Solubility and Precipitation
The amount of solute, usually expressed in grams, that dissolves in a certain amount of solvent (usually 100 g), to obtain a saturated solution, is called solubility.
It is said that a body is very soluble, soluble, slightly soluble or insoluble, depending on its solubility, in a particular solvent, very large, regular, small or negligible. Therefore we have:
- Very soluble: its solubility is greater than 0,1M
- Soluble: its solubility is equal to 0,1M
- Little soluble: its solubility is between 0,1M and 0,001M
- Insoluble: its solubility does not reach 0,001M
The solubility is very variable from one body to another. It depends on the temperature, the pressure (in gases) and, above all, on the nature of the solute and the solvent, as we can see some examples in the following table:
Internal factors that influence in the solubility
Several factors are responsible for the solubility of a substance in a given solvent. First, there are attractive intermolecular forces, which is probably the most important factor in explaining solubility. In general, when a solute that we will designate with the letter S, is mixed with a solvent D, there are three types of attractive forces that must be considered. On the one hand, there are the attractive forces between the particles of solute with each other, S-S, or those of solvent between them, D-D, and on the other hand, the attractive forces between the particles of solute with those of solvent, S-D.
When one of the first two is much larger than the third, the S body will not dissolve in the solvent D. Only when both types of attraction (SS and DD on the one hand and SD on the other) are of the same order will it take place the solution. This is the basis of the general rule that “like dissolves like”, known since ancient times.
In this way it is easily explained that sugar dissolves in water, while it is insoluble in ether, benzene, carbon tetrachloride, etc. Sugar molecules have many O-H groups (dipolar in nature), which are strongly attracted to water O-H groups, but do not show any attraction with the non-polar molecules of ether, benzene, carbon tetrachloride, etc.
It is therefore also easy to understand that oil does not dissolve in water, due to the strong attraction of water molecules to each other (through hydrogen bonds) and the very weak attraction that exists between bipolar water molecules and non-polar molecules. -pools of oil.
Effect of pressure and temperature
In addition to the internal factors, which depend only on the nature of the solute and the solvent, the solubility is also influenced by other external factors, the two most important being pressure and temperature.
The variation of the pressure has very little influence on the solubility of solids or liquids, but it has a great effect on the solubility of gases, which increases markedly as the pressure increases. This increase is governed by Henry’s law, named in honor of its discoverer, and can be stated as follows: “At constant temperature, the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas” . This law is not met when the gas reacts chemically with the solvent as it happens, for example with hydrogen chloride or ammonia, when they dissolve in water. The formation of foam that is observed when opening a bottle of champagne, soda, beer or any other carbonated beverage, is a direct consequence of Henry’s law.
The carbonated drinks are bottled at a pressure higher than atmospheric, to increase the solubility of carbon dioxide; when the bottle is opened, the pressure is reduced and carbon dioxide escapes from the solution, its bubbles forming an abundant foam.
Solubility of ionic compounds
As we have already mentioned, due to the intense attractive forces between the ions of opposite charge, which keep them held in the knots of the crystalline lattice, the ionic compounds are insoluble in almost all the solvents. Only some very polar solvents, such as water, liquid ammonia, sulfuric acid, etc., are capable of dissolving the ionic compounds. These solvents form electrically conductive solutions not only with ionic compounds, but also with other more or less covalent solutes, such as ClH or SO4H2, which is why they are called ionizing solvents.
The dissolution process is carried out thanks to the strong attraction that is exerted between the ions and the polar molecules of the solvent, which causes them to be oriented around the ions of the faces of the crystal, tending to remove the ions from the crystal and pass them to the dissolution, as it is the case of the ions in a solution of a salt (Cl- Na +) that we can see next:
Once in solution, the ions are still surrounded by a layer of solvent molecules, which prevents the recombination of the ions, with which recrystallization is prevented. These ions surrounded by solvent molecules are said to be solvated and, in particular, hydrated, when the solvent is water.
The attractive force between the ions and the molecules of the solvent depends on the polarity of these, that is, the dipole moment. The greater the dipole moment, the greater will be said forces of attraction and the dissolution process will be easier. But not all substances dissolve the same in all solvents.
Many ionic compounds are quite soluble in water. When they dissolve they dissociate completely in their ions. This is the case, for example, of sodium iodide, where the dissociation process is complete, the reaction will be:
As we already know, any calculation referring to this reaction implies taking into account only its stoichiometry. That is, for this example, one mole of NaI would provide one mole of Na+ ions and one mole of I– ions.
However, many other tonic substances have a very small solubility; They are practically insoluble. In these cases we can speak of a state of equilibrium between dissolved ions, liquid phase (aqueous), and salt without dissolving or precipitated, the solid phase.
For a compound of general formula AnBm, the solubility balance can be represented by:
Any calculation necessarily involves, in addition to the stoichiometry of the reaction, the use of the equilibrium constant that is called the constant of the solubility product, Ks, or simply a product of solubility.
The precipitation of a poorly soluble ionic compound can take place when two solutions are mixed in which each of them contains one of the ions that form said compound.
Thus, the silver chloride, AgCl, is a very insoluble white salt. When a silver nitrate solution, AgNO3, which contains the Ag+ ion, is mixed with another sodium chloride solution containing the Cl– ion, precipitation of the AgCl occurs.
In the reaction, the NO3– and Na+ ions are not written since they do not intervene in the process, they are spectator ions.
Constant of solubility product
Even in the most insoluble substances there is always a small proportion of particles that pass into solution. This equilibrium, in very little soluble substances, is clearly displaced towards the solid, undissociated form.
As in any heterogeneous equilibrium, the concentration of the solid species can be considered practically constant and included in the value of the equilibrium constant, which takes the following form:
Being a saturase solution of barium sulfate:
The expression of the equilibrium constant would be:
“The product of solubility, like any equilibrium constant, depends solely and exclusively on temperature.”
By means of the concept of reaction quotient, Q, we can establish in what conditions the precipitation process occurs. If:
Relation between solubility and solubility product
If we call S to the solubility in mol/L the compound AnBm, we will have:
The solubility product will be:
And when we clear S we have:
This expression allows us to calculate the solubility of the compound from the value of Ks.