Solubility

solubility

Dissolution of hydrosoluble carotene (solute, brown) in water (solvent).

The solubility is the ability of a substance to dissolve in another substance called a solvent. It also refers to the mass of solute that can be dissolved in a certain mass of solvent, under certain temperature conditions, and even pressure (in case of a gaseous solute). The solubility can be found in different mixtures, for example, in the common ion it is very difficult to find since the common ion is the main factor in solubility. If no more solute can be dissolved in a solution, the solution is said to be saturated. Under certain conditions the solubility can exceed this maximum and it is called a supersaturated solution. On the contrary, if the solution admits even more solute, it is said to be unsaturated.

Not all substances dissolve in the same solvent. For example, in water, alcohol and salt dissolve, while oil and gasoline do not dissolve in water. In the solubility, the polar or apolar character of the substance influences a lot, since, due to this character, the substance will be more or less soluble; For example, compounds with more than one functional group have great polarity, so they are not soluble in ethyl ether. The less reactive compounds, such as paraffins, aromatic compounds and halogenated derivatives have lower solubility.

The term solubility is used both to designate the qualitative phenomenon of the dissolution process and to quantitatively express the concentration of the solutions. The solubility of a substance depends on the nature of the solvent and the solute, as well as the temperature and pressure of the system.

Factors Affecting Solubility

Solubility is defined for specific phases. For example, the solubility of aragonite and calcite in water are expected to differ, even though both are polymorphs of calcium carbonate and have the same molecular formula.

The solubility of one substance in another is determined by the balance of intermolecular forces between the solvent and the solute, and the change in entropy that accompanies solvation. Factors such as temperature and pressure influence this balance, thus changing the solubility.

Solubility also depends to a large extent on the presence of other substances dissolved in the solvent, such as the existence of metal complexes in liquids. The solubility will also depend on the excess or deficiency of some common ion, with the solute, in the solution; such a phenomenon is known as the common ion effect. To a lesser extent, the solubility will depend on the ionic strength of the solutions. The last two mentioned effects can be quantified using the solubility equilibrium equation.

For a solid that dissolves in a redox reaction, the solubility is expected to depend on the potentials (within the range of potentials at which the solid remains thermodynamically stable phase). For example, the solubility of gold in water at high temperature is observed to be almost an order of magnitude higher when the redox potential is controlled by a highly oxidizing Fe₃O _{redox buffer. 4}-Fe₂O₃ than with a moderately oxidizing Ni-NiO buffer.

Solubility (metastable) also depends on the physical size of the crystal grain or more strictly speaking, on the specific (or molar) surface area of the solute. To assess quantification, one should look at the equation in the article on solubility equilibrium. For crystals highly defective in their structure, the solubility may increase with increasing degree of disorder. Both effects occur due to the constant solubility dependence on the so-called Gibbs free energy associated with the crystal. The last two effects, although often difficult to measure, are of relevant importance in practice ^{[citation needed]} as they provide the driving force for determining the degree of precipitation, since that crystal size grows spontaneously with time.

Temperature

The solubility of a solute in a certain solvent mainly depends on the temperature. For many dissolved solids in liquid water, the solubility increases with temperature up to 100 °C, although there are cases that present an inverse behavior. In most cases in liquid water at high temperatures the solubility of ionic solutes tends to increase due to the change in the properties and structure of liquid water, which reduces the dielectric constant of a less polar solvent.

Gaseous solutes show more complex behavior with temperature. As the temperature rises, gases generally become less soluble in water (the minimum being below 120 °C for most gases), but more soluble in organic solvents.

The graph shows the solubility curve of some typical solid inorganic salts. Many salts behave like barium nitrate and acid disodium arsenate, showing a large increase in solubility with temperature. Some solutes (for example, sodium chloride (NaCl) in water) exhibit solubility that is quite independent of temperature. A few, such as cerium(III) sulfate and lithium carbonate, become less soluble in water as the temperature increases. This temperature dependence is sometimes referred to as "retrograde" or "inverse solubility." Sometimes a more complex pattern is seen, such as with sodium sulfate, where the less soluble decahydrate crystal loses water of crystallization at 32 °C to form a less soluble anhydrous phase. ^{[citation required]}

The solubility of organic compounds almost always increases with temperature. The recrystallization technique, used for the purification of solids, depends on a solute of different solubilities in a hot and cold solvent. There are some exceptions, such as certain cyclodextrins.

Pressure

The solubility of gases varies not only with temperature but also with the pressure exerted on it. In this way, the amount of a gaseous solute that can be dissolved in a certain solvent increases when subjected to a higher partial pressure (see Henry's Law). At an industrial level, this can be observed in the packaging of soft drinks, for example, where the solubility of carbon dioxide is increased by exerting a pressure of around 4 atm. ^{[citation required]}

How is solubility quantified?

A very common way to find the values that quantitatively describe the solute solubility is to find the maximum number of grams of solute that can be dissolved in a given amount of solvent, taking this into account we can express the solubility in moles per liter (this is known as molar solubility) only if the molar mass of the substance is known. In the particular case of ionic salts that are only slightly soluble, their solubility is usually quantified by studying the following equilibrium:

<math alttext="{displaystyle {ce {MX(s) M+(ac) + X-(ac)}}}" xmlns="http://www.w3.org/1998/Math/MathML">MX(s) − − − − M+(ac)+X− − (ac){displaystyle {ce {MX(s)}}}<img alt="{displaystyle {ce {MX(s) M+(ac) + X-(ac)}}}" aria-hidden="true" class="mwe-math-fallback-image-inline" src="https://wikimedia.org/api/rest_v1/media/math/render/svg/3a3a8de0d1b74bdf9c51d75da1efe07611eb6e38" style="vertical-align: -0.838ex; width:29.289ex; height:3.176ex;"/>

When a solid substance participates in an equilibrium, its concentration does not appear in the expression of the equilibrium constant, since it remains constant. This occurs with the concentration of MX, so the expression remains:

K[chuckles]MX]=Kps=[chuckles]M+][chuckles]X− − ]{displaystyle {ce {K[MX] = Kps = [M+][X-]}}}

The solubility product of an ionic compound is the product of the molar concentrations of the constituent ions, each raised to the power of its stoichiometric coefficient in the equilibrium equation.