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 body fluids in plants and animals are also solutions. The process of digesting food 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 industries, metal processing, in mining, in the production of plastics and fibers. That is why it is important to understand what they are, to know their properties and distinguishing features.
Signs of true solutions
By solutions we mean 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 forming 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. The following features are characteristic of such molecularly dispersed systems:
- affinity (interaction);
- spontaneous education;
- constancy of concentration;
- homogeneity;
- sustainability.
In other words, they can form if there is an interaction between the components, which leads to spontaneous separation of the substance into tiny particles without the efforts exerted from the outside. The resulting solutions should be single-phase, that is, there should not be an interface between the components. The last sign is the most important, since the spontaneous dissolution process can proceed only if it is energetically beneficial for the system. In this case, there is a decrease in free energy, and the system becomes equilibrium. Given all these features, the following definition can be formulated:
A true solution is a stable equilibrium system of interacting particles of two or more substances, the sizes of which do not exceed 10 -7 cm, that is, are proportional to atoms, molecules and ions.
One of the substances is a solvent (as a rule, it is the component whose concentration is higher), and the rest are dissolved substances. If the starting materials were in different states of aggregation, then the solvent that is not changed is taken as the solvent.
Types of True Solutions
According to the state of aggregation, the solutions are liquid, gaseous, and solid. The most common liquid systems, and they are also divided into several types depending on the initial state of the dissolved substance:
- solid in liquid, for example, sugar or salt in water;
- liquid in liquid, for example sulfuric or hydrochloric acid in water;
- gaseous in liquid, for example oxygen or carbon dioxide in water.
However, the solvent may be not only water. 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.
By electrical conductivity, solutions are divided into electrolytes and non-electrolytes. Electrolytes are compounds with a predominantly ionic crystalline bond, which upon dissociation in solution form ions. Non-electrolytes dissolve into atoms or molecules upon dissolution.
In true solutions, two opposite processes simultaneously occur - the dissolution of the substance and its crystallization. Depending on the equilibrium position in the "dissolved substance - solution" system, the following types of solutions are distinguished:
- saturated when the dissolution rate of a substance is equal to its crystallization rate, 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 solute in comparison with saturated, and one crystal of it is enough to start active crystallization.
As a quantitative characteristics reflecting the content of a component in solution, a concentration is used. Solutions with a low content of solute are called diluted, and with a high - concentrated.
Ways to Express Concentration
Mass fraction (ω) - the mass of the substance (m in-va ), referred to the mass of the solution (m r-ra ). In this case, the mass of the solution is taken as the sum of the masses of the substance and the solvent (m p-la ).
Molar fraction (N) - the number of moles of solute (N -va ), referred to the total number of moles of substances that form the solution (ΣN).
Molarity (C m ) - the number of moles of solute (N in-va ), referred to the mass of the solvent (m p-la ).
Molar concentration (C m ) is the mass of the solute (m in-va ), referred to the volume of the whole solution (V).
Normality, or equivalent concentration, (C n ) is the number of equivalents (E) of the solute, referred to the volume of the solution.
The titer (T) is the mass of the substance (m in-va ), dissolved in a given volume of solution.
The volume fraction (ϕ) of a gaseous substance is the volume of the substance (V in-va ), referred to the volume of the solution (V r-ra ).
Solution properties
Considering this issue, they most often speak of dilute solutions of non-electrolytes. This is due, firstly, to the fact that the degree of interaction between the 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 above the solution is described by Raoul’s law, which states that the decrease in the saturated vapor pressure of the solvent ΔP above the solution is directly proportional to the molar fraction of the solute (T in-va ) and the vapor pressure above the pure solvent (P 0 p-la ):
? P = P o A p-∙ T-va
An increase in the boiling points Δk and the freezing temperatures Δ of solutions is directly proportional to the molal concentration of the substances C m dissolved in them:
Δ = ∙ m , where is an ebulioscopic constant;
ΔT z = K ∙ C m , where K is a cryoscopic constant.
The osmotic pressure π is calculated by the equation:
π = P ∙ E ∙ X in-v / V r-la ,
where X in-va - the molar fraction of the solute, V p-la - the volume of solvent.
The value of solutions in the everyday life of any person is difficult to overestimate. Natural water contains dissolved gases - 2 and 2 , various salts - NaCl, CaSO4, MgCO3, KCl, etc. But without these impurities in the body, water-salt metabolism and the cardiovascular system could be disturbed. Another example of true solutions is metal alloy. It can be brass or jewelry gold, but, most importantly, after mixing the molten components and cooling the resulting solution, one solid phase is formed. Metal alloys are used everywhere, starting with cutlery, and ending with electronics.