A manual on chemistry for those entering higher educational institutions. Lesson “Dissolution. Solubility of substances in water Solubility of liquid substances in water
Let us first get acquainted with the process of dissolving solids in water, for which we turn again to our glass of water and see what will happen if we pour a spoonful of table salt into it.
Water molecules in continuous motion, when colliding with salt crystals, will, as it were, tear off individual salt molecules from their surface, which, once in water, will begin to move randomly, like water molecules.
In this case, however, they will tend to be distributed evenly throughout the volume of water. This property of substances is called diffusion, and since it is closely related to the process of dissolution, it is necessary to dwell on it in some detail.
Diffusion is the property of a substance to spread in any medium, i.e., its desire to penetrate from where it is to where it is not, and this process occurs solely due to the thermal random movement of the molecules of the medium.
Let us imagine that a certain layer of water containing molecules of sodium chloride has formed directly near the bottom of the glass.
Let us denote them conditionally by points, as shown in Fig. 9, while these molecules will naturally be especially numerous directly near the surface of the salt crystals, then, as they move upwards, their number should be less.
How will these salt molecules behave? After all, as we already know, their movement, due to the random movement of water molecules, will be just as random and, therefore, they will move in the water in a variety of directions - sometimes down, sometimes up, and sometimes sideways or obliquely.
However, as strange as it may seem at first glance, despite the completely random movement of salt molecules, there will be a gradual regular upward movement from places with a higher concentration of them to places with a lower concentration, until finally the salt molecules spread evenly. throughout the volume of water in the glass.
To explain the reason for this seemingly unexpected process, which is called diffusion, let us consider what will happen to salt molecules at the boundary of the section a-a conventionally taken in a glass (Fig. 9).
The diffusion process is not associated with any force that allegedly causes salt molecules to move upward, i.e., to an area with a lower concentration in water.
Each salt molecule behaves independently of other salt molecules, with which it rarely occurs.
Each salt molecule, no matter where it is located - below the section a-a or above it, experiences continuous shocks from the side of water molecules, as a result of which it can move down from this section or up from it.
But here comes into force the theory of probability and its basic law of large numbers, which is currently widely used by the natural sciences (primarily physics and chemistry) in the study of the properties of bodies consisting of a huge number of individual particles (molecules, atoms, ions, etc.). ).
The accuracy of the statistical law of large numbers increases as the number of particles participating in a given phenomenon increases and, conversely, decreases with their decrease, to the point that for a certain number of them this law becomes inapplicable and we pass into the region of pure chance.
To clarify this situation, one can resort to a simple public experience. Let's take two identical in size, but different in color balls: white and black.
Let's put them in some urn or just in a hat and we will successively take out one of these balls, each time returning the taken out ball back.
Since the marbles are the same size, there seems to be an equal chance for each of them to be drawn from the urn. But this identical possibility will become more and more apparent as the number of experiments increases.
If we carry out two or three or even five experiments, then it is possible that only a white or only a black ball will be taken out 2-3 or even 5 times.
But for a hundred experiments, such a probability becomes impossible, the number of white and black balls taken out will approach fifty.
At the same time, the law of probability states that the inaccuracy with which we can determine the average number of cases in which a given phenomenon occurs is equal to the square root of the number of these cases.
Let's return now to our glass with water and the molecules of salt dissolved in it. According to the theory of probability, the possibilities of moving salt molecules down or up from the section a-a will be the same due to the fact that each salt molecule is surrounded by a huge number of water molecules, from which it experiences a huge number of pushes both up and down.
But if all the salt molecules that are in a glass of water near the section a-a will move with the same probability both up and down from this section, then that is why the salt molecules will more often cross the section a-a from the bottom up than from top to bottom, because below this cross section the concentration of salt molecules is greater than above it.
Such a preferential upward movement of salt molecules will occur until they are evenly distributed throughout the entire volume of water.
Simultaneously with the process of salt dissolution, the reverse process of its crystallization occurs, since as a result of the random movement of salt molecules, some of them, located near the surface of salt crystals, when colliding with it, can linger on it, thus restoring the crystal partially destroyed as a result of the dissolution process. .
Obviously, this possibility of the reverse process will increase as the concentration of the solution increases.
But as we pour more portions of table salt into our glass, there will come a moment when its dissolution seems to stop, i.e., when the rate of both processes (dissolution and crystallization) equalizes, while there will be the same number of molecules per unit time go into solution, how many of them stand out on the salt crystals. Solutions having such a limiting concentration of a solute are called saturated solutions.
When such a state is reached in our glass, the so-called dynamic equilibrium between the solid salt and its saturated solution in water will come, as a result of which it will seem to us that the dissolution process has stopped.
To make sure that the processes of dissolution of a solid in water and its reverse separation from water do not stop in saturated aqueous solutions, it is sufficient to carry out the following experiment.
After obtaining a saturated solution of sodium chloride in our glass, we will add to it a certain amount of crystals of this salt containing radioactive sodium.
Then, after a few minutes, we will find with the help of a special counter (Geiger-Muller) that radioactive sodium atoms have appeared in the solution, and their number will gradually increase, reaching the maximum value in a few tens of minutes.
This experiment convincingly shows that in a saturated solution, the crystals are constantly being renewed, i.e., the transition of sodium chloride molecules from the surface of the crystal to the saturated solution and the transition of salt molecules from the solution to their place.
The diffusion process in solutions proceeds relatively slowly, as a result of which the water layer immediately adjacent to the salt crystals quickly becomes saturated, after which further dissolution occurs only as dissolved salt molecules diffuse upward from this layer.
Thus, the process of salt dissolution falls off rapidly and proceeds as slowly as the diffusion of dissolved salt molecules.
Therefore, to accelerate the dissolution, artificial acceleration of diffusion is resorted to by stirring the solution.
The dissolution of gases in water occurs basically in the same way as the dissolution of a solid, with the only difference that the penetration of solid molecules into water occurs by tearing them off by water molecules from salt crystals in water, and the molecules of a gaseous substance enter the water as a result of their random movement above the surface of the water, as a result of which some of them fall directly on the surface of the water and, being subjected to the action of the attractive forces of water molecules, are drawn inward.
This retraction of gas molecules into water is one of the essential moments in the process of dissolution of gases in water.
The further fate of the gas molecules that have fallen into the depths of the water is similar to the behavior of the dissolved salt molecules, which, experiencing various collisions with the water molecules surrounding them, also perform random movements.
Some gas molecules as a result of this movement between water molecules can again find themselves on its surface.
With a favorable push of this molecule towards the surface of the water, it can even fly away from the water, or, finding itself on the surface of the water, this gas molecule can be released as a result of a successful push that it will receive from some other gas molecule that has flown up, otherwise this the gas molecule will again be drawn into the depths of the water.
Thus, if we have water and some gas above it, for example oxygen, then two opposite processes will occur simultaneously: the penetration of oxygen molecules into the water, i.e. its dissolution in water, and the reverse process - the escape of oxygen molecules from water.
As the number of oxygen molecules dissolved in water increases, the opportunity for some of them to escape from the water will correspondingly increase.
Finally, the moment will come when the number of oxygen molecules entering the water becomes equal to the number of oxygen molecules leaving the water.
Consequently, similar to the system of salt crystals - a saturated solution, the so-called dynamic equilibrium will come, in which the process of dissolving oxygen in water, although it will continue, but the number of gas molecules in water will remain unchanged.
However, there is also a significant difference between the system of salt crystals - its saturated solution in water and the gas system - a gas solution in water.
The fact is that the maximum number of gas molecules in our case - oxygen, which can be dissolved in water, will be the greater, the more of these molecules will be above the surface of the water and, therefore, the more favorable collisions of gas molecules with water will be created and their penetration into its depths.
In fact, let's return to our system oxygen - a solution of oxygen in water, when dynamic equilibrium has come in it.
What happens if we somehow increase the amount of oxygen above the solution, that is, if we increase the number of oxygen molecules per unit volume of space above the solution?
Then the number of oxygen molecules entering the solution will increase, while the number of molecules leaving it remains the same.
Consequently, the dynamic equilibrium will be disturbed and further dissolution of oxygen molecules will begin, until, as a result of their increase in water, a new dynamic equilibrium occurs, which will differ from the first one in that the number of oxygen molecules dissolved in water will increase.
So, we have established a relationship between the amount of oxygen per unit volume over a solution and the solubility of oxygen in water.
But according to the molecular kinetic theory, the pressure of a gas produced by it on the walls of the vessel in which it is located is directly proportional to the number of molecules per unit volume, i.e., the more gas molecules per unit volume, the more often these molecules will hit the walls of the vessel, and hence the more pressure they will experience.
From this we can say that the solubility of a gas is directly proportional to its pressure. This relationship between the pressure of a gas and its solubility is called the Henry-Dalton law.
In practice, in most cases, we will deal not with any one gas, but with a mixture of several gases, and above all with air, which is a mixture of nitrogen, oxygen, carbon dioxide, etc.
How will they dissolve in water under these conditions?
It is quite obvious that the probability of penetration of oxygen molecules into water will, as before, be the greater, the more of these molecules there are in a unit volume of space above water, regardless of the number of molecules of other gases, i.e., the same Henry's law will again apply. Dalton.
But the pressure of a mixture of gases is made up of the pressures of the individual gases, determined respectively by the number of molecules of each gas.
In this case, the share of the total pressure of such a mixture of gases per individual gas is called its partial pressure.
Therefore, generalizing the Henry-Dalton law for a mixture of gases, we can say that the solubility of gases is proportional to their partial pressure.
Let us take a brief look at the question of the influence of temperature on the solubility. For aqueous solutions of solids, in the vast majority of cases, the solubility more or less increases with increasing temperature (substances with a positive solubility coefficient).
However, some substances have a negative solubility coefficient, that is, their solubility in water decreases with increasing temperature.
Such substances, in particular, include: calcium oxide Ca(OH) 2 hydrate and calcium sulfate CaSO 4 *.
* From temperatures of 40°C and above.
With an increase in temperature in the system, gas and its solution in water will, as we already know, increase the intensity of the movement of molecules, i.e., an increase in the number of fast molecules, which in turn will have two consequences.
On the one hand, this will contribute to an increase in the number of gas molecules penetrating into the water, at the same time, the number of molecules that can escape from the water will increase.
Ultimately, this will lead to a decrease in the solubility of the gas. Above the water is always a mixture of gases, including a certain amount of water vapor.
When water is heated, the amount of water vapor above it begins to increase, due to which the amount of other gases decreases, and therefore their partial pressure also decreases, as a result of which the solubility of other gases in water decreases markedly, and the more, the closer the temperature of the water to its boiling point .
When boiling, there will be essentially only one gas above the water - water vapor, and, therefore, the partial pressure of other gases will be close to zero. Therefore, when water boils, all gases dissolved in it are almost completely removed.
The amount of solute per unit volume or weight of the solvent is called the concentration of solutions.
The concentration of aqueous solutions is usually expressed as the number of grams of a solute in 1 liter of water and is abbreviated as g / l, or in 1 m 3 of water - g / m 3, and for poorly soluble substances - in milligrams of a solute, i.e. mg / l.
They also express the concentration of solutions in percent, more often in weight percent, i.e., indicate how many weight parts of an anhydrous substance are dissolved in 100 weight parts of a solvent or how many weight parts of an anhydrous substance are dissolved in 100 weight parts of a solution.
In water chemistry, a convenient measure of the concentration of substances is widespread, expressed as the number of grams or milligrams of a substance in 1 liter of a solution, numerically equal to its equivalent weight and abbreviated, respectively, g-eq / l or mg-eq / l.
This measure of concentration is convenient because the chemical elements are combined with each other in equivalent quantities.
The solubility of a given substance in water is the limiting amount of this substance that can be dissolved in water under given conditions, that is, when this solution becomes saturated.
Therefore, the solubility of any substance is determined by the concentration of its saturated solution.
Solubility (R, χ or k s) – this is a characteristic of a saturated solution, which shows what mass of a solute can be dissolved in 100 g of solvent as much as possible. The solubility dimension is g/ 100 g water. Since we are determining the mass of salt that falls on 100 g of water, we add a factor of 100 to the solubility formula:
here m r.v. is the mass of the dissolved substance, g
m r-la is the mass of the solvent, g
Sometimes the designation is used solubility factor kS .
Solubility tasks, as a rule, cause difficulties, since this physical quantity is not very familiar to schoolchildren.
The solubility of substances in various solvents varies widely.
The table shows the solubility of some substances in water at 20 o C:
Substance | Substance | Solubility, g per 100 g H 2 O |
|
NH4NO3 | H3BO3 | ||
NaCl | CaCO3 | 0,0006 |
|
NaHCO3 | 0,0000002 |
What does the solubility of substances depend on? From a number of factors: from the nature of the solute and solvent, from temperature and pressure. The reference tables suggest substances are divided into highly soluble, slightly soluble and insoluble. This division is very conditional, since there are no absolutely insoluble substances. Even silver and gold are soluble in water, but their solubility is so low as to be negligible.
Dependence of solubility on the nature of the solute and solvent*
Solubility of solids in liquids depends on the structure of the solid (on the type of crystal lattice of the solid). For example, substances with metallic crystal lattices (iron, copper, etc.) are very slightly soluble in water. Substances with an ionic crystal lattice, as a rule, are highly soluble in water.
There is a wonderful rule: like dissolves in like". Substances with an ionic or polar type of bond dissolve well in polar solvents.For example salts are highly soluble in water. At the same time, non-polar substances, as a rule, dissolve well in non-polar solvents.
Most alkali metal and ammonium salts are highly soluble in water. Almost all nitrates, nitrites and many halides (except silver, mercury, lead and thallium halides) and sulfates (except alkaline earth metal, silver and lead sulfates) are highly soluble. Transition metals are characterized by a low solubility of their sulfides, phosphates, carbonates, and some other salts.
The solubility of gases in liquids also depends on their nature. For example, in 100 volumes of water at 20 o C dissolves 2 volumes of hydrogen, 3 volumes of oxygen. Under the same conditions, 700 volumes of ammonia dissolve in 1 volume of H 2 O.
Effect of temperature on the solubility of gases, solids and liquids*
The dissolution of gases in water due to the hydration of the dissolved gas molecules is accompanied by the release of heat. Therefore, as the temperature rises, the solubility of gases decreases.
Temperature affects the solubility of solids in water in various ways. In most cases solubility of solids increases with temperature. For example, the solubility of sodium nitrate NaNO 3 and potassium nitrate KNO 3 increases when heated (the dissolution process proceeds with the absorption of heat). The solubility of NaCl increases slightly with increasing temperature, which is due to the almost zero thermal effect of the dissolution of table salt.
Effect of pressure on the solubility of gases, solids and liquids*
The solubility of solid and liquid substances in liquids is practically not affected by pressure, since the change in volume during dissolution is small. When gaseous substances are dissolved in a liquid, the volume of the system decreases, therefore, an increase in pressure leads to an increase in the solubility of gases. In general, the dependence of the solubility of gases on pressure obeys W. Henry's law(England, 1803): the solubility of a gas at constant temperature is directly proportional to its pressure over the liquid.
Henry's law is valid only at low pressures for gases whose solubility is relatively low and provided there is no chemical interaction between the molecules of the dissolved gas and the solvent.
Influence of foreign substances on solubility*
In the presence of other substances (salts, acids and alkalis) in water, the solubility of gases decreases. The solubility of gaseous chlorine in a saturated aqueous solution of table salt is 10 times less. than pure water.
The effect of decreasing solubility in the presence of salts is called salting out. The decrease in solubility is due to the hydration of salts, which causes a decrease in the number of free water molecules. Water molecules associated with electrolyte ions are no longer a solvent for other substances.
Examples of problems for solubility
Task 1. The mass fraction of a substance in a saturated solution is 24% at a certain temperature. Determine the solubility coefficient of this substance at a given temperature.
Solution:
To determine the solubility of a substance, we take the mass of the solution equal to 100 g. Then the mass of salt is equal to:
m r.v. = m r-ra ⋅ω r.v. = 100⋅0.24 = 24 g
The mass of water is:
m water \u003d m solution - m r.v. = 100 - 24 = 76 g
Determine the solubility:
χ = m r.v. /m p-la ⋅100 = 24/76⋅100 = 31.6 g of substance per 100 g of water.
Answer: χ = 31.6 g
A few more similar issues:
2. The mass fraction of salt in a saturated solution at a certain temperature is 28.5%. Determine the solubility coefficient of the substance at this temperature.
3. Determine the solubility coefficient of potassium nitrate at a certain temperature, if the mass fraction of salt at this temperature is 0.48.
4. What mass of water and salt will be required to prepare 500 g of a solution of potassium nitrate saturated at a certain temperature, if its solubility coefficient at this temperature is 63.9 g of salt per 100 g of water?
Answer: 194.95 g
5. The solubility coefficient of sodium chloride at a certain temperature is 36g of salt in 100g of water. Determine the molar concentration of a saturated solution of this salt if the density of the solution is 1.2 g/ml.
Answer: 5.49M
6. What mass of salt and 5% of its solution will be required to prepare 450 g of a solution of potassium sulfate saturated at a certain temperature, if its solubility coefficient at this temperature is 439 g / 1000 g of water?
7. What mass of barium nitrate will be released from a solution saturated at 100ºС and cooled to 0ºС if there was 150 ml of water in the solution taken? The solubility coefficient of barium nitrate at temperatures of 0ºС and 100ºС is 50 g and 342 g in 100 g of water, respectively.
8. The solubility coefficient of potassium chloride at 90ºС is 500g/l of water. How many grams of this substance can be dissolved in 500 g of water at 90ºC and what is its mass fraction in a saturated solution at this temperature?
9. 300 g of ammonium chloride are dissolved in 500 g of water when heated. What mass of ammonium chloride will be released from the solution when it is cooled to 50ºС, if the solubility coefficient of the salt at this temperature is 50 g/l of water?
* Materials of the portal onx.distant.ru
In everyday life, people rarely encounter pure substances. Most objects are mixtures of substances.
A solution is a homogeneous mixture in which the components are uniformly mixed. There are several types according to particle size: coarse systems, molecular solutions and colloidal systems, which are often called sols. This article deals with molecular (or true) solutions. The solubility of substances in water is one of the main conditions affecting the formation of compounds.
Solubility of substances: what is it and why is it needed
To understand this topic, you need to know what solutions and solubility of substances are. In simple terms, this is the ability of a substance to combine with another and form a homogeneous mixture.
From a scientific point of view, a more complex definition can be considered.
The solubility of substances is their ability to form homogeneous (or heterogeneous) compositions with one or more substances with a dispersed distribution of components. There are several classes of substances and compounds:
- soluble;
- sparingly soluble;
- insoluble.
What is the measure of the solubility of a substance
a substance in a saturated mixture is a measure of its solubility. As mentioned above, for all substances it is different. Soluble are those that can dissolve more than 10g of themselves in 100g of water. The second category is less than 1 g under the same conditions. Practically insoluble are those in the mixture of which less than 0.01 g of the component passes. In this case, the substance cannot transfer its molecules to water.
What is the solubility coefficient
The solubility coefficient (k) is an indicator of the maximum mass of a substance (g) that can be dissolved in 100 g of water or another substance.
Solvents
This process involves a solvent and a solute. The first differs in that initially it is in the same state of aggregation as the final mixture. As a rule, it is taken in larger quantities.
However, many people know that water occupies a special place in chemistry. There are separate rules for it. A solution in which H2O is present is called an aqueous solution.
When talking about them, the liquid is an extractant even when it is in a smaller amount. An example is an 80% solution of nitric acid in water.
The proportions here are not equal. Although the proportion of water is less than that of acids, it is incorrect to call the substance a 20% solution of water in nitric acid.There are mixtures that do not contain H2O. They will bear the name seine. Such electrolyte solutions are ionic conductors. They contain single or mixtures of extractants. They are composed of ions and molecules. They are used in industries such as medicine, the production of household chemicals, cosmetics and other areas.
They can combine several desired substances with different solubility. The components of many products that are applied externally are hydrophobic. In other words, they do not interact well with water. In such mixtures, the solvents may be volatile, non-volatile, or combined.
Organic substances in the first case dissolve fats well. The volatiles include alcohols, hydrocarbons, aldehydes, and others. They are often included in household chemicals. Non-volatile are most often used for the manufacture of ointments. These are fatty oils, liquid paraffin, glycerin and others.
Combined is a mixture of volatile and non-volatile, for example, ethanol with glycerin, glycerin with dimexide. They may also contain water.
A saturated solution is a mixture of chemicals that contains the maximum concentration of one substance in a solvent at a certain temperature. It will not breed further.
In the preparation of a solid substance, precipitation is noticeable, which is in dynamic equilibrium with it.
This concept means a state that persists in time due to its flow simultaneously in two opposite directions (forward and reverse reactions) at the same speed.
If a substance can still decompose at a constant temperature, then this solution is unsaturated. They are stable. But if you continue to add a substance to them, then it will be diluted in water (or other liquid) until it reaches its maximum concentration.
Another type is oversaturated. It contains more solute than can be at a constant temperature. Due to the fact that they are in an unstable equilibrium, crystallization occurs when they are physically affected.
How can you tell a saturated solution from an unsaturated one?
This is easy enough to do. If the substance is a solid, then a precipitate can be seen in a saturated solution.
In this case, the extractant can thicken, as, for example, in a saturated composition, water to which sugar has been added.
But if you change the conditions, increase the temperature, then it will no longer be considered saturated, since at a higher temperature the maximum concentration of this substance will be different.
Theories of interaction of components of solutions
There are three theories regarding the interaction of elements in a mixture: physical, chemical and modern. The authors of the first one are Svante August Arrhenius and Wilhelm Friedrich Ostwald.
They assumed that, due to diffusion, the particles of the solvent and the solute were evenly distributed throughout the volume of the mixture, but there was no interaction between them. The chemical theory put forward by Dmitri Ivanovich Mendeleev is the opposite of it.
According to it, as a result of chemical interaction between them, unstable compounds of constant or variable composition are formed, which are called solvates.
At present, the unified theory of Vladimir Aleksandrovich Kistyakovsky and Ivan Alekseevich Kablukov is used. It combines physical and chemical. The modern theory says that in a solution there are both non-interacting particles of substances and the products of their interaction - solvates, the existence of which Mendeleev proved.When the extractant is water, they are called hydrates. The phenomenon in which solvates (hydrates) are formed is called solvation (hydration). It affects all physical and chemical processes and changes the properties of the molecules in the mixture.
Solvation occurs due to the fact that the solvation shell, consisting of molecules of the extractant closely associated with it, surrounds the solute molecule.
Factors affecting the solubility of substances
Chemical composition of substances. The rule “like attracts like” applies to reagents as well. Substances that are similar in physical and chemical properties can mutually dissolve faster. For example, non-polar compounds interact well with non-polar ones.
Substances with polar molecules or an ionic structure are diluted in polar ones, for example, in water. Salts, alkalis and other components decompose in it, while non-polar ones do the opposite. A simple example can be given. To prepare a saturated solution of sugar in water, a larger amount of substance is required than in the case of salt.
What does it mean? Simply put, you can dilute much more sugar in water than salt.
Temperature. To increase the solubility of solids in liquids, you need to increase the temperature of the extractant (works in most cases). An example can be shown. If you put a pinch of sodium chloride (salt) in cold water, this process will take a long time.
If you do the same with a hot medium, then the dissolution will be much faster. This is explained by the fact that as a result of an increase in temperature, kinetic energy increases, a significant amount of which is often spent on the destruction of bonds between molecules and ions of a solid.
However, when the temperature rises in the case of lithium, magnesium, aluminum and alkali salts, their solubility decreases.
Pressure. This factor only affects gases. Their solubility increases with increasing pressure. After all, the volume of gases is reduced.
Changing the dissolution rate
Do not confuse this indicator with solubility. After all, different factors influence the change in these two indicators.
The degree of fragmentation of the solute.
This factor affects the solubility of solids in liquids. In the whole (lumpy) state, the composition is diluted longer than the one that is broken into small pieces. Let's take an example.
A solid block of salt will take much longer to dissolve in water than salt in the form of sand.
Stirring speed. As is known, this process can be catalyzed by stirring. Its speed is also important, because the faster it is, the faster the substance will dissolve in the liquid.
Why is it important to know the solubility of solids in water?
First of all, such schemes are needed to correctly solve chemical equations. In the solubility table there are charges of all substances. They need to be known in order to correctly record the reagents and draw up the equation of a chemical reaction. Solubility in water indicates whether the salt or base can dissociate.
Aqueous compounds that conduct current have strong electrolytes in their composition. There is another type. Those that conduct current poorly are considered weak electrolytes. In the first case, the components are substances that are completely ionized in water.
Whereas weak electrolytes show this indicator only to a small extent.
Chemical reaction equations
There are several types of equations: molecular, complete ionic and short ionic. In fact, the last option is a shortened form of molecular. This is the final answer. The complete equation contains the reactants and products of the reaction. Now comes the turn of the solubility table of substances.
First you need to check whether the reaction is feasible, that is, whether one of the conditions for the reaction is met. There are only 3 of them: the formation of water, the release of gas, precipitation. If the first two conditions are not met, you need to check the last one.
To do this, you need to look at the solubility table and find out if there is an insoluble salt or base in the reaction products. If it is, then this will be the sediment. Further, the table will be required to write the ionic equation.
Since all soluble salts and bases are strong electrolytes, they will decompose into cations and anions. Further, unbound ions are reduced, and the equation is written in a short form. Example:- K2SO4+BaCl2=BaSO4↓+2HCl,
- 2K+2SO4+Ba+2Cl=BaSO4↓+2K+2Cl,
- Ba+SO4=BaSO4↓.
Thus, the table of solubility of substances is one of the key conditions for solving ionic equations.
A detailed table helps you find out how much component you need to take to prepare a rich mixture.
Solubility table
This is what the usual incomplete table looks like. It is important that the temperature of the water is indicated here, as it is one of the factors that we have already mentioned above.
How to use the table of solubility of substances?
The table of solubility of substances in water is one of the main assistants of a chemist. It shows how various substances and compounds interact with water. The solubility of solids in a liquid is an indicator without which many chemical manipulations are impossible.
The table is very easy to use. Cations (positively charged particles) are written on the first line, anions (negatively charged particles) are written on the second line. Most of the table is occupied by a grid with certain symbols in each cell.
These are the letters "P", "M", "H" and the signs "-" and "?".
- "P" - the compound is dissolved;
- "M" - dissolves a little;
- "H" - does not dissolve;
- "-" - connection does not exist;
- "?" - no information about the existence of the connection.
There is one empty cell in this table - this is water.
Simple example
Now about how to work with such material. Suppose you need to find out if a salt is soluble in water - MgSo4 (magnesium sulfate). To do this, you need to find the Mg2+ column and go down it to the SO42- line. At their intersection is the letter P, which means the compound is soluble.
Conclusion
So, we have studied the issue of the solubility of substances in water and not only. Without a doubt, this knowledge will be useful in the further study of chemistry. After all, the solubility of substances plays an important role there. It is useful in solving chemical equations and various problems.
Solubility of various substances in water
The ability of a given substance to dissolve in a given solvent is called solubility.
On the quantitative side, the solubility of a solid characterizes the solubility coefficient or simple solubility - this is the maximum amount of a substance that can dissolve in 100 g or 1000 g of water under given conditions to form a saturated solution.
Since most solids absorb energy when dissolved in water, according to Le Chatelier's principle, the solubility of many solids increases with increasing temperature.
The solubility of gases in a liquid characterizes absorption coefficient- the maximum volume of gas that can dissolve at n.o. in one volume of solvent.
When dissolving gases, heat is released, therefore, with increasing temperature, their solubility decreases (for example, the solubility of NH3 at 0 ° C is 1100 dm3 / 1 dm3 of water, and at 25 ° C - 700 dm3 / 1 dm3 of water).
The dependence of gas solubility on pressure obeys Henry's law: The mass of dissolved gas at constant temperature is directly proportional to pressure.
Expression of the quantitative composition of solutions
Along with temperature and pressure, the main parameter of the state of a solution is the concentration of the dissolved substance in it.
solution concentration called the content of a solute in a certain mass or in a certain volume of a solution or solvent. The concentration of a solution can be expressed in different ways. In chemical practice, the following methods of expressing concentrations are most commonly used:
but) mass fraction of a solute shows the number of grams (mass units) of a solute contained in 100 g (mass units) of a solution (ω, %)
b) molar volume concentration, or molarity , shows the number of moles (amount) of the dissolved substance contained in 1 dm3 of the solution (s or M, mol / dm3)
in) equivalent concentration, or normality , shows the number of equivalents of a solute contained in 1 dm3 of a solution (ce or n, mol / dm3)
G) molar mass concentration, or molality , shows the number of moles of a solute contained in 1000 g of solvent (cm, mol / 1000 g)
e) titer solution is the number of grams of solute in 1 cm3 of solution (T, g / cm3)
In addition, the composition of the solution is expressed in terms of dimensionless relative values - fractions.
Volume fraction - the ratio of the volume of the solute to the volume of the solution; mass fraction - the ratio of the mass of the solute to the volume of the solution; mole fraction is the ratio of the amount of a dissolved substance (number of moles) to the total amount of all components of the solution.The most commonly used value is the mole fraction (N) - the ratio of the amount of dissolved substance (ν1) to the total amount of all components of the solution, that is, ν1 + ν2 (where ν2 is the amount of solvent)
Nr.v.= ν1/(ν1+ ν2)= mr.v./Mr.v./(mr.v./Mr.v+mr-l./Mr-l).
Dilute solutions of non-electrolytes and their properties
In the formation of solutions, the nature of the interaction of the components is determined by their chemical nature, which makes it difficult to identify general patterns. Therefore, it is convenient to resort to some idealized solution model, the so-called ideal solution.
A solution whose formation is not associated with a change in volume and thermal effect is called ideal solution.
However, most solutions do not fully possess the properties of ideality and general patterns can be described using examples of so-called dilute solutions, that is, solutions in which the content of the solute is very small compared to the content of the solvent and the interaction of molecules of the solute with the solvent can be neglected. Solutions have olligative properties are the properties of solutions that depend on the number of particles of the solute. The colligative properties of solutions include:
- osmotic pressure;
- saturated steam pressure. Raoult's law;
- increase in boiling point;
- freezing temperature drop.
Osmosis. Osmotic pressure.
Let there be a vessel divided by a semi-permeable partition (dotted line in the figure) into two parts filled to the same level O-O. Solvent is placed on the left side, solution is placed on the right side.
solvent solution
The concept of osmosis
Due to the difference in solvent concentrations on both sides of the partition, the solvent spontaneously (in accordance with the Le Chatelier principle) penetrates through the semi-permeable partition into the solution, diluting it.
The driving force for the predominant diffusion of the solvent into the solution is the difference between the free energies of the pure solvent and the solvent in the solution. When the solution is diluted due to spontaneous diffusion of the solvent, the volume of the solution increases and the level moves from position O to position II.
One-way diffusion of a certain kind of particles in solution through a semi-permeable partition is called osmosis.
It is possible to quantitatively characterize the osmotic properties of a solution (with respect to a pure solvent) by introducing the concept of osmotic pressure.
The latter is a measure of the tendency of the solvent to pass through the semi-permeable partition into the given solution.
It is equal to the additional pressure that must be applied to the solution so that osmosis stops (the action of pressure is reduced to an increase in the release of solvent molecules from the solution).
Solutions with the same osmotic pressure are called isotonic. In biology, solutions with an osmotic pressure greater than that of the intracellular contents are called hypertensive, with less hypotonic.The same solution is hypertonic for one cell type, isotonic for another, and hypotonic for the third.
Most of the tissues of organisms have the properties of semi-permeability. Therefore, osmotic phenomena are of great importance for the vital activity of animal and plant organisms. The processes of digestion, metabolism, etc.are closely related to the different permeability of tissues for water and certain solutes. The phenomena of osmosis explain some of the issues related to the relationship of the organism to the environment.
For example, they are due to the fact that freshwater fish cannot live in sea water, and marine fish in river water.
Van't Hoff showed that the osmotic pressure in a non-electrolyte solution is proportional to the molar concentration of the solute
Rosm=cRT,
where Rosm is the osmotic pressure, kPa; c is the molar concentration, mol/dm3; R is the gas constant equal to 8.314 J/mol∙K; T is temperature, K.
This expression is similar in form to the Mendeleev-Clapeyron equation for ideal gases, but these equations describe different processes. Osmotic pressure occurs in a solution when an additional amount of solvent penetrates into it through a semi-permeable partition. This pressure is the force that prevents further equalization of concentrations.
Van't Hoff formulated legal cosmic pressure The osmotic pressure is equal to the pressure that a solute would produce if it, in the form of an ideal gas, occupied the same volume as a solution at the same temperature.
Saturated steam pressure. Raul's Law.
Consider a dilute solution of a non-volatile (solid) substance A in a volatile liquid solvent B. In this case, the total saturation vapor pressure over the solution is determined by the partial vapor pressure of the solvent, since the vapor pressure of the solute can be neglected.
Raul showed that the pressure of a saturated vapor solvent over a solution P is less than over a pure solvent P °. The difference P ° - P \u003d P is called the absolute decrease in vapor pressure over the solution. This value, referred to the vapor pressure of a pure solvent, that is, (P ° - P) / P ° \u003d P / P °, is called the relative decrease in vapor pressure.According to Raoult's law, the relative decrease in the saturated vapor pressure of the solvent over the solution is equal to the mole fraction of the dissolved non-volatile substance
(Р°-Р)/Р°= N= ν1/(ν1+ ν2)= mr.v./Mr.v./(mr.v./Mr.v+mr-la./Mr-la)= XA
where XA is the mole fraction of the solute. And since ν1 \u003d mr.v. / Mr.v, using this law, you can determine the molar mass of the solute.
Consequence of Raoult's law. The decrease in vapor pressure over a solution of a non-volatile substance, for example in water, can be explained using the principle of Le Chatelier's equilibrium shift.
Indeed, with an increase in the concentration of a non-volatile component in a solution, the equilibrium in the water-saturated steam system shifts towards the condensation of a part of the vapor (the reaction of the system to a decrease in the water concentration when the substance is dissolved), which causes a decrease in the vapor pressure.
A decrease in vapor pressure over a solution compared to a pure solvent causes an increase in the boiling point and a decrease in the freezing point of solutions compared to a pure solvent (t). These values \u200b\u200bare proportional to the molar concentration of the solute - non-electrolyte, that is:
t= K∙sT= K∙t∙1000/M∙a,
where cm is the molar concentration of the solution; a is the mass of the solvent. Proportionality factor TO , when the boiling point rises, it is called ebullioscopic constant for a given solvent (E ), and to lower the freezing temperature - cryoscopic constant(TO ).
These constants, numerically different for the same solvent, characterize an increase in the boiling point and a decrease in the freezing point of a one molar solution, i.e. by dissolving 1 mol of non-volatile non-electrolyte in 1000 g of solvent. Therefore, they are often referred to as the molar increase in the boiling point and the molar decrease in the freezing point of the solution.
The criscopic and ebullioscopic constants do not depend on the concentration and nature of the dissolved substance, but depend only on the nature of the solvent and are characterized by the dimension kg∙deg/mol.
The concept of solutions. Solubility of substances
Solutions- homogeneous (homogeneous) systems of variable composition, which contain two or more components.
Liquid solutions are the most common. They consist of a solvent (liquid) and solutes (gaseous, liquid, solid):
Liquid solutions may be aqueous or non-aqueous. Aqueous solutions are solutions in which the solvent is water. Non-aqueous solutions- these are solutions in which other liquids are solvents (benzene, alcohol, ether, etc.). In practice, aqueous solutions are most often used.
Dissolution of substances
Dissolution is a complex physical and chemical process. The destruction of the structure of the solute and the distribution of its particles between solvent molecules is a physical process. At the same time, the solvent molecules interact with the particles of the dissolved substance, i.e. chemical process. As a result of this interaction, solvates are formed.
solvates- products of variable composition, which are formed during the chemical interaction of particles of a solute with solvent molecules.
If the solvent is water, then the resulting solvates are called hydrates. The process of formation of solvates is called solvation. The process of hydrate formation is called hydration. Hydrates of some substances can be isolated in crystalline form by evaporating solutions. For example:
What is a blue crystalline substance and how is it formed? When copper (II) sulfate is dissolved in water, it dissociates into ions:
The resulting ions interact with water molecules:
When the solution is evaporated, copper sulfate (II) crystalline hydrate - CuSO4 5H2O is formed.
Crystalline substances containing water molecules are called crystalline hydrates. The water included in their composition is called water of crystallization. Examples of crystalline hydrates:
For the first time, the idea of the chemical nature of the dissolution process was expressed by D. I. Mendeleev in his chemical (hydrate) theory of solutions(1887). The proof of the physicochemical nature of the dissolution process is the thermal effects during dissolution, i.e., the release or absorption of heat.
The thermal effect of dissolution is equal to the sum of the thermal effects of physical and chemical processes. The physical process proceeds with the absorption of heat, the chemical - with the release.
If as a result of hydration (solvation) more heat is released than it is absorbed during the destruction of the structure of the substance, then dissolution is an exothermic process. The release of heat is observed, for example, when such substances as NaOH, AgNO3, H2SO4, ZnSO4, etc. are dissolved in water.
If more heat is needed to destroy the structure of a substance than it is generated during hydration, then dissolution is an endothermic process. This happens, for example, when NaNO3, KCl, K2SO4, KNO2, NH4Cl, etc. are dissolved in water.
Solubility of substances
We know that some substances dissolve well, others poorly. When substances are dissolved, saturated and unsaturated solutions are formed.
saturated solution is the solution that contains the maximum amount of solute at a given temperature.
unsaturated solution is a solution that contains less solute than saturated at a given temperature.
The quantitative characteristic of solubility is solubility factor. The solubility coefficient shows what is the maximum mass of a substance that can be dissolved in 1000 ml of solvent at a given temperature.
Solubility is expressed in grams per liter (g/l).
By solubility in water, substances are divided into 3 groups:
Table of solubility of salts, acids and bases in water:
The solubility of substances depends on the nature of the solvent, on the nature of the solute, temperature, pressure (for gases). The solubility of gases decreases with increasing temperature, and increases with increasing pressure.
The dependence of the solubility of solids on temperature is shown by solubility curves. The solubility of many solids increases with increasing temperature.Solubility curves can be used to determine: 1) the coefficient of solubility of substances at different temperatures; 2) the mass of the solute that precipitates when the solution is cooled from t1oC to t2oC.
The process of isolating a substance by evaporating or cooling its saturated solution is called recrystallization. Recrystallization is used to purify substances.
A solution is a homogeneous system consisting of two or more substances, the content of which can be changed within certain limits without violating homogeneity.
Aquatic solutions are made up of water(solvent) and solute. The state of substances in an aqueous solution, if necessary, is indicated by a subscript (p), for example, KNO 3 in solution - KNO 3 (p) .
Solutions that contain a small amount of solute are often referred to as diluted while solutions with high solute content concentrated. A solution in which further dissolution of a substance is possible is called unsaturated and a solution in which a substance ceases to dissolve under given conditions is saturated. The last solution is always in contact (in heterogeneous equilibrium) with the undissolved substance (one or more crystals).
Under special conditions, such as gentle (without stirring) cooling of a hot unsaturated solution solid substances can form supersaturated solution. When a crystal of a substance is introduced, such a solution is separated into a saturated solution and a precipitate of the substance.
In accordance with chemical theory of solutions D. I. Mendeleev, the dissolution of a substance in water is accompanied, firstly, destruction chemical bonds between molecules (intermolecular bonds in covalent substances) or between ions (in ionic substances), and thus the particles of the substance mix with water (in which some of the hydrogen bonds between molecules are also destroyed). Chemical bonds are broken due to the thermal energy of the movement of water molecules, and in this case cost energy in the form of heat.
Secondly, once in the water, the particles (molecules or ions) of the substance are subjected to hydration. As a result, hydrates- compounds of indeterminate composition between particles of a substance and water molecules (the internal composition of the particles of a substance itself does not change when dissolved). This process is accompanied highlighting energy in the form of heat due to the formation of new chemical bonds in hydrates.
In general, a solution cools down(if the cost of heat exceeds its release), or heats up (otherwise); sometimes - if the cost of heat and its release are equal - the temperature of the solution remains unchanged.
Many hydrates are so stable that they do not break down even when the solution is completely evaporated. So, solid crystal hydrates of salts CuSO 4 5H 2 O, Na 2 CO 3 10H 2 O, KAl (SO 4) 2 12H 2 O, etc. are known.
The content of a substance in a saturated solution at T= const quantifies solubility this substance. Solubility is usually expressed as the mass of solute per 100 g of water, for example 65.2 g KBr/100 g H 2 O at 20 °C. Therefore, if 70 g of solid potassium bromide is introduced into 100 g of water at 20 °C, then 65.2 g of salt will go into solution (which will be saturated), and 4.8 g of solid KBr (excess) will remain at the bottom of the beaker.
It should be remembered that the solute content in rich solution equals, in unsaturated solution less and in supersaturated solution more its solubility at a given temperature. So, a solution prepared at 20 ° C from 100 g of water and sodium sulfate Na 2 SO 4 (solubility 19.2 g / 100 g H 2 O), with a content
15.7 g of salt - unsaturated;
19.2 g salt - saturated;
2O.3 g of salt is supersaturated.
The solubility of solids (Table 14) usually increases with increasing temperature (KBr, NaCl), and only for some substances (CaSO 4 , Li 2 CO 3) is the opposite observed.
The solubility of gases decreases with increasing temperature, and increases with increasing pressure; for example, at a pressure of 1 atm, the solubility of ammonia is 52.6 (20 ° C) and 15.4 g / 100 g H 2 O (80 ° C), and at 20 ° C and 9 atm it is 93.5 g / 100 g H 2 O.
In accordance with the solubility values, substances are distinguished:
– well soluble, the mass of which in a saturated solution is commensurate with the mass of water (for example, KBr - at 20 ° C the solubility is 65.2 g / 100 g H 2 O; 4.6 M solution), they form saturated solutions with a molarity of more than 0.1 M;
– sparingly soluble, the mass of which in a saturated solution is much less than the mass of water (for example, CaSO 4 - at 20 ° C, the solubility is 0.206 g / 100 g H 2 O; 0.015 M solution), they form saturated solutions with a molarity of 0.1–0.001 M;
– practically insoluble the mass of which in a saturated solution is negligible compared to the mass of the solvent (for example, AgCl - at 20 ° C, the solubility is 0.00019 g per 100 g of H 2 O; 0.0000134 M solution), they form saturated solutions with a molarity of less than 0.001 M.
Compiled according to reference data solubility table common acids, bases and salts (Table 15), in which the type of solubility is indicated, substances are noted that are not known to science (not obtained) or completely decomposed by water.
Solutions play a key role in nature, science and technology. Water is the basis of life, always contains dissolved substances. Fresh water of rivers and lakes contains few dissolved substances, while sea water contains about 3.5% of dissolved salts.
The primordial ocean (during the birth of life on Earth) is thought to have contained only 1% dissolved salts.
“It was in this environment that living organisms first developed, from this solution they scooped up the ions and molecules that are necessary for their further growth and development ... Over time, living organisms developed and transformed, so they were able to leave the aquatic environment and move to land and then rise to air. They obtained these abilities by preserving in their organisms an aqueous solution in the form of liquids that contain a vital supply of ions and molecules, ”the famous American chemist, Nobel Prize winner Linus Pauling describes the role of solutions in nature in these words. Inside each of us, in every cell of our body, there are memories of the primordial ocean, the place where life originated, an aqueous solution that provides life itself.
In any living organism, an unusual solution constantly flows through the vessels - arteries, veins and capillaries, which forms the basis of blood, the mass fraction of salts in it is the same as in the primary ocean - 0.9%. Complex physicochemical processes occurring in the human and animal body also interact in solutions. The process of assimilation of food is associated with the transfer of highly nutritious substances into solution. Natural aqueous solutions are directly related to the processes of soil formation, the supply of plants with nutrients. Such technological processes in the chemical and many other industries, such as the production of fertilizers, metals, acids, paper, occur in solutions. Modern science deals with the study of the properties of solutions. Let's find out what is a solution?
Solutions differ from other mixtures in that the particles of the constituents are evenly distributed in them, and in any microvolume of such a mixture the composition will be the same.
That is why solutions were understood as homogeneous mixtures, which consist of two or more homogeneous parts. This idea was based on the physical theory of solutions.
Adherents of the physical theory of solutions, which van't Hoff, Arrhenius and Ostwald were engaged in, believed that the dissolution process is the result of diffusion.
D. I. Mendeleev and supporters of the chemical theory believed that dissolution is the result of the chemical interaction of a solute with water molecules. Thus, it will be more accurate to define a solution as a homogeneous system that consists of particles of a solute, a solvent, and also the products of their interaction.
Due to the chemical interaction of a solute with water, compounds are formed - hydrates. Chemical interaction is usually accompanied by thermal phenomena. For example, the dissolution of sulfuric acid in water takes place with the release of such an enormous amount of heat that the solution can boil, which is why acid is poured into water, and not vice versa. The dissolution of substances such as sodium chloride, ammonium nitrate, accompanied by the absorption of heat.
M. V. Lomonosov proved that solutions turn into ice at a lower temperature than the solvent.
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