Concept of a system and basic characteristics of solutions
In chemistry, the concept of a "solution" goes far beyond the everyday notion of a transparent liquid. To properly describe such objects, the fundamental term system is introduced. This is absolutely any object that contains a multitude of structural units, has real or conditional boundaries, and is isolated from the environment for study. A chemical flask, an aerosol can, a crystal, a plant, and even a loaf of bread can all be considered systems.
From a scientific point of view, a solution is a homogeneous system, which is necessarily characterized by three features:
- Multicomponentness. There are always at least two substances present in the composition (a solvent and a dissolved compound).
- Interaction. The particles within the volume are not merely mechanically mixed, but actively contact and interact with one another.
- Variable composition. The proportions of the components can be varied smoothly, without leaps, within certain limits while maintaining homogeneity.
According to their state of aggregation, mixtures can be liquid (aqueous and non-aqueous, for example, pharmacy alcoholic iodine tincture) and solid (various metal alloys). However, in medical practice, when speaking of solutions, liquid forms are most commonly meant.
Changes in physical properties
A key feature of homogeneous mixtures is that their final physical properties differ cardinally from the characteristics of the initial individual substances.
- Cryoscopy (change in freezing point). Pure water turns into ice at 0 °C while expanding in volume. But if ethylene glycol (HOCH₂CH₂OH) is dissolved in it, antifreeze is formed. Depending on the concentration, this liquid does not freeze down to temperatures from –40 °C to –65 °C.
- Electrical conductivity. Chemically pure distilled water and dry crystalline sodium chloride (NaCl) do not conduct current (they are dielectrics). However, their aqueous solution becomes a powerful electrolyte with high conductivity.
- Solid solutions. Metal alloys demonstrate strength, plasticity, and melting points that are completely atypical for the pure metals from which they were synthesized.
Kinetics and safety of chemical reactions
The widespread use of liquid media in laboratories and industry is due to their unique kinetic and thermodynamic advantages.
Rate and completeness of interaction. In liquids, all molecules move freely. This ensures constant and effective collisions of particles throughout the entire volume of the flask—the reaction proceeds quickly and to completion. For comparison, in solids, atoms are rigidly fixed in the crystal lattice. Interaction there is possible exclusively at the points of surface contact, while the internal volume of the crystal remains inert. Mechanical grinding or pressing accelerates the process, but the completeness of the reaction is achieved extremely slowly. A clear example is effervescent tablets. In dry form they are stable, and begin to vigorously release gas only when a solvent (water) is added.
Thermodynamic safety. Solid-phase reactions are often accompanied by structural destabilization and avalanche-like energy release, which is fraught with dangerous overheating and even explosion. In liquid media, a huge mass of solvent acts as a reliable buffer: it accumulates heat, preventing thermal catastrophes.
Advantages over the gas phase. In gases, particles are also maximally mobile, but not all reactants can be converted into a vapor state. Many molecules are thermolabile and break down upon heating to the boiling point. The solvent allows reactions with such unstable, non-volatile substances to be carried out under mild conditions.
Medical and pharmaceutical significance
Liquid homogeneous systems play a fundamental role in biology, ensuring the continuous flow of all metabolic transformations.
Dissolution makes it possible to delicately regulate the strength of action (activity) of a substance, which is critically important in medicine. The dependence of the effect on concentration is clearly seen in the example of potassium permanganate (KMnO₄):
- Bright pink: used externally as an antiseptic for treating abrasions.
- Pale pink: prepared by strong dilution with water and safely used per os for gastric lavage in case of food poisoning.
- Dark purple (concentrated): strictly not used for therapeutic purposes, as its high activity causes severe chemical burns of the skin and mucous membranes.
Also, liquid forms are indispensable in pharmaceutical technology. Measuring out a drug by volume is much faster and technologically simpler than weighing dry microdoses. Factories prepare huge vats of liquid with an ideally calibrated concentration, and then automatic machines instantly pour it into ampoules (for example, 2 ml with 40 mg of the active substance in each), eliminating the risk of error.
Briefly: answers to topical questions
What states of aggregation can solutions have?
Solutions can be liquid and solid.
- Liquid solutions are aqueous and non-aqueous systems, such as pharmacy tincture of iodine.
- Solid solutions are metal alloys whose properties differ from those of individual metals.
In medical and chemical practice, the term "solution" most often refers to liquid systems. They serve as a mobile medium for conducting chemical processes.
Which colligative properties of solutions are of the greatest importance for physiology and medicine?
Of the greatest importance for physiology and medicine is the phenomenon of osmotic pressure and its component part—oncotic pressure. There are four colligative properties in total:
- Lowering of the solvent vapor pressure — depends on the mole fraction of the solute.
- Elevation of the solvent boiling point — ebullioscopy.
- Lowering of the solvent freezing point — cryoscopy.
- Phenomenon of osmotic pressure — ensures the constancy of metabolic processes in tissues, supports water-salt metabolism, and is the basis of the kidney mechanism.
What methods of expressing solution concentration are used in medical chemistry?
In medical chemistry, methods of expressing concentration via mass fraction, molar, and osmolar concentrations are used.
- Mass fraction — the ratio of the mass of the solute to the total mass of the solution (expressed in fractions of a unit or percentages).
- Molar concentration — the ratio of the amount of substance to the volume of the solution (the main unit is mol/L). For low concentrations often used in medicine, mmol/L is used.
- Osmolar concentration — used to assess osmotic phenomena in cells and biological fluids.
What types of solutions are distinguished by their osmotic pressure relative to blood plasma?
According to the magnitude of osmotic pressure relative to cells, three types of solutions are distinguished.
| Type of solution | Osmotic pressure | Effect on cells |
|---|---|---|
| Isotonic | Equal to the osmotic pressure in cells | Cell volume does not change (isoosmia) |
| Hypertonic | Higher than in the cell | Cells decrease in volume; erythrocytes shrink |
| Hypotonic | Lower than in the cell | Cells increase in volume; erythrocytes swell and may rupture, which is called hemolysis |
An isotonic solution, such as 0.9% NaCl, is used for intravenous injections and volume replacement in blood loss.
Remember the three characteristics of a solution by the acronym VPI: Variable composition, Poly-component nature, Interaction of particles.
Class and exam questions
What is the main danger of carrying out chemical reactions in the solid phase?
In solid substances, processes can lead to the destabilization of the crystal structure and an avalanche-like release of energy, causing dangerous overheating and a risk of explosion.
Why cannot concentrated potassium permanganate solution be used in medicine?
Dark purple KMnO₄ concentrate has excessive chemical activity. Instead of a therapeutic effect, it causes severe chemical burns to mucous membranes and skin.
Why do pharmaceutical manufacturers prefer liquid forms for ampoules rather than powders?
Volumetric dosing of a liquid substance allows for rapid automation. This is technologically simpler and more precise than individual weighing of dry powder microdoses for each separate ampoule.
Can a solution have a non-liquid state of aggregation?
Yes, various metal alloys serve as a classic example of solid solutions, and their properties differ from those of the individual metals they comprise.
What else to study on this topic
- Mechanisms of chemical interaction of particles within solutions
- Thermodynamics and energetics of the dissolution process
- Colligative properties: detailed analysis of cryoscopy and ebullioscopy
- Methods of expressing solution concentration in medical practice
- Specifics of metabolic processes in the body's aqueous environments
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Other topics in "Chapter 4. Solutions"
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