, a cup of black coffee can be compared to one with a bubbly surface. The second would be less homogeneous than the first, since it does not present a uniform distribution of its bubbles. But, if the two coffees taste the same, and lack sugar crystals (most important variables), then they are both equally homogeneous.

Coffees with whipped cream, or with artistic drawings on their surface, can be taken by heterogeneous systems (even if the mixture is homogeneous with respect to coffee).

Characteristics of a homogeneous system

What characteristics should a homogeneous system have?

-It must have a single material phase (liquid, solid or gas).

-When it is a mixture, its components must be able to form a single uniform phase. This is the case with coffee and sugar. If there are undissolved sugar crystals at the bottom of the glass or cup, they constitute a second phase.

-Its intensive properties (density, viscosity, molar volume, boiling point, etc.) must be the same at all points within the system. This also applies to the organoleptic properties (taste, color, smell, etc.). Thus, a single flavor meringue is a homogeneous system as long as it does not have another element (such as chopped fruits).

-The components of your mixtures are arranged in space in a homogeneous and symmetrical way.

The chessboard and subjectivity

The latter feature can trigger confusion and conflicting views.

The chess board (without the pieces), for example, represents a point where different opinions arise about it. Is it homogeneous or heterogeneous? And if the black and white squares alternate in rows (one white, one black, and so on), what would the response be in that scenario?

Because the boxes are differentiated from each other by color, this is the main variable. There is a noticeable difference between white and black, which alternates throughout the board.

Each color represents a component, and the mixture is homogeneous if their physical arrangement is oriented in such a way as to minimize the differences in their properties. Therefore, the colors should be arranged as evenly and symmetrically as possible.

From this reasoning, the chessboard is homogeneous, because despite being heterogeneous with respect to its colors, their difference alternates uniformly. While with the colors displayed in rows, the “black and white phases” are evident, which would be equivalent to having two phases and entering into the definition of a heterogeneous system.

Classification

Homogeneous systems can have many classifications, which depend on which branch of knowledge they belong to. In chemistry, it is not enough to superficially observe a system, but to find which particles make it up and what they do in it.

Solutions

Unsaturated solutions are homogeneous mixtures or systems present not only in chemistry but in everyday life. The sea and the oceans are gigantic masses of unsaturated salt water. Solvent molecules, usually in the liquid phase, surround solute molecules and prevent them from aggregating to form a solid or bubble.

Almost all solutions fall into this classification. Impure alcohols, acids, bases, a mixture of organic solvents, indicator solutions or transition metal reagents; all contained in volumetric balloons or glass or plastic containers are classified as homogeneous systems.

Faced with less formation of a second phase in any of these solutions, the system is no longer homogeneous.

Pure substances

The phrase “impure alcohols” was written above, referring to the fact that they are usually mixed with water. However, pure alcohols, as well as any other liquid compound, are homogeneous systems. This applies not only to liquids, but also to solids and gases.

Why? Because when you have only one type of particle in a system, you speak of high homogeneity. They are all the same, and the only variation is in the way they vibrate or move; but, in relation to its physical or chemical properties, there is no difference in any part of the system.

This means that a cube of pure iron is a homogeneous system because it only has iron atoms. If a fragment were torn from any of its vertices, and its properties were determined, the same results would be obtained; that is, the homogeneity of its properties is fulfilled.

If it was impure, its properties would fluctuate within a range of values. This is the effect of impurities on iron, and on any other substance or compound.

If, on the other hand, the iron cube has rusted parts (red) and metallic parts (grayish), then it is a heterogeneous system.

Homogeneous reactions

Homogeneous reactions are perhaps the most important homogeneous chemical systems. In them, all the reactants are in the same phase, especially the liquid or the gas. They are characterized by greater contact and molecular collisions between the reactants.

Since there is only one phase, the particles move with greater freedom and speeds. On the one hand this is a great benefit; But on the other hand, unwanted products can be formed or some reagents move so fast that they do not collide efficiently.

The reaction of hot gases with oxygen to create fire is an emblematic example of this type of reaction.

Any other system where reactants with different phases participate, such as the oxidation of metals, is considered a heterogeneous reaction.

Fractionation methods

In principle, given their uniformity, it is not possible to separate the components of homogeneous systems by mechanical methods; much less if it is a pure substance or compound, from whose fractionations its elemental atoms are obtained.

For example, it is easier (or faster) to separate the components of a pizza (heterogeneous system), than those of a coffee (homogeneous system). In the first one, it is enough to use your hands to remove the ingredients; while with the second, it will take more than hands to separate the coffee from the water.

The methods vary according to the complexity of the system and its material phases.

Evaporation

Evaporation consists of heating a solution until the solvent evaporates completely, leaving the solute settled. Therefore, this method is applied to homogeneous liquid-solid systems.

For example, when dissolving a pigment in a container of water, the system is initially heterogeneous, since the crystals of the pigment have not yet diffused throughout the volume. After a while, all the water turns the same color, which is indicative of a homogenization.

To recover the added pigment, the entire volume of water must be heated until it evaporates. Thus, H ₂ O molecules increase their average kinetic energy thanks to the energy supplied by heat. This leads to them escaping into the gaseous phase, leaving pigment crystals at the bottom (and on the walls of the container).

The same happens with sea water, from which its salts can be extracted as white stones when heated.

On the other hand, evaporation is also used to remove volatile solutes such as gaseous molecules (O ₂, CO ₂, N ₂, etc.). When the solution is heated, the gases begin to gather to form bubbles, the pressure of which, if it exceeds the external pressure, will rise to escape the liquid.

Rotaevaporation

This method makes it possible to recover organic solvents by applying a vacuum. It is very useful, especially when extracting oils or fats from organic matter.

In this way, the solvent can be reused for future extractions. These experiments are very common in the study of natural oils obtained from any organic matter (merey, seeds, flowers, fruit shells, etc.).

Distillation

Distillation allows the separation of components of a homogeneous liquid-liquid system. It is based on the difference of the boiling points of each component (ΔT _eb); the greater the difference, the easier it will be to separate them.

It requires a cooling column that promotes the condensation of the most volatile liquid, which will then flow into a collecting balloon. The type of distillation varies depending on the values ​​of ΔT _eb and the substances involved.

This method is widely used when purifying homogeneous mixtures; such as, for example, recovering a gaseous product from a homogeneous reaction. However, it also has application for heterogeneous mixtures, as occurs in the refining processes of crude oil to obtain fossil fuels and other products.

Liquefaction

And what about homogeneous gaseous systems? They are composed of more than one type of gaseous molecules or atoms, which differ in their molecular structures, masses, and atomic radii.

Therefore, they have their own physical properties and behave differently in the face of an increase in pressure and a decrease in temperature.

When both T and P vary, some gases tend to interact more strongly than others; with enough force to condense into a liquid phase. If, on the other hand, the entire system condenses, then distillation of the condensate components is used.

If A and B are gases, through liquefaction they condense into a homogeneous mixture, which is then subjected to distillation. In this way, pure A and B are obtained in different vessels (such as separate liquid oxygen and nitrogen).

Examples

Additional examples of homogeneous systems are listed below.

Of daily life

-White toothpaste.

-Vinegar, as well as commercial alcohol and liquid detergents.

-The blood plasma.

-The air. Clouds can also be considered homogeneous systems, although they actually contain micro droplets of water.

-Alcoholic drinks without ice.

-Perfumes.

-Gelatins, milk and honey. However, microscopically they are heterogeneous systems, despite showing a single phase to the naked eye.

-Any solid object with visible uniform characteristics, such as color, brightness, dimensions, etc. For example, symmetrical and metallic nuggets, or faceted blocks of a mineral or salt. Mirrors also fall within this range of objects.

Chemicals

-Steel and metal alloys. Its metallic atoms are arranged in a crystalline arrangement where the metallic bond participates. If the distribution of the atoms is uniform, without "layers" of atoms of a metal X or Y.

-All solutions prepared inside or outside the laboratory.

-Pure hydrocarbons (butane, propane, cyclohexane, benzene, etc.).

-All syntheses or productions where the reagents or raw material are in a single phase.

Homogeneous catalysis

Some reactions are accelerated by adding homogeneous catalysts, which are substances that participate according to a very specific mechanism in the same phase of the reactants; that is, in reactions carried out in aqueous solutions, these catalysts must be soluble.

In general, homogeneous catalysis is very selective, although not very active or stable.

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