We explain what solubility is in chemistry and what factors affect it. Also, what is the solubility product and various examples.
What is solubility?
In chemistry, solubility is the capacity of a body or a substance determined (call solute) to dissolve in a certain medium (called solvent); that is, it is the maximum amount of a solute that a solvent can receive under certain environmental conditions.
The solute is the substance that dissolves in a certain solvent. It can be a solid, a liquid or a gas. In general, the solute is found in less quantity than the solvent in a dissolution.
The solvent or solvent is the substance in which a certain solute dissolves. Solvent is generally found in greater quantity than solute in solution.
Solubility can be expressed in units of concentration, such as molarity or molality, for example.
Molar concentration (referred to molarity) is defined as the number of moles of solute per liter of solution (or equivalent unit), and is calculated as follows:
Where:
- M (X). Molarity of the substance X expressed in mol / L.
- n (X). Substance amount of substance X expressed in moles (mol).
- V (X). Volume of dissolution expressed in liters (L) or equivalent units.
Molal concentration (referred to molality) is defined as the number of moles of solute in one kilogram of solvent, and is calculated as follows:
Where:
- m (X). It is the molality of the substance X expressed in mol / (kg of solvent).
- n (X). It is the amount of substance of the substance X expressed in moles (mol).
- m (solvent expressed in kg). Is the mass solvent expressed in kg.
However, solubility is not a universal characteristic of all substances. Some dissolve easily, others more difficultly, and some simply don't dissolve.
It all also depends on what substances we are mixing. The Water, commonly referred to as the universal solvent, cannot completely dissolve oil, for example.
But even when a solvent manages to dissolve a solute, it does so to a certain extent, due to which solutions can be classified into:
- Saturated. When no more solute can be dissolved, that is, when the solution has the maximum solute that the solvent supports.
- Unsaturated When you can continue to dissolve more solute in the solution.
- Oversaturated When the solution has more solute than it can dissolve. A supersaturated solution can be achieved by modifying certain conditions, such as temperature, so that more solute dissolves than the maximum that can be dissolved.
Factors Affecting Solubility
In principle, the solubility of a substance depends on which one we are mixing it with. Broadly speaking, substances are classified into:
- Water soluble. They are those that can dissolve more easily (or completely) in water.
- Fat soluble. They are those that can dissolve more easily in oils.
On the other hand, the solubility of substances depends on the following factors:
Temperature. Most solids increase their solubility in water with increasing temperature, although there are some exceptions. Also organic compounds, in general, increase their solubility with increasing temperature. This increase in solubility with increasing temperature is due to the increased interactions between the particles of solute and solvent, so the intermolecular forces between them can be broken. On the other hand, gaseous solutes have a different behavior, since when the temperature increases, their solubility in organic solvents increases, but it decreases in water because the gas tends to escape from the liquid with the increase in temperature.
For example, a glass of water dissolves a certain amount of sugar, until the excess begins to settle to the bottom. If we heat this glass of water, we will notice how the excess begins to disappear, increasing the solubility of the solute in the solvent.
Pressure. Pressure influences the solubility of gaseous solutes mainly. By increasing the pressure of a gaseous solute, its solubility in a certain solvent increases.
Nature of the solute and the solvent. Substances with the same polarity are soluble in each other, hence the phrase: "similar dissolves similar." However, when a solute and a solvent have different polarities, they are completely insoluble in each other, although there is always a range of intermediate polarities in which a solute and a solvent can be partially soluble.
Polarity is a property of chemical compounds They have the tendency to separate electrical charges in their structure.
The molecules Polar molecules are made up of atoms whose electronegativity is very different, while nonpolar molecules are made up of atoms with equal electronegativity.
But the polarity of a molecule is also determined by the symmetry of its structure, so there may be molecules made up of atoms whose electronegativity is different, but they are arranged in such a way in the molecular structure that their dipoles and finally the molecule cancel. is apolar.
Agitation. Shaking or stirring the solutions increases the solubility of the solute, as it contributes to a greater interaction between the solute and the solvent.
Solubility product
When we talk about solubility product or ionic product (abbreviated Konly Ks), we refer to the product of the molar concentrations of the ions that form a compound, raised to their respective stoichiometric indices of the equilibrium equation. Thus, the greater the Ksol, the more soluble the compound will be. This is expressed with the following formula, considering the equilibrium equation:
Where:
- Ksol. It is the solubility product.
- [Cn +] m. It is the molar concentration of the cation raised to the stoichiometric coefficient m.
- [Am-] n. It is the molar concentration of the anion raised to the stoichiometric coefficient n.
Examples of solubility
- Salt dissolved in water. Common salt (sodium chloride, NaCl) dissolves easily in water, at a rate of 360 grams per liter, as long as the water is at 20ºC. If we increase the temperature of the solvent, the amount of salt that we can dissolve will increase.
- Fizzy drinks The canned or bottled sodas that we consume every day have an amount of carbon dioxide (CO2) gaseous dissolved in their interior, and therefore they have their characteristic bubbling. To achieve this, industries oversaturate the mixture at very high pressure conditions. Therefore, when we uncover them, the pressure balances and a gas leak begins.
- Iodine solutions. We often use iodine solutions to heal superficial wounds, which cannot be made with water, as iodine is not soluble in it. That is why they use alcohol, whose solubility rate improves and allows the mixture to be produced.
- Coffee with milk. To prepare a coffee with milk, we add the milk to the infusion and observe its change in colors how they mix. This is always done with hot coffee, since the solubility rate of both substances increases with temperature. If we wait for the substances to cool, however, we will notice the formation of cream on the surface, since the solution has become saturated more quickly.