In everyday life, people rarely encounter pure substances. Most items are mixtures of substances.
A solution is a homogeneous mixture in which the components are evenly 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:
- instant;
- poorly soluble;
- insoluble.
What the measure of the solubility of a substance says
The content of 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 dilute 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 diluted 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 the acid, it is incorrect to call the substance a 20% solution of water in nitric acid.
There are mixtures that lack H2O. They will bear the namenon-aqueous. 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.
Types of solutions by degree of saturation
A saturated solution is a mixture of chemicals containing 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 the substanceat a constant temperature can still decompose, 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 look - oversaturated. It contains more solute than can be at a constant temperature. Due to the fact that they are in unstable equilibrium, physical impact on them causes crystallization.
How do 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 other.
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, unstablecompounds of constant or variable composition, which are called solvates.
Currently, the unified theory of Vladimir Aleksandrovich Kistyakovsky and Ivan Alekseevich Kablukov is used. It combines physical and chemical. The modern theory says that in the solution there are both non-interacting particles of substances and the products of their interaction - solvates, the existence of which Mendeleev proved. In the case 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. S alts, 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 s alt. What does it mean? Simply put, you can breed a lot moresugar in water than s alt.
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 (s alt) 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 s alts, their solubility decreases.
Pressure. This factor only affects gases. Their solubility increases with increasing pressure. After all, the volume of gases is reduced.
Change 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 dissolved substance. 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 s alt will take much longer to dissolve in water than s alt in the form of sand.
Stir speed. As is known, this process can be catalyzed by stirring. Its speed is also important, because the larger it is, the faster it will dissolve.substance in liquid.
Why do we need to know the solubility of solids in water?
First of all, such schemes are needed to solve chemical equations correctly. 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 s alt 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, full 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 s alt 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 s alts and bases are strong electrolytes,then 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 solubility table of substances is one of the key conditions for solving ionic equations.
Detailed table helps you find out how much component you need to take to prepare a rich mixture.
Solubility table
This is the usual incomplete table. It is important that the water temperature is indicated here, as it is one of the factors that we have already discussed above.
How to use the solubility table?
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 in the first line, anions (negatively charged particles) are written in 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" - compound dissolves;
- "M" - dissolves a little;
- "H" - does not dissolve;
- "-" - no connection exists;
- "?" - there is 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 s alt is soluble in water - MgSo4 (magnesium sulfate). To do this, you need to find the column Mg2+ and go down to the line SO42-. At their intersection is the letter P, which means the compound is soluble.
Conclusion
So, we have studied the question 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 will be useful for solving chemical equations and various problems.
