Pure substances practically do not occur in nature . Basically, they are presented in the form of mixtures that are capable of forming homogeneous or heterogeneous systems.
Features of true solutions
True solutions are a type of disperse systems that have greater strength between the dispersion medium and the dispersed phase.
Any chemical can have crystals of different sizes. In any case, they will have the same internal structure: ionic or molecular crystal lattice.
Dissolution
In the process of dissolving grains of sodium chloride and sugar in water, an ionic and molecular solution forms. Depending on the degree of fragmentation, the substance is able to be in the form of:
- visible macroscopic particles larger than 0.2 mm;
- microscopic particles having a size less than 0.2 mm, they can be caught only with a microscope.
True and colloidal solutions differ in particle size of the solute. Crystals invisible under a microscope are called colloidal particles, and the resulting state is called a colloidal solution.
Solution phase
In many cases, true solutions are fragmented (dispersed) systems of a homogeneous form. They have a continuous continuous phase - a dispersion medium, and fragmented particles of a certain shape and size (dispersed phase). What is the difference between colloidal solutions from true systems?
The main difference is in particle sizes . Colloidal-dispersed systems are considered heterogeneous, since it is impossible to detect the phase boundary in a light microscope.
True solutions - this is the option when in the environment the substance is presented in the form of ions or molecules. They belong to single-phase homogeneous solutions.
As a prerequisite for the formation of disperse systems, the mutual dissolution of the dispersion medium and dispersible substance is considered. For example, sodium chloride and sucrose are insoluble in benzene and kerosene, so colloidal solutions will not form in such a solvent.
Classification of Dispersed Systems
How are disperse systems divided? True solutions, colloidal systems differ in several ways.
There is a division of disperse systems according to the state of aggregation of the medium and the dispersed phase, the formation or absence of interaction between them.
Characteristics
There are certain quantitative characteristics of the dispersion of a substance. First of all, the degree of dispersion is distinguished . This value is the reciprocal of the particle size. It characterizes the number of particles that can be placed in a row at a distance of one centimeter.
In the case when all particles have the same size, a monodisperse system is formed. With unequal particles of the dispersed phase, a polydisperse system is formed.
With an increase in the dispersion of a substance, processes that occur in the interphase surface increase in it. For example, the specific surface of the dispersed phase increases, and the physicochemical effect of the medium at the interface between the two phases increases.
Dispersed System Options
Depending on which phase the solute will be in, different varieties of dispersed systems are isolated.
Aerosols are dispersed systems in which a dispersed medium is presented in gaseous form. Mists are aerosols having a liquid dispersed phase. Smoke and dust are formed by a solid dispersed phase.
Foams are a dispersion of a gaseous substance in a liquid. Fluids in foams degenerate into films that separate gas bubbles.
Emulsions are called disperse systems, where one liquid is distributed over the volume of another without dissolving in it.
Suspensions or suspensions are low dispersion systems in which solid particles are in a liquid. Colloidal solutions or sols in an aqueous dispersed system are called hydrosols.
Depending on the presence (absence) between the particles of the dispersed phase, free-dispersed or coherent-dispersed systems are distinguished. The first group includes lyosols, aerosols, emulsions, suspensions. In such systems, there are no contacts between particles and the dispersed phase. They move without limitation in solution under the action of gravity.
Coherently dispersed systems arise in the case of contact of particles with a dispersed phase, resulting in the formation of structures in the form of a grid or a frame. Such colloidal systems are called gels.
The gelation (gelation) process is the transformation of a sol into a gel, based on a decrease in the stability of the initial sol. Examples of coherent systems are suspensions, emulsions, powders, foams. They also include soil formed during the interaction of organic (humic) substances and soil minerals.
Capillary-dispersed systems are distinguished by a continuous mass of substance penetrating the capillaries and pores. They consider fabrics, different membranes, wood, cardboard, paper.
True solutions are homogeneous systems consisting of two components. They can exist in solvents different in the state of aggregation. A solvent taken in excess is considered a solvent. A component that is not taken in sufficient quantities is considered a dissolved substance.
Features of solutions
Hard alloys are also solutions in which various metals act as a dispersed medium and component. From a practical point of view, liquid mixtures in which the liquid acts as a solvent are of particular interest.
Of the many inorganic solvents, water is of particular interest. Almost always, a true solution is formed when particles of a soluble substance are mixed with water.
Among organic compounds, the following substances are excellent solvents: ethanol, methanol, benzene, carbon tetrachloride, acetone. Due to the chaotic movement of molecules or ions of the dissolved component, their partial transition into solution occurs, the formation of a new homogeneous system.
Substances differ in their ability to form solutions. Some can be mixed with each other in unlimited quantities. An example is the dissolution of salt crystals in water.
The essence of the dissolution process from the point of view of molecular kinetic theory is that after the introduction of crystals of sodium chloride into the solvent, it dissociates into sodium cations and chlorine anions. Charged particles undergo oscillatory motion, collisions with particles of the solvent itself lead to the transition of ions into the solvent (binding). Gradually, other particles are connected to the process, the surface layer is destroyed, the salt crystal dissolves in water. Diffusion allows you to distribute particles of a substance by volume of solvent.
Types of True Solutions
A true solution is a system that is divided into several types. There is a classification of such systems into aqueous and non-aqueous by type of solvent. They are also classified according to the version of the solute on alkali, acid, salt.
There are various types of true solutions with respect to electric current: non-electrolytes, electrolytes. Depending on the concentration of the solute, they can be diluted or concentrated.
From the thermodynamic point of view, true solutions of low molecular weight substances are divided into real and ideal.
Such solutions can be ion dispersed as well as molecular dispersed systems.
Saturation of solutions
Depending on how many particles go into solution, there are supersaturated, unsaturated, saturated solutions. The solution is a liquid or solid homogeneous system, which consists of several components. In any such system, a solvent is necessarily present, as well as a dissolved substance. When some substances are dissolved, heat is generated.
A similar process confirms the theory of solutions, according to which, dissolution is considered as a physicochemical process. There is a division of the solubility process into three groups. The first is composed of substances that are capable of dissolving in an amount of 10 g per 100 g of solvent, they are called well soluble.
Materials are considered insoluble if less than 10 g is dissolved in 100 g of the component, the rest are called insoluble.
Conclusion
Systems consisting of different in aggregate state, particle sizes, are necessary for normal human life. True, colloidal solutions, discussed above, are used for the manufacture of drugs, the creation of food products. Having an idea of the concentration of the solute, you can independently prepare the necessary solution, for example, ethyl alcohol or acetic acid, for various purposes in everyday life. Depending on the state of aggregation of the soluble substance and solvent, the resulting systems have certain physical and chemical characteristics.