Solutions are homogeneous (uniform) mixtures consisting of two or more components ( components). The difference between a solution and other mixtures is that the molecules of substances are distributed evenly in it and in any microvolume of such a mixture its composition is the same. In the language of chemical thermodynamics, such a mixture is called single-phase. Like individual (pure) substances, solutions can be in the liquid, solid or gas phase (see Phases). For example, air is a solution of various gases - nitrogen, oxygen, hydrogen, carbon dioxide, water vapor, etc. At the same time, dust particles and liquid droplets (fog) are not components of a gas solution, since inside a speck of dust we would only find solid substance, and inside the fog droplets there is only liquid, water. Thus, both dust and fog are solid and liquid phases scattered (dispersed) in a solution of gases. The difference between a solution and a pure substance is that an individual substance has certain physical constants, for example, melting and boiling points, a certain chemical composition, while the physical constants and composition of solutions depend on the ratio of their components. Thus, the density of the salt solution in water increases, and the freezing point decreases with increasing salt content.
Pure substances, when their phase state changes, do not change their chemical composition, and upon returning to the initial phase state, they acquire the original characteristics. The components of solutions can separate when the phase state of the system changes. Thus, the evaporation of water from a saline solution (an operation that has long been used in the extraction of salt) leads, on the one hand, to an increase in the content (concentration) of salt in the remaining solution, and on the other, the condensed water is a pure substance. Further evaporation of water will lead to the precipitation of the solid phase - salt crystals.
The process of solution formation - dissolution - consists in the destruction of the interaction between the molecules of individual substances and the formation of new intermolecular bonds between the components of the solution. Dissolution is possible only when the interaction energy between the components of the solution is greater than the sum of the interaction energies in the original substances.
When an ionic crystal of table salt is dissolved in water, the polar molecules of the solvent cover the ions with a coat of dipoles (electric charges equal in magnitude and opposite in sign). This so-called solvation shell completely separates the ions. The general name for this interaction with a solvent is solvation. Solvation leads to the formation of a variety of bonds between molecules in solution: ion-dipole, which is described above, dipole-dipole (for example, chloroform dipoles interact with ethanol dipoles) or the formation of hydrogen bonds (see Chemical bond). The last interaction is one of the strongest and plays an important role in the dissolution of organic and inorganic substances.
The dissolution of organic substances in each other is facilitated by the similarity of their structures. Antique chemical rule- like dissolves in like - is explained by the fact that in this case the interactions between different molecules are similar in type and close in energy to the interactions in the original substances. Thus, the formation of hydrogen bonds between water and alcohol molecules easily compensates for the destruction of hydrogen bonds in the starting substances when mixing these liquids. Non-polar hydrocarbon molecules cannot insert themselves between water molecules connected by hydrogen bonds, which prevents their dissolution. Often, dissolution does not completely destroy the intermolecular bonds within individual substances, and they remain partially connected (associated). For example, organic acids are mostly present in organic nonpolar solvents as dimers linked by hydrogen bonds. Such associates are destroyed upon further dilution. As the solution is concentrated, the association becomes stronger, and there are not enough solvent molecules to separate the solute molecules or ions. In this case, a system of intermolecular bonds of the original individual substance is formed inside the solution, which is released into a separate phase. The remaining solution, which is in equilibrium with the released component, is called saturated. By increasing the temperature, it is possible to destroy the association and transfer the precipitated component into the solution. However, this is not always possible.
Inorganic substances can also decrease their ability to dissolve (solubility) with increasing temperature. Solubility solids in a liquid is determined by the heat of dissolution, which can be positive (heat is released during dissolution, and the substance dissolves worse with increasing temperature) or negative (heat during dissolution is absorbed, and solubility increases with increasing temperature). Since there are no intermolecular interactions in gases, their ability to dissolve mutually is unlimited. Their solubility in liquids decreases with increasing temperature, since the intermolecular interactions of gas molecules with the solvent are weakened.
Solid solutions also exist in nature. These are mainly metal alloys. The physical reason for such dissolution is the introduction of atoms of one metal into the crystal lattice of another and the construction of a common crystal lattice.
Methods of expressing the composition of solutions
The composition of solutions is usually expressed quantitatively through dimensionless relative quantities - fractions (mass, volume, molar) and dimensional quantities - concentrations. Concentration shows the ratio of the mass or amount of solute to the volume of solution.
Molar concentration is the ratio of the amount of solute B to the volume of solution:
The unit of molar concentration is mol/m3 or mol/L (the latter is much more commonly used). To denote the unit of molar concentration, the symbol M is usually used, for example: - one-molar solution (mol/l); - centimolar solution (mol/l).
K category: Selection of building materials
Mortars
Mortars are mineral mixtures that harden and bond firmly to the stone. The solution must contain a binder (cement, gypsum or lime), aggregate (gravel or sand) and clean water.
Depending on the purpose and use of mortar additives, the following solutions are prepared:
1. Construction, for bricklaying.
2. Plastering.
3. Plaster.
4. Cement.
The mortar for masonry should consist of sand and lime in a ratio of 3: 1 or 4: 1. 1 or 2 shovels of cement can be added to the mortar. This is especially necessary when constructing walls that bear a special load. Sand and cement in this case are mixed in a ratio of 3:1 -6:1.
To prepare the plaster mortar, you can use both hydraulic lime and air lime. It also contains sand. There is a difference between plaster mortar for external work and plaster mortar for internal work.
In the first case, hydraulic lime and sand are taken in a ratio of 1:3; air and lime - 1: 2. In the second case, hydraulic lime and sand are mixed in a ratio of 1: 5, and air lime - 1: 3.
Gypsum mortar differs from cement and lime mortar in its high strength and ease of preparation. To do this, all you need to do is take a container, pour it into water, pour in the plaster and mix everything thoroughly so that there are no lumps, which could later cause cracks to appear. Dilute the plaster with water immediately before working with it, because it may thicken ahead of time, then you will not be able to work with it. To prevent this from happening, you can mix a little sifted sand (2:1) into the plaster, but be aware that this will significantly reduce the strength of the gypsum.
Cement mortar necessary for the preparation of durable plaster. To do this, take pure cement and water in a ratio of 1: 2 (1: 3).
Mortar additives are necessary to improve the quality of solutions. They significantly improve the physical and mechanical properties of solutions, their color, and frost resistance.
When coloring solutions, in addition to the usual additives, you can only use paints of bright colors that do not contain gypsum and barite impurities. Frost resistance is achieved by adding chlorides to the solution. They allow you to work with the solution at fairly low sub-zero temperatures. Chlorides and other means of protection from exposure low temperatures are used with the utmost caution, because an overdose of substances, as a rule, leads to the formation of unsightly smudges.
Mortars characterized by three main parameters: density, type binder and its purpose.
Depending on the density (in a dry state), heavy (density 1500 kg/m3 or more) and light (density less than 1500 kg/m3) solutions are distinguished. To make heavy solutions, heavy quartz or other sands are used; fillers in light solutions are light porous sands made from pumice, tuff, slag, expanded clay, etc. Light solutions are also obtained using foaming additives - porous solutions.
Based on the type of binder, construction mortars are divided into cement (based on Portland cement or its varieties), lime (based on air or hydraulic lime), gypsum (based on gypsum binders) and mixed (based on cement-lime, cement-clay, lime-gypsum binders) . Solutions prepared with one binder are called simple, and solutions prepared with several binders are called mixed (complex).
According to their intended purpose, mortars can be masonry (for masonry, installation of walls from large-sized elements), finishing (for plastering premises, applying decorative layers to wall blocks and panels), special ones with special properties (waterproofing, acoustic, X-ray protective).
The choice of binder depends on the purpose of the solution, the requirements for it, the temperature and humidity conditions of hardening and the operating conditions of the building. Portland cements, pozzolanic Portland cements, slag Portland cements, special low-grade cements, lime, and gypsum binders are used as binders. To save hydraulic binders and improve the technological properties of mortars, mixed binders are widely used. Lime in mortars is used in the form of lime paste or milk. Gypsum is mainly used in plaster solutions as an additive to lime.
The water used for solutions should not contain impurities that have a harmful effect on the hardening of the binder. Tap water is suitable for these purposes.
If the solution is used in winter conditions, hardening accelerators are added to its composition, as well as additives that reduce the freezing point of water (calcium chloride, sodium chloride, potash, sodium nitrate, etc.).
The composition of a mortar is indicated by the quantity (by mass or volume) of materials per 1 m3 of mortar or by the relative ratio (by mass or volume) of the original dry materials. In this case, the binder consumption is taken as 1. For simple solutions, consisting of a binder (cement or lime) and not containing mineral additives, the composition is designated 1: 4, that is, for 1 mass part of cement there are 4 mass parts of sand. Mixed mortars consisting of two binders or containing mineral additives are designated by three numbers, for example 1: 3: 4 (cement: lime: sand).
The quality of mortar mixtures is characterized by their workability - the ability to be laid without special compaction on the base in a thin layer, filling all its unevenness. Workability is determined by the mobility and water-holding capacity of mortar mixtures.
Mobility is the ability of a mortar mixture to spread under the influence of its own mass. Mobility is determined (in cm) by the depth of immersion in the mortar mixture of a standard cone weighing 300 g with an apex angle of 30° and a height of 15 cm. The deeper the cone is immersed in the mortar mixture, the greater mobility it has.
The degree of mobility of the mixture depends on the amount of water, the composition and properties of the starting materials. To increase the mobility of mortar mixtures, plasticizing additives and surfactants are added to them.
The mobility of mortars, depending on their purpose and method of installation, should be as follows.
Laying walls made of bricks, concrete stones, light rock stones: 9-11
Laying hollow brick walls, ceramic stones: 7-8.
Filling horizontal joints when installing walls made of concrete blocks and panels; Joining vertical and horizontal seams: 5-7.
Rubble masonry: 4-6.
Filling voids in rubble masonry: 13-15.
Water retention capacity is the ability of a solution to retain water when laid on a porous base. If the mortar has good water-holding capacity, partial suction of water compacts it in the masonry, which increases the strength of the mortar. The water holding capacity depends on the ratio of the components of the mortar mixture. It increases with increasing cement consumption and frequent replacement! cement, the introduction of additives (ash, clay, etc.), as well as some surfactants. The strength of the hardened mortar depends on the activity of the binder, the water-cement ratio, the duration and conditions of hardening (temperature and humidity environment). When laying mortar mixtures n; a porous base capable of intensively sucking out water, the hardening strength of solutions is significantly higher than the same solutions laid on a dense base.
The strength of the mortar depends on its brand, which is determined by the compressive strength after 28 days of hardening at air temperature; 5-25° C. The following brands of solutions are available: 4, 10, 15, 50, 75, 100, 150, 2t)0 and 300
The frost resistance of solutions is determined by the number of cycles of alternating freezing and thawing until the loss of 15% of the original strength (or 5% of the mass). PS frost resistance solutions are divided into Mrz grades from 10 to 300.
- Construction solutions
What are substitutional and interstitial solid solutions? Give examples.
Solid solutions are phases in which one of the components of the alloy retains its crystal lattice, and the atoms of other (or other) components are located in the lattice of the first component (solvent), changing its dimensions (periods).
Thus, a solid solution consisting of two or more components has one type of lattice and represents one phase.
There are interstitial solid solutions and substitutional solid solutions.
During the formation of interstitial solid solutions, the atoms of the dissolved component B are located between the atoms of the solvent A in its crystal lattice. During the formation of substitutional solid solutions, atoms of the dissolved component B replace part of the atoms of the solvent (component A) in its crystal lattice.
Since the sizes of dissolved atoms differ from the sizes of solvent atoms, the formation of a solid solution is accompanied by distortion of the crystal lattice of the solvent. 
a – the atom of the dissolved component is larger than the solvent atom
b – the atom of the dissolved component is smaller than the solvent atom
Substitutional solid solutions can have limited or unlimited solubility. In solid solutions with limited solubility, the concentration of the dissolved component is possible up to certain limits.
In solid solutions with unlimited solubility, any concentration of the dissolved component is possible (from 0 to 100%). Solid solutions with unlimited solubility are formed if following conditions: 1) the components must have the same type of crystal lattices; 2) the difference in the atomic radii of the components should not exceed 9% for iron-based alloys, and 15% for copper-based alloys; 3) components must be close physical and chemical properties. However, compliance with these properties does not always lead to the formation of substitutional solid solutions with unlimited solubility. In practice, as a rule, solid solutions with limited solubility are formed.
Interstitial solid solutions can only be of limited concentration, since the number of pores in the lattice is limited, and the atoms of the main component are retained at lattice sites.
Substitutional solid solutions with unlimited solubility based on components: Ag and Au, Ni and Cu, Mo and W, V and Ti, etc.
Substitutional solid solutions with limited solubility based on components: Al and Cu, Cu and Zn, etc.
Interstitial solid solutions: when non-metallic elements such as carbon, boron, nitrogen and oxygen are dissolved in metals. For example: Fe and C.
How and why do the properties of metals change during cold plastic deformation?
Cold deformation is one that is carried out at a temperature below the recrystallization temperature. Therefore, cold deformation is accompanied by hardening (hardening) of the metal.
The shape of the workpiece during pressure treatment changes under the influence of external forces due to plastic deformation of each crystallite in accordance with the scheme of main deformations. The main change in the shape of crystallites is that they are elongated in the direction of the main tensile strain (for example, in the direction of rolling or drawing). As the degree of cold deformation increases, the grains become more and more elongated and the structure becomes fibrous.
Strengthening of the metal during plastic deformation (hardening) is explained by an increase in the number of defects in the crystal structure (dislocations, vacancies, interstitial atoms). An increase in the density of defects in the crystalline structure impedes the movement of individual new dislocations, and therefore increases the resistance to deformation and reduces ductility. Highest value has an increase in the density of dislocations, since the interaction that arises between them inhibits their further movement.
With an increase in the degree of cold deformation, the indicators of resistance to deformation (tensile strength, yield strength and hardness) increase, and the indicators of plasticity (relative elongation and contraction) decrease.
Draw an iron-iron carbide phase diagram, indicate the structural components in all areas of the diagram, describe the transformations and construct a cooling curve (using the phase rule) for an alloy containing 0.8% C. What is the structure of this alloy at room temperature and what is this alloy called?
Primary crystallization of alloys of the iron-carbon system begins upon reaching temperatures corresponding to the ABCD line (liquidus line) and ends at temperatures forming the AHJECF line (solidus line).
When alloys crystallize along the AB line, crystals of a solid solution of carbon in α-iron (δ-solution) are released from a liquid solution. The crystallization process of alloys with a carbon content of up to 0.1% ends along the AN line with the formation of an α (δ) solid solution. On the HJB line, a peritectic transformation occurs, as a result of which a solid solution of carbon in γ-iron, i.e., austenite, is formed. The process of primary crystallization of steels ends along the AHJE line.
At temperatures corresponding to the BC line, austenite crystallizes from the liquid solution. In alloys containing from 4.3% to 6.67% carbon, at temperatures corresponding to the CD line, crystals of primary cementite begin to precipitate. Cementite that crystallizes from the liquid phase is called primary. At point C at a temperature of 1147°C and a carbon concentration in a liquid solution of 4.3%, a eutectic is formed, which is called ledeburite. The eutectic transformation with the formation of ledeburite can be written by the formula ЖР4,3->Л[А2,14+Ц6,67]. The process of primary crystallization of cast iron ends along the ECF line with the formation of ledeburite.
Thus, the structure of cast irons below 1147°C will be: hypoeutectic - austenite + ledeburite, eutectic - ledeburite and hypereutectic - cementite (primary) + ledeburite.
Transformations occurring in the solid state are called secondary crystallization. They are associated with the transition of γ-iron to α-iron upon cooling and the decomposition of austenite.
The GS line corresponds to the temperatures at which the transformation of austenite into ferrite begins. Below the GS line, the alloys consist of ferrite and austenite.
The ES line shows the temperature at which cementite begins to separate from austenite due to a decrease in the solubility of carbon in austenite with decreasing temperature. Cementite released from austenite is called secondary cementite.
At point S at a temperature of 727°C and a carbon concentration in austenite of 0.8%, a eutectoid mixture consisting of ferrite and cementite, called pearlite, is formed. Pearlite is obtained as a result of the simultaneous precipitation of ferrite and cementite particles from austenite. The process of transformation of austenite into pearlite can be written by the formula A0.8->P[F0.03+Ts6.67].
The PQ line indicates a decrease in the solubility of carbon in ferrite upon cooling and the release of cementite, which is called tertiary cementite.
Consequently, alloys containing less than 0.008% carbon (point Q) are single-phase and have the structure of pure ferrite, and alloys containing carbon from 0.008 to 0.03% have a ferrite + tertiary cementite structure and are called technical iron.
Hypoeutectoid steels at temperatures below 727ºC have a ferrite + pearlite structure, and hypereutectoid steels have pearlite + secondary cementite in the form of a network along the grain boundaries.
In hypoeutectic cast irons in the temperature range 1147–727ºС, upon cooling, secondary cementite is released from austenite due to a decrease in carbon solubility (ES line). Upon reaching a temperature of 727ºС (PSK line), austenite, depleted in carbon to 0.8% (point S), turns into pearlite. Thus, after final cooling, the structure of hypoeutectic cast iron consists of pearlite, secondary cementite and converted ledeburite (perlite + cementite).
The structure of eutectic cast irons at temperatures below 727ºС consists of converted ledeburite. Hypereutectic cast iron at temperatures below 727ºС consists of converted ledeburite and primary cementite.
The phase rule establishes the relationship between the number of degrees of freedom, the number of components and the number of phases and is expressed by the equation:
C = K + 1 – Ф,
where C is the number of degrees of freedom of the system;
K – number of components forming the system;
1 – number of external factors ( external factor we consider only the temperature, since pressure, except for very high pressure, has little effect on the phase equilibrium of alloys in solid and liquid states);
Ф – number of phases in equilibrium.
An alloy of iron and carbon containing 0.8% C is called eutectoid steel. Its structure at room temperature is pearlite. 
Using the iron-iron carbide state diagram and the graph of hardness versus tempering temperature, assign a mode heat treatment(quenching temperature, cooling medium and tempering temperature) of products made of steel 50, which must have a hardness of 230...250 HB. Describe the microstructure and properties of 50 steel after heat treatment.
Critical points for St50: AC1=725ºС, AC3=760ºС.
When heated to 700ºC, allotropic transformations do not occur in steel 50 and we have the same structure - pearlite + ferrite, quickly cooling (since hardening), we also have after cooling pearlite + ferrite with the same mechanical properties (approximately) as in the original state before heating for quenching.
If hypoeutectoid steel is heated above Ac1, but below Ac3, then in its structure after quenching, along with martensite, there will be sections of ferrite. The presence of ferrite as a soft component reduces the hardness of steel after hardening. This type of hardening is called incomplete. It provides good mechanical properties and stampability. At heating temperature the structure is austenite + ferrite. When cooling at a rate above the critical rate, a martensitic transformation occurs: γ->M. As a result, we obtain a ferrite + martensite structure.
Optimal heating mode for hardening for hypoeutectoid steels (%C<0,8%) составляет АС3+(30÷50º), т.е. для Ст50 – 800-820ºС. При этом после закалки имеем мелкое зерно, обеспечивающее наилучшие механические свойства стали 50.
Heating and holding steel 50 above a temperature of 820ºС before hardening leads to grain growth and deterioration of the mechanical properties of steel after heat treatment. The coarse grain structure causes increased brittleness of steel.
To ensure a cooling rate above the critical one, we choose water as the cooling medium. The structure of steel 50 at the heating temperature for quenching is austenite, after cooling at a rate above critical it is martensite.
Tempering is the heating of steel to a temperature below Ac1, holding at a given temperature and subsequent cooling at a given speed (usually in air). Tempering is the final operation of heat treatment, carried out after hardening to reduce internal stresses and obtain a more balanced structure. The higher the tempering temperature, the more completely the stress in hardened products is relieved.
To obtain a hardness of 230...250 HB with a workpiece diameter of 20 mm, tempering of steel 50 must be carried out at a temperature of 500ºC. The cooling medium is water. When tempered at high temperatures, a structure is formed which is called tempering sorbitol. Tempered sorbitol consists of a ferritic base penetrated with cementite particles.
Properties of steel 50 after heat treatment: σt=680-780 MPa, σw=870-970 MPa, δ=13-11%, ψ=61-57%, an=120-80, HB=230-250.
Steel 40 was hardened at temperatures of 760 and 840 ºС. Using the iron-cementite phase diagram, indicate the selected heating temperatures and describe the transformations that occurred under the two quenching conditions. Which mode should be preferred and why?
Hardening of hypoeutectoid steel consists of heating the steel to a temperature above the critical temperature (Ac3), holding it and then cooling it at a rate exceeding the critical one.
The temperature of the Ac3 point for steel 40 is 790°C.
If hypoeutectoid steel is heated above Ac1, but below Ac3, then in its structure after quenching, along with martensite, there will be sections of ferrite. The presence of ferrite as a soft component reduces the hardness of steel after hardening. When heated to a temperature of 760°C (below point Ac3), the structure of steel 40 is austenite + ferrite; after cooling at a rate above the critical rate, the structure of steel is martensite + ferrite. 
Figure 5 – Fragment of the iron-carbon diagram
Austenite is heterogeneous in chemical composition. In those places where there were cementite plates, austenite is richer in carbon, and where ferrite plates are poorer. Therefore, during heat treatment to level the chemical composition of austenite grains, the steel is heated slightly above the critical point Ac3 (by 30-50°C) and held for some time at this temperature. The austenitization process proceeds faster the higher the actual heating temperature for hardening is above the Ac3 temperature. For complete hardening, hypoeutectoid steels should be heated to a temperature 30-50°C above Ac3. The heating temperature of steel 40 for full hardening is therefore 820-840°C. The structure of steel 40 at the heating temperature for quenching is austenite, after cooling at a rate above critical it is martensite.
If you heat above this temperature, small austenite grains begin to connect with each other, and the higher the heating temperature, the more intense the size increases. The coarse-grained structure worsens the mechanical properties of steel.
Therefore, preference should be given to hardening at a temperature of 840 ºС.
) of one substance are evenly distributed between the molecules of another substance.
1. General characteristics
A solution is a single-phase, homogeneous, multicomponent system of variable chemical composition. Almost all liquids found in nature are solutions. In addition to the solution, there are gas (gas) solutions - they are usually called gas mixtures (for example, air) and solid solutions (for example, some alloys). As a rule, a solution is understood as a liquid molecular dispersed system (the so-called true solutions, English. true solution). A solvent is a component whose concentration is significantly greater than the concentration of other components. A solvent in its pure form has the same state of aggregation as a solution. The process of solution formation consists in the destruction of bonds between the molecules (ions) of the original substance and the formation of new bonds between the molecules (ions) of the solute and the solvent. Based on the concentration of the dissolved substance, solutions are divided into saturated, unsaturated and supersaturated. Based on the presence or absence of electrolytic dissociation of the dissolved substance into ions, solutions of electrolytes and solutions of non-electrolytes are distinguished. In addition, polymer solutions are isolated, the main feature of which is the very large difference in the sizes of the molecules of the solvent and the dissolved substance.
Many natural and industrial processes take place in solutions. They are associated with the formation of deposits of a number of minerals, their extraction and processing, separation of substances, deep purification, etc.
In terms of their properties, solutions occupy an intermediate place between mechanical mixtures and chemical compounds. They differ from mechanical mixtures mainly in their homogeneity and the release or absorption of heat during formation, and from chemical compounds in that their composition is stable and can vary within fairly wide limits.
2. Properties
Solutions are also characterized by a number of specific properties that differ from the properties of their constituent parts. In particular, they differ from their constituent parts in density, freezing and boiling points, and other properties. Solutions can be in liquid, solid and gaseous states. An example of the former are solutions of sugar, salt and alcohol in water. Solid solutions are different metal alloys: copper or silver in gold, nickel in copper, etc. Gaseous solutions are mixtures of various gases, such as air.
3. Solvent and Solute
A solvent is a component of a solution, the state of aggregation of which does not change during the formation of the solution, or the content of which prevails over the content of other components. The components of a solution are: solvent and dissolving substance.
Solvent and solute. Every solution consists of a solvent and a solute. A solvent is usually a substance that serves as a medium for a dissolved substance and, in its pure form, is in the same state of aggregation as the solution being created. However, sometimes it is difficult to tell which substance is a solvent and which is a solute, especially when both substances are mutually soluble in each other in unlimited quantities (like alcohol and water). In such cases, the solvent is the substance that is more abundant in the solution.
The most common and practically important solvent is water. The water of the seas and oceans is a natural solution that has a salty and bitter taste. On average, 1 kg of sea water contains 35 g of dissolved substances - the average salinity of sea water is 35? The composition of sea water includes more than a hundred substances formed from almost all chemical elements known in nature. Other substances are also used as solvents: acetone, gasoline, alcohol, etc., but much less frequently.
4. The importance of solutions
Aqueous solutions play a huge role in nature and practical human activity. Suffice it to say that plants take from the soil all the nutrients necessary for their growth only in the form of aqueous solutions. Therefore, the timely supply of water to the soil is of such great importance for the normal development of plants and ensuring a high yield of agricultural crops. The processes of digestion and assimilation of food by humans and all animals are also associated with the transfer of nutrients into solution.
Solutions play a huge role in technology. Most chemical processes in industry are carried out in solutions. Fields of technology such as leather and paper production, the production of sugar, mineral fertilizers, medicinal substances and many others are inextricably linked with the widespread use of aqueous solutions.
5. Saturation of solutions
5.1. Saturated solution
In a certain amount of water at a given temperature, only a certain amount of a substance can dissolve, and its excess remains insoluble. A solution in which a substance is taken at a given temperature no longer dissolves is called saturated.
When preparing a saturated solution, such an amount of dissolved substance is usually added to the solvent so that part of it remains insoluble, no matter how much the solution is stirred and stirred. However, in practice, they usually use unsaturated solutions, that is, those in which, at a given temperature, the substance can still be dissolved (before the formation of a saturated solution).
5.2. Saturated solution
In addition to saturated and unsaturated solutions, so-called supersaturated solutions are also known, in which there are more dissolved substances in the dissolved state than are needed to obtain a saturated solution. But supersaturated solutions are relatively rare, and they form only some substances, for example, sodium sulfate decahydrate - Na 2 SO 4? 10H 2 O, sodium pentahydrate thiosulfate - Na 2 S 2 O 3? 5H 2 O, etc.. Saturated solutions are very unstable and decompose quite easily with the release of excess solute and the formation of a saturated solution.
5.3. Concentrated and diluted solutions
The concepts of saturated and unsaturated solutions should not be confused with the concepts of concentrated and dilute. The names concentrated and diluted only indicate the degree of dissolution of the substance contained in a given amount of solvent, and do not indicate anything about the degree of its saturation.
A concentrated solution can be either saturated or unsaturated. For example, if in 100 g of water at 100? C dissolve 200 g of potassium nitrate KNO 3, then such a solution will be quite concentrated, but unsaturated, since to obtain a saturated solution under these conditions it is necessary to dissolve not 200, but 245 g of this salt. Second example: if 0.10 g of Ca (OH) 2 is dissolved in 100 g of water at ordinary temperature in one case, and 0.16 g in the second, then both solutions will be very dilute and at the same time the first of them will be unsaturated, and the second - saturated.
Every experienced builder has several recipes for mortars that can be used for certain jobs. Each mortar has its own characteristics, composition, advantages and disadvantages. The release of dry mixtures has greatly simplified the preparation of this substance, because now it is enough to add the required amount of water to the dry powder and mix the ingredients well. But still, those who plan to engage in construction or renovation need to know basic information about this area.
What is mortar? This is a mixture of several components. The required components are fine-grained filler, a binder, and water. This solution is often confused with concrete, to which, in addition to the listed components, coarse aggregate (gravel, crushed stone) is also added. Professionals know that these are different substances with their own areas of application.
Fine-grained mortar has been used in construction and repair work for a very long time; its variety was discovered even during the study of the Egyptian pyramids. Modern products are subject to detailed classification, identifying types intended for different jobs.
For those who do not have a professional education, it is important to know that according to the scope of application, mortars are divided into masonry, finishing and special.
- Masonry, as the name suggests, are used for laying walls made of brick and stone. You can prepare such a solution from a ready-made dry mixture (which is very convenient and reduces time), as well as from cement, sand and water. The size and purity of the sand and the quality of the cement are of great importance.
- The finishing substance is used by plasterers. Mixtures can also have additional properties, for example, they can be used to decorate walls.
- Special solutions with additional additives have sound-proofing and heat-insulating properties. Basically, these are mixtures of a modern look; their use improves the quality of construction and speaks of high professionalism. Mixtures with plasticizing additives are popular; they make the solution more plastic and easier to use. Walls plastered with this finish will be smoother and neater. There are also additives for working in winter, they speed up hardening. Frost resistance is indicated by special markings.
Solutions are also classified according to the type of binder. Mixtures of cement, lime, gypsum, and mixed types are produced. If there is only one type of binder component, such a solution is considered simple; if there are several, it is considered complex. The type of components affects the method of preparing a solution from a dry mixture. Whoever will prepare and use the substance must observe the necessary proportions and cooking time. For work safety, you should choose products from well-known, reputable manufacturers that contain only environmentally friendly substances. And even if this condition is met, you need to prepare a solution from dry mixtures in a protective mask so that when mixing the powder does not get into the respiratory system.