Solutions, as well as the process of their formation, are of great importance in the world around us. Water and air are two of their representatives, without which life on Earth is impossible. Most biological fluids in plants and animals are also solutions. The process of digestion is inextricably linked with the dissolution of nutrients.
Any production is associated with the use of certain types of solutions. They are used in the textile, food, pharmaceutical, metalworking, mining, plastics and fiber industries. That is why it is important to understand what they are, to know their properties and distinguishing features.
Signs of true solutions
Solutions are understood as multicomponent homogeneous systems formed during the distribution of one component in another. They are also called dispersed systems, which, depending on the size of the particles that form them, are divided into colloidal systems, suspensions and true solutions.
In the latter, the components are in a state of separation into molecules, atoms or ions. Such molecular-dispersed systems are characterized by the following features:
- affinity (interaction);
- spontaneity of education;
- constancy of concentration;
- homogeneity;
- sustainability.
In other words, they can be formed if there is an interaction between the components, which leads to the spontaneous separation of the substance into tiny particles without external efforts. The resulting solutions should be single-phase, that is, there should be no interface between the constituent parts. The last sign is the most important, since the dissolution process can proceed spontaneously only if it is energetically favorable for the system. In this case, the free energy decreases, and the system becomes equilibrium. Taking into account all these features, we can formulate the following definition:
A true solution is a stable equilibrium system of interacting particles of two or more substances, the size of which does not exceed 10-7cm, that is, they are commensurate with atoms, molecules and ions.
One of the substances is a solvent (as a rule, this is the component whose concentration is higher), and the rest are solutes. If the original substances were in different states of aggregation, then the solvent is taken as the one that did not change it.
Types of true solutions
According to the state of aggregation, solutions are liquid, gaseous and solid. Liquid systems are the most common, and they are also divided into several types depending on the initial state.solute:
- solid in liquid, like sugar or s alt in water;
- liquid in liquid, such as sulfuric or hydrochloric acid in water;
- gaseous to liquid, like oxygen or carbon dioxide in water.
However, not only water can be a solvent. And by the nature of the solvent, all liquid solutions are divided into aqueous, if the substances are dissolved in water, and non-aqueous, if the substances are dissolved in ether, ethanol, benzene, etc.
According to electrical conductivity, solutions are divided into electrolytes and non-electrolytes. Electrolytes are compounds with a predominantly ionic crystalline bond, which, when dissociated in solution, form ions. When dissolved, non-electrolytes break down into atoms or molecules.
In true solutions, two opposite processes occur simultaneously - the dissolution of a substance and its crystallization. Depending on the position of equilibrium in the "solute-solution" system, the following types of solutions are distinguished:
- saturated, when the rate of dissolution of a certain substance is equal to the rate of its own crystallization, that is, the solution is in equilibrium with the solvent;
- unsaturated if they contain less solute than saturated at the same temperature;
- supersaturated, which contain an excess of a dissolved substance in comparison with a saturated one, and one crystal of it is enough to start active crystallization.
As a quantitativecharacteristics, reflecting the content of a particular component in solutions, use the concentration. Solutions with a low content of a solute are called dilute, and with a high content - concentrated.
Ways to Express Concentration
Mass fraction (ω) - the mass of a substance (mv-va), referred to the mass of the solution (mp-ra). In this case, the mass of the solution is taken as the sum of the masses of the substance and the solvent (mp-la).
Mole fraction (N) - the number of moles of a solute (Nv-va) divided by the total number of moles of substances that form a solution (ΣN).
Molality (Cm) - the number of moles of a solute (Nv-va) divided by the mass of the solvent (m r-la).
Molar concentration (Cm) - the mass of the solute (mv-va) referred to the volume of the entire solution (V).
Normality, or equivalent concentration, (Cn) - the number of equivalents (E) of the solute, referred to the volume of the solution.
Titer (T) - the mass of a substance (m in-va) dissolved in a given volume of solution.
Volume fraction (ϕ) of a gaseous substance - the volume of the substance (Vv-va) divided by the volume of the solution (Vp-ra).
Properties of solutions
Considering this issue, most often they talk about dilute solutions of non-electrolytes. This is due, firstly, to the fact that the degree of interaction between particles brings them closer to ideal gases. And secondly,their properties are due to the interconnectedness of all particles and are proportional to the content of the components. Such properties of true solutions are called colligative. The vapor pressure of the solvent over the solution is described by Raoult's law, which states that the decrease in saturated vapor pressure of the solvent ΔР over the solution is directly proportional to the molar fraction of the solute (Tv-va) and the vapor pressure over the pure solvent (R0r-la):
ΔР=Рor-la∙ Tv-va
The increase in boiling points ΔТк and freezing points ΔТз of solutions is directly proportional to the molar concentrations of substances dissolved in them Сm:
ΔTk=E ∙ Cm, where E is the ebullioscopic constant;
ΔTz=K ∙ Cm, where K is the cryoscopic constant.
Osmotic pressure π is calculated by the equation:
π=R∙E∙Xv-va / Vr-la, where Xv-va is the molar fraction of the solute, Vr-la is the volume of the solvent.
The importance of solutions in the everyday life of any person is difficult to overestimate. Natural water contains dissolved gases - CO2 and O2, various s alts - NaCl, CaSO4, MgCO3, KCl, etc. But without these impurities in the body could disrupt the water-s alt metabolism and the work of the cardiovascular system. Another example of true solutions is an alloy of metals. It can be brass or jewelry gold, but, most importantly, after mixingmelted components and cooling of the resulting solution, one solid phase is formed. Metal alloys are used everywhere, from cutlery to electronics.
