If crystals of table salt, sugar or potassium permanganate (potassium permanganate) are placed in a vessel with water, then we can observe how the amount of solid substance gradually decreases. At the same time, the water into which the crystals were added acquires new properties: it has a salty or sweet taste (in the case of potassium permanganate, a raspberry color appears), the density, freezing temperature, etc. change. The resulting liquids can no longer be called water, even if they are indistinguishable from water in appearance (as is the case with salt and sugar). It - solutions .
Solutions - homogeneous multicomponent system consisting of a solvent, dissolved substances and products of their interaction.
Solutions do not settle and remain homogeneous all the time. If the solution is filtered through the densest filter, then neither salt, nor sugar, nor potassium permanganate can be separated from water. Therefore, these substances in water are fragmented to the smallest particles - molecules. Molecules can again be collected in crystals only when we evaporate water. So solutions are molecular mixtures.
According to the state of aggregation, solutions can be liquid(sea water) , gaseous (air) or solid (many metal alloys).
Particle sizes in true solutions are less than 10 -9 m (on the order of the size of the molecules).
Any solution consists of solvent and solute . In the above examples, the solvent is water. But not always necessarily water is a solvent. For example, you can get a solution of water in sulfuric acid. Here, the solvent is acid. You can also prepare solutions of acid in water.
Of the two or more components of the solution, the solvent is one that is taken in larger quantities and has the same state of aggregation as the solution as a whole.
** There are solutions not only liquid, but also gas and even solid. For example, air is a solution of oxygen and several more gases in nitrogen. Metal alloys are solid solutions of metals in each other. Gases, as we already know, are capable of dissolving in water.
Let's understand how the dissolution of substances occurs. To do this, we observe how the sugar added to tea dissolves. If tea is cold, then sugar dissolves slowly. On the contrary, if the tea is hot and stirred with a spoon, then dissolution occurs quickly.
Once in water, sugar molecules located on the surface of granulated sugar crystals form donor-acceptor (hydrogen) bonds with water molecules. At the same time, several water molecules bind to one sugar molecule. The thermal movement of water molecules causes the sugar molecules associated with them to break away from the crystal and pass into the thickness of the solvent molecules (Fig. 7-2).
Sugar molecules transferred from a crystal to a solution can move along the entire volume of the solution together with water molecules due to thermal motion. This phenomenon is called diffusion . Diffusion occurs slowly, therefore, near the surface of the crystals there is an excess of sugar molecules already detached from the crystal, but not yet diffused into the solution.
They prevent new water molecules from approaching the surface of the crystal in order to contact its molecules with hydrogen bonds. If the solution is mixed, then diffusion occurs more intensively and the dissolution of sugar is faster. Sugar molecules are evenly distributed and the solution becomes equally sweet throughout.
The number of molecules capable of passing into solution is often limited. Molecules of a substance not only leave the crystal, but also reattach to the crystal from solution. While there are relatively few crystals, more molecules pass into the solution than return from it - there is a dissolution. But if the solvent is in contact with a large number of crystals, then the number of leaving and returning molecules becomes the same and dissolution stops for an external observer.
Unsaturated, Saturated, and Oversaturated Solutions
If molecular or ionic particles distributed in a liquid solution are present in it in such an amount that under these conditions there is no further dissolution of the substance, the solution is called saturated. (For example, if you place 50 g of NaCl in 100 g of H2 O, then at 20ºC only 36 g of salt will dissolve).
Saturated called a solution that is in dynamic equilibrium with an excess of solute.
By placing in 100 g of water at 20ºC less than 36 g of NaCl we get unsaturated solution .
When the mixture of salt and water is heated to 100 ° C, 39.8 g of NaCl in 100 g of water will dissolve. If now insoluble salt is removed from the solution and the solution is carefully cooled to 20ºC, excess salt does not always precipitate. In this case, we are dealing with supersaturated solution . Oversaturated solutions are very unstable. Stirring, shaking, adding grains of salt can cause crystallization of excess salt and a transition to a saturated stable state.
Unsaturated solution - a solution containing less substance than saturated.
Supersaturated solution - a solution containing more substance than saturated.
Dissolution as a physicochemical process
Solutions are formed by the interaction of a solvent and a dissolved substance. The process of interaction of a solvent and a dissolved substance is called solvation (if the solvent is water - hydration ).
Dissolution proceeds with the formation of products of various shapes and strengths - hydrates. In this case, forces of both physical and chemical nature are involved. The dissolution process due to this kind of component interactions is accompanied by various thermal phenomena.
The energy characteristic of dissolution is heat of formationsolution , considered as the algebraic sum of the thermal effects of all endo- and exothermic stages of the process. The most significant among them are:
– heat absorbing processes - destruction of the crystal lattice, breaks in chemical bonds in molecules,
– heat-generating processes - the formation of products of the interaction of a dissolved substance with a solvent (hydrates), etc.
If the energy of destruction of the crystal lattice is less than the energy of hydration of the dissolved substance, then the dissolution proceeds with the release of heat (heating is observed). So, the dissolution of NaOH is an exothermic process: 884 kJ / mol is spent on the destruction of the crystal lattice, and 422 and 510 kJ / mol, respectively, are released during the formation of hydrated Na + and OH ions.
If the energy of the crystal lattice is greater than the hydration energy, then the dissolution proceeds with the absorption of heat (in the preparation of an aqueous solution of NH4 NO3 a decrease in temperature is observed).
We say: "sugar dissolves in water well" or "chalk does not dissolve well in water." But it is possible to quantify the ability of a substance to dissolve or, in other words, solubility substances.
Solubility - called the ability of a substance to dissolve in a particular solvent. A measure of the solubility of a substance under these conditions is its content in a saturated solution.
If more than 10 g of a substance is dissolved in 100 g of water, then such a substance is called OKsoluble . If less than 1 g of the substance is dissolved, the substance sparingly soluble . Finally, the substance is considered practically insoluble if less than 0.01 g of substance passes into the solution. Absolutely insoluble substances do not exist.
Solubility expressed by the mass of a substance that can dissolve in 100 g of water at a given temperature is also called solubility coefficient.
As an example, we give the solubility (in grams of a substance per 100 g of water at room temperature) of several substances: solid, liquid and gaseous, among which many have similar chemical formulas (table 7-2).
Table 7- 2. The solubility of some substances in water at room temperature, the solubility of most (but not all!) Solids increases with increasing temperature, and the solubility of gases, on the contrary, decreases. This is primarily due to the fact that gas molecules during thermal motion are able to leave the solution much easier than solid molecules.
** If you measure the solubility of substances at different temperatures, you will find that some substances noticeably change their solubility depending on temperature, others - not very much (see table 7-3).
|Substance name||Formula||State of aggregation||Solubility (g / 100 g water)|
|Sulphuric acid||H2 SO4||liquid||any quantity|
|Copper sulphate||CuSO4 5H2 O||crystal||20,7|
If the values obtained in the experiments are plotted on the coordinate axis, the so-called solubility curves of various substances are obtained (Fig. 7-3). These curves are of practical importance. It’s easy to find out how much substance (for example, KNO3 ) will precipitate upon cooling to 20 ° C of a saturated solution prepared at 80 ° C.