In everyday life, people rarely come across pure substances. Most items are mixtures of substances.
A solution is a homogeneous mixture in which the components are mixed evenly. There are several types of particle size: coarse systems, molecular solutions and colloidal systems, often called sols. This article is about 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 necessary
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 solubility of a substance?
The content of a substance in a saturated mixture is a measure of its solubility. As stated above, it is different for all substances. Soluble are those that can dilute more than 10 g of themselves per 100 g of water. The second category is less than 1 g under the same conditions. Almost insoluble are those in which less than 0.01 g of the component passes into the mixture. 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), which can be dissolved in 100 g of water or another substance.
Solvents
In this process, a solvent and a dissolved substance are involved. The first is different in that it initially resides 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 has a special place in chemistry. There are separate rules for her. The solution in which H 2 O is present is called aqueous. When it is said 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 are not equal here. Although the proportion of water is less than acid, it is incorrect to call a substance a 20% solution of water in nitric acid.
There are mixtures in which H 2 O is absent. They will bear the name non-aqueous. Such electrolyte solutions are ionic conductors. They contain one or a mixture of extractants. Their composition includes ions and molecules. They are used in industries such as medicine, the production of household chemicals, cosmetics, and in other areas. They can combine several necessary substances with different solubilities. The components of many agents that are applied externally are hydrophobic. In other words, they interact poorly with water. In such mixtures, the solvents may be volatile, non-volatile and combined. Organic substances in the first case dissolve fats well. Volatiles include alcohols, hydrocarbons, aldehydes and others. They are often part of 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 be bred further. In a solid preparation, precipitation is noticeable, which is in dynamic equilibrium with it. This concept means a state that persists in time due to its occurrence simultaneously in two opposite directions (forward and reverse reactions) at the same speed.
If the substance can still decompose at a constant temperature, then this solution is unsaturated. They are stable. But if you continue to add substance to them, then it will be diluted in water (or another liquid) until it reaches its maximum concentration.
Another species is oversaturated. It contains more solute than can be at a constant temperature. Due to the fact that they are in unstable equilibrium, crystallization occurs when they are physically exposed.
How to distinguish a saturated solution from an unsaturated one?
This is easy enough to do. If the substance is solid, then a precipitate can be seen in the saturated solution. In this case, the extractant may thicken, as, for example, in a saturated composition of water, in which sugar was added.
But if you change the conditions, increase the temperature, then it will cease to be considered saturated, since at a higher temperature the maximum concentration of this substance will be different.
Theories of the interaction of solution components
There are three theories regarding the interaction of elements in a mixture: physical, chemical, and modern. The authors of the first are Svante August Arrhenius and Wilhelm Friedrich Ostwald. They suggested that, due to diffusion, the particles of the solvent and the solute were evenly distributed throughout the mixture, but there was no interaction between them. The chemical theory put forward by Dmitry Ivanovich Mendeleev is the opposite of it. According to her, as a result of chemical interaction between them, unstable compounds of constant or variable composition are formed, which are called solvates.
Currently, the combined theory of Vladimir Alexandrovich Kistyakovsky and Ivan Alekseevich Kablukov is used. It combines the 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 was proved by Mendeleev. 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 physicochemical processes and changes the properties of molecules in a mixture. Solvation occurs due to the fact that the solvation shell, consisting of extractant molecules closely connected with it, surrounds the molecule of the dissolved substance.
Factors Affecting the Solubility of Substances
The chemical composition of substances. The rule “like attracts like” applies to reagents. Substances similar in physical and chemical properties can mutually dissolve faster. For example, non-polar compounds interact well with non-polar. Substances with polar molecules or ionic structure are bred in polar, for example, in water. It decomposes salts, alkalis and other components, and non-polar ones - on the contrary. You can give a simple example. To prepare a saturated solution of sugar in water, a larger amount of substance will be 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). You can demonstrate such an example. If you put a pinch of sodium chloride (salt) in cold water, then this process will take a lot of time. If you do the same with a hot medium, then the dissolution will be much faster. This is due to the fact that due to 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 substance. However, when the temperature rises in the case of lithium, magnesium, aluminum salts and alkalis, their solubility decreases.
Pressure. This factor only affects gases. Their solubility increases with increasing pressure. After all, the volume of gases is reduced.
Dissolution rate change
Do not confuse this indicator with solubility. Indeed, 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. We give an example. A whole piece of salt will dissolve in water much longer than salt in the form of sand.
Stir speed. As you know, this process can be catalyzed by stirring. Its speed is also important, because the larger it is, the faster the substance will dissolve in the liquid.
Why do I need 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 must be known for the correct recording of reagents and the preparation of the chemical reaction equation. Solubility in water indicates whether a salt or base can dissociate. Water 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 equation
There are several types of equations: molecular, full ion, and short ion. In fact, the last option is an abbreviated molecular form. This is the final answer. The complete equation contains the reagents and reaction products. Now comes the turn of the solubility table. First you need to check whether the reaction is feasible, that is, whether one of the conditions for the reaction is fulfilled. There are only 3 of them: water formation, gas evolution, precipitation. If the first two conditions are not met, you need to check the last. To do this, look at the solubility table and find out if the reaction products have an insoluble salt or base. If it is, then this will be the precipitate. Next, a table will be needed to write the ion equation. Since all soluble salts and bases are strong electrolytes, they will decay into cations and anions. Further, unbound ions are reduced, and the equation is written in short form. Example:
- K 2 SO 4 + BaCl 2 = BaSO 4 ↓ + 2HCl,
- 2K + 2SO 4 + Ba + 2Cl = BaSO 4 ↓ + 2K + 2Cl,
- Ba + SO4 = BaSO 4 ↓.
Thus, the solubility table of substances is one of the key conditions for solving ionic equations.
A detailed table helps to find out how much component you need to take to prepare a saturated mixture.
Solubility table
This is what a familiar 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 solubility table?
The table of solubility of substances in water is one of the main assistants to a chemist. It shows how various substances and compounds interact with water. The solubility of solids in liquids is an indicator without which many chemical manipulations are impossible.
The table is very easy to use. The first line contains cations (positively charged particles), the second - anions (negatively charged particles). Most of the table is occupied by a grid with specific characters in each cell. These are the letters "P", "M", "H" and the signs "-" and "?".
- "P" - the compound is dissolved;
- "M" - slightly soluble;
- "N" - does not dissolve;
- "-" - connection does not exist;
- "?" - 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 the salt is soluble in water - MgSo 4 (magnesium sulfate). To do this, find the Mg 2+ column and go down to the SO 4 2- line . At their intersection is the letter P, which means the compound is soluble.
Conclusion
So, we studied the issue of solubility of substances in water and not only. Without a doubt, this knowledge will come in handy 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.