Concept of systems and solutions in chemistry

Why does chemistry need the concept of a system?

In chemistry, any object under study — whether a biological tissue, a flask with reagents, or a mixture of substances — is viewed as a system. The main criterion of a system is that it must consist of a giant number of structural units. Single molecules or atoms are not considered systems.

The difference between the micro- and macro-level is colossal. Let us consider the classic reaction between hydrogen and chlorine ($H_2 + Cl_2$).

• At the system level (in a gas mixture): we can state with absolute certainty that hydrogen chloride ($HCl$) will form as a result of the reaction. This is a statistical regularity.
• At the micro-level (in a vacuum): if we collide just two molecules, the outcome is unpredictable. They may simply bounce off each other, dissociate into individual atoms, or interact.

This is precisely why all chemical predictions and calculations apply exclusively to systems.

Homogeneous and heterogeneous systems

Chemical systems are generally divided into two major classes depending on their uniformity:

1. Homogeneous systems. In these, the composition and physicochemical properties are completely identical at any, even the smallest, point of the volume. There are no surfaces separating one part of the system from another.
2. Heterogeneous systems. They consist of multiple phases — parts that differ from one another in composition or properties. There is always a physical interface between these phases.

A striking example of a heterogeneous system is a beaker with an aqueous sodium chloride solution containing floating ice cubes. In this case, we have two distinct phases (liquid solution and solid ice) with a visible boundary between them.

True solutions and their components

A special place among chemical systems is occupied by true solutions. These are exclusively homogeneous systems formed as a result of the spontaneous breakdown of a substance into the smallest structural units — ions, atoms, or molecules.

Any solution consists of at least two components:

• Solvent — the substance acting as the medium.
• Solute — the substance distributed within that medium.

It is important to remember that solutions can exist not only in the liquid, but also in the solid state of aggregation (e.g., metal alloys). However, gas mixtures are excluded from this category. The distances between molecules in gases are so large that they practically do not interact with one another. The role of the medium in a gas is played by empty space, not a solvent.

Heterogeneous dispersions

If a substance in a system is distributed not as individual molecules or ions, but as larger entities (droplets, sand grains, powder), such a system is not a true solution.

Such heterogeneous systems include:

• Suspensions (solid particles in a liquid);
• Emulsions (liquid in liquid);
• Colloidal solutions.

This leads to terminological confusion. For example, a construction mixture of cement, sand, and water is colloquially called a "solution". However, from the perspective of chemistry, this is a classic heterogeneous suspension, since it is not homogeneous and has not been broken down to the molecular level.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of spontaneous substance breakdown in solutions
• Differences between colloidal solutions, emulsions, and suspensions
• Solid solutions: specifics and examples
• Statistical reliability of chemical processes

Other topics in "Chapter 4. Solutions"

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