Solubility is the property of a
solidSolid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...
,
liquidLiquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
, or gaseous
chemical substanceIn chemistry, a chemical substance is a form of matter that has constant chemical composition and characteristic properties. It cannot be separated into components by physical separation methods, i.e. without breaking chemical bonds. They can be solids, liquids or gases.Chemical substances are...
called
soluteIn chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
to
dissolveDissolution is the process by which a solid, liquid or gas forms a solution in a solvent. In solids this can be explained as the breakdown of the crystal lattice into individual ions, atoms or molecules and their transport into the solvent. For liquids and gases, the molecules must be compatible...
in a
solidSolid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...
,
liquidLiquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
, or gaseous
solventA solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
to form a homogeneous
solutionIn chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on temperature and pressure. The extent of the solubility of a substance in a specific solvent is measured as the
saturationIn chemistry, saturation has six different meanings, all based on reaching a maximum capacity...
concentration where adding more solute does not increase the concentration of the solution.
Most often, the solvent is a liquid, which can be a pure substance or a
mixtureIn chemistry, a mixture is a material system made up by two or more different substances which are mixed together but are not combined chemically...
. One may also speak of
solid solutionA solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase...
, but rarely of solution in a gas (see
vapor-liquid equilibriumVapor–liquid equilibrium is a condition where a liquid and its vapor are in equilibrium with each other, a condition or state where the rate of evaporation equals the rate of condensation on a molecular level such that there is no net vapor-liquid interconversion...
instead).
The extent of solubility ranges widely, from infinitely soluble (fully miscible ) such as
ethanolEthanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...
in
waterWater is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
, to poorly soluble, such as
silver chlorideSilver chloride is a chemical compound with the chemical formula AgCl. This white crystalline solid is well known for its low solubility in water . Upon illumination or heating, silver chloride converts to silver , which is signalled by greyish or purplish coloration to some samples...
in water. The term
insoluble is often applied to poorly or very poorly soluble compounds.
Under certain conditions, the
equilibrium solubilitySolubility equilibrium is a type of dynamic equilibrium. It exists when a chemical compound in the solid state is in chemical equilibrium with a solution of that compound. The solid may dissolve unchanged, with dissociation or with chemical reaction with another constituent of the solvent, such as...
can be exceeded to give a so-called
supersaturatedThe term supersaturation refers to a solution that contains more of the dissolved material than could be dissolved by the solvent under normal circumstances...
solution, which is metastable.
Solubility is not to be confused with the ability to dissolve or liquefy a substance, because the solution might occur not only because of dissolution but also because of a chemical reaction. For example, zinc is insoluble in hydrochloric acid, but does dissolve in it by chemical reaction into zinc chloride and hydrogen, where zinc chloride is then soluble in hydrochloric acid. Solubility does not also depend on particle size or other
kineticChemical kinetics, also known as reaction kinetics, is the study of rates of chemical processes. Chemical kinetics includes investigations of how different experimental conditions can influence the speed of a chemical reaction and yield information about the reaction's mechanism and transition...
factors; given enough time, even large particles will eventually dissolve.
IUPAC definition
According to an IUPAC definition, solubility is the analytical composition of a saturated solution expressed as a proportion of a designated solute in a designated solvent. Solubility may be stated in units of concentration, molality, mole fraction, mole ratio, and other units.
Molecular view
Solubility occurs under dynamic equilibrium, which means that solubility results from the simultaneous and opposing processes of
dissolutionSolvation, also sometimes called dissolution, is the process of attraction and association of molecules of a solvent with molecules or ions of a solute...
and phase joining (e.g.,
precipitationPrecipitation is the formation of a solid in a solution or inside anothersolid during a chemical reaction or by diffusion in a solid. When the reaction occurs in a liquid, the solid formed is called the precipitate, or when compacted by a centrifuge, a pellet. The liquid remaining above the solid...
of solids). The solubility equilibrium occurs when the two processes proceed at a constant rate.
The term
solubility is also used in some fields where the solute is altered by
solvolysisSolvolysis is a special type of nucleophilic substitution or elimination where the nucleophile is a solvent molecule. For certain nucleophiles, there are specific terms for the type of solvolysis reaction...
. For example, many metals and their
oxideAn oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
s are said to be "soluble in hydrochloric acid," whereas the aqueous acid degrades the solid to irreversibly give soluble products. It is also true that most ionic solids are degraded by polar solvents, but such processes are reversible. In those cases where the solute is not recovered upon evaporation of the solvent, the process is referred to as solvolysis. The thermodynamic concept of solubility does not apply straightforwardly to solvolysis.
When a solute dissolves, it may form several species in the solution. For example, an aqueous
suspensionIn chemistry, a suspension is a heterogeneous fluid containing solid particles that are sufficiently large for sedimentation. Usually they must be larger than 1 micrometer. The internal phase is dispersed throughout the external phase through mechanical agitation, with the use of certain...
of
ferrous hydroxideIron hydroxide or ferrous hydroxide is a compound produced when iron ions, from a compound such as iron sulfate, react with hydroxide ions. Iron hydroxide itself is practically white, but even traces of oxygen impart it with a greenish tinge...
, , will contain the series [
6 − x(OH)
x]
(2 − x)+ as well as other
oligomerIn chemistry, an oligomer is a molecule that consists of a few monomer units , in contrast to a polymer that, at least in principle, consists of an unlimited number of monomers. Dimers, trimers, and tetramers are oligomers. Many oils are oligomeric, such as liquid paraffin...
ic species. Furthermore, the solubility of ferrous hydroxide and the composition of its soluble components depends on
pHIn chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
. In general, solubility in the solvent phase can be given only for a specific solute that is thermodynamically stable, and the value of the solubility will include all the species in the solution (in the example above, all the iron-containing complexes).
Factors affecting solubility
Solubility is defined for specific
phasesIn the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, and chemical composition...
. For example, the solubility of
aragoniteAragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...
and
calciteCalcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
in water are expected to differ, even though they are both
polymorphsPolymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...
of
calcium carbonateCalcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
and have the same
chemical formulaA chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
.
The solubility of one substance in another is determined by the balance of
intermolecular forceIntermolecular forces are forces of attraction or repulsion which act between neighboring particles: atoms, molecules or ions. They are weak compared to the intramolecular forces, the forces which keep a molecule together...
s between the solvent and solute, and the
entropyEntropy is a thermodynamic property that can be used to determine the energy available for useful work in a thermodynamic process, such as in energy conversion devices, engines, or machines. Such devices can only be driven by convertible energy, and have a theoretical maximum efficiency when...
change that accompanies the solvation. Factors such as temperature and pressure will alter this balance, thus changing the solubility.
Solubility may also strongly depend on the presence of other species dissolved in the solvent, for example,
complex-In chemistry, a coordination complex or metal complex, is an atom or ion , bonded to a surrounding array of molecules or anions, that are in turn known as ligands or complexing agents...
forming anions (
ligandIn coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s) in liquids. Solubility will also depend on the excess or deficiency of a common ion in the solution, a phenomenon known as the
common-ion effectThe common ion effect is an effect which results when two substances, which both ionize to give the same ion, are involved in a chemical equilibrium.-Solubility effects:...
. To a lesser extent, solubility will depend on the
ionic strengthThe ionic strength of a solution is a measure of the concentration of ions in that solution. Ionic compounds, when dissolved in water, dissociate into ions. The total electrolyte concentration in solution will affect important properties such as the dissociation or the solubility of different salts...
of solutions. The last two effects can be quantified using the equation for
solubility equilibriumSolubility equilibrium is a type of dynamic equilibrium. It exists when a chemical compound in the solid state is in chemical equilibrium with a solution of that compound. The solid may dissolve unchanged, with dissociation or with chemical reaction with another constituent of the solvent, such as...
.
For a solid that dissolves in a redox reaction, solubility is expected to depend on the potential (within the range of potentials under which the solid remains the thermodynamically stable phase). For example, solubility of gold in high-temperature water is observed to be almost an order of magnitude higher when the redox potential is controlled using a highly-oxidizing Fe
3O
4-Fe
2O
3 redox buffer than with a moderately-oxidizing Ni-NiO buffer.
Solubility (metastable) also depends on the physical size of the crystal or droplet of solute (or, strictly speaking, on the specific or molar surface area of the solute). For quantification, see the equation in the article on solubility equilibrium. For highly defective crystals, solubility may increase with the increasing degree of disorder. Both of these effects occur because of the dependence of solubility constant on the Gibbs energy of the crystal. The last two effects, although often difficult to measure, are of practical importance. For example, they provide the driving force for
precipitate agingright|thumb|300px|Basic schematic of the Ostwald ripening process Ostwald ripening is an observed phenomenon in solid solutions or liquid sols which describes the change of an inhomogeneous structure over time...
(the crystal size spontaneously increasing with time).
Temperature
The solubility of a given solute in a given solvent typically depends on temperature. For many solids dissolved in liquid water, the solubility increases with temperature up to 100 °C. In liquid water at high temperatures, (e.g., that approaching the critical temperature), the solubility of ionic solutes tends to decrease due to the change of properties and structure of liquid water; the lower
dielectric constantThe relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...
results in a less polar solvent.
GasGas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
eous solutes exhibit more complex behavior with temperature. As the temperature is raised, gases usually become less soluble in water (to minimum, which is below 120 °C for most permanent gases), but more soluble in organic solvents.
The chart shows solubility curves for some typical solid inorganic
saltIn chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...
s (temperature is in degrees
CelsiusCelsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
). Many salts behave like
barium nitrateBarium nitrate with chemical formula Ba2 is a salt of barium and the nitrate ion.Barium nitrate exists as a white solid at room temperature. It is soluble in water, and like other soluble barium compounds, is toxic and should be handled with care...
and disodium hydrogen arsenate, and show a large increase in solubility with temperature. Some solutes (e.g., NaCl in water) exhibit solubility that is fairly independent of temperature. A few, such as cerium(III) sulfate, become less soluble in water as temperature increases. This temperature dependence is sometimes referred to as "retrograde" or "inverse" solubility. Occasionally, a more complex pattern is observed, as with
sodium sulfateSodium sulfate is the sodium salt of sulfuric acid. When anhydrous, it is a white crystalline solid of formula Na2SO4 known as the mineral thenardite; the decahydrate Na2SO4·10H2O has been known as Glauber's salt or, historically, sal mirabilis since the 17th century. Another solid is the...
, where the less soluble deca
hydrateHydrate is a term used in inorganic chemistry and organic chemistry to indicate that a substance contains water. The chemical state of the water varies widely between hydrates, some of which were so labeled before their chemical structure was understood....
crystal loses
water of crystallizationIn crystallography, water of crystallization or water of hydration or crystallization water is water that occurs in crystals. Water of crystallization is necessary for the maintenance of crystalline properties, but capable of being removed by sufficient heat...
at 32 °C to form a more soluble
anhydrousAs a general term, a substance is said to be anhydrous if it contains no water. The way of achieving the anhydrous form differs from one substance to another...
phase.
The solubility of organic compounds nearly always increases with temperature. The technique of
recrystallization-Chemistry:In chemistry, recrystallization is a procedure for purifying compounds. The most typical situation is that a desired "compound A" is contaminated by a small amount of "impurity B". There are various methods of purification that may be attempted , which includes recrystallization...
, used for purification of solids, depends on a solute's different solubilities in hot and cold solvent. A few exceptions exist, such as certain
cyclodextrinCyclodextrins are a family of compounds made up of sugar molecules bound together in a ring ....
s.
Pressure
For condensed phases (solids and liquids), the pressure dependence of solubility is typically weak and usually neglected in practice. Assuming an ideal solution, the dependence can be quantified as:
where the index i iterates the components, N
i is the mole fraction of the i
th component in the solution, P is the pressure, the index T refers to constant temperature, V
i,aq is the partial molar volume of the i
th component in the solution, V
i,cr is the partial molar volume of the i
th component in the dissolving solid, and R is the universal gas constant.
The pressure dependence of solubility does occasionally have practical significance. For example, precipitation fouling of oil fields and wells by
calcium sulfateCalcium sulfate is a common laboratory and industrial chemical. In the form of γ-anhydrite , it is used as a desiccant. It is also used as a coagulant in products like tofu. In the natural state, unrefined calcium sulfate is a translucent, crystalline white rock...
(which decreases its solubility with decreasing pressure) can result in decreased productivity with time.
Solubility of gases
Henry's lawIn physics, Henry's law is one of the gas laws formulated by William Henry in 1803. It states that:An equivalent way of stating the law is that the solubility of a gas in a liquid at a particular temperature is proportional to the pressure of that gas above the liquid...
is used to quantify the solubility of gases in solvents. The solubility of a gas in a solvent is directly proportional to the
partial pressureIn a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
of that gas above the solvent. This relationship is written as:
where
kH is a temperature-dependent constant (for example, 769.2
Lpic|200px|right|thumb|One litre is equivalent to this cubeEach side is 10 cm1 litre water = 1 kilogram water The litre is a metric system unit of volume equal to 1 cubic decimetre , to 1,000 cubic centimetres , and to 1/1,000 cubic metre...
·
atmThe standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of pressure. For practical purposes it has been replaced by the bar which is 105 Pa...
/
molThe mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...
for dioxygen (O
2) in water at 298 K),
p is the partial pressure (atm), and
c is the
concentrationIn chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...
of the dissolved gas in the liquid (mol/L).
The solubility of gases is sometimes also quantified using
Bunsen solubility coefficientThe Bunsen solubility coefficient or Bunsen absorption coefficient , named for Robert Bunsen, is one of a number of units used to describe the solubility of gases in liquids...
.
In the presence of small
bubbleA bubble is a globule of one substance in another, usually gas in a liquid.Due to the Marangoni effect, bubbles may remain intact when they reach the surface of the immersive substance.-Common examples:...
s, the solubility of the gas does not depend on the bubble radius in any other way than through the effect of the radius on pressure (i.e., the solubility of gas in the liquid in contact with small bubbles is increased due to pressure increase by Δp = 2γ/r; see
Young–Laplace equationIn physics, the Young–Laplace equation is a nonlinear partial differential equation that describes the capillary pressure difference sustained across the interface between two static fluids, such as water and air, due to the phenomenon of surface tension or wall tension, although usage on the...
).
Polarity
A popular
aphorismAn aphorism is an original thought, spoken or written in a laconic and memorable form.The term was first used in the Aphorisms of Hippocrates...
used for predicting solubility is "
like dissolves like". This statement indicates that a solute will dissolve best in a solvent that has a similar
chemical structureA chemical structure includes molecular geometry, electronic structure and crystal structure of molecules. Molecular geometry refers to the spatial arrangement of atoms in a molecule and the chemical bonds that hold the atoms together. Molecular geometry can range from the very simple, such as...
to itself. This view is simplistic, but it is a useful rule of thumb. The overall solvation capacity of a solvent depends primarily on its
polarityIn chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in...
. For example, a very polar (
hydrophilicA hydrophile, from the Greek "water" and φιλια "love," is a molecule or other molecular entity that is attracted to, and tends to be dissolved by water. A hydrophilic molecule or portion of a molecule is one that has a tendency to interact with or be dissolved by, water and other polar substances...
) solute such as
ureaUrea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....
is very soluble in highly polar water, less soluble in fairly polar
methanolMethanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
, and practically insoluble in non-polar solvents such as
benzeneBenzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
. In contrast, a non-polar or lipophilic solute such as
naphthaleneNaphthalene is an organic compound with formula . It is a white crystalline solid with a characteristic odor that is detectable at concentrations as low as 0.08 ppm by mass. As an aromatic hydrocarbon, naphthalene's structure consists of a fused pair of benzene rings...
is insoluble in water, fairly soluble in methanol, and highly soluble in non-polar benzene.
The solubility is favored by
entropy of mixingIn thermodynamics the entropy of mixing is the increase in the total entropy of a compound system, when different and chemically non-reacting chemical substances or material components are mixed by removing partition between the system's initially separate volumes...
and depends on enthalpy of dissolution and the
hydrophobic effectThe hydrophobic effect is the observed tendency of nonpolar substances to aggregate in aqueous solution and exclude water molecules. The name, literally meaning "water-fearing," describes the segregation and apparent repulsion between water and nonpolar substances...
.
Synthetic chemists often exploit differences in solubilities to separate and purify compounds from reaction mixtures, using the technique of
liquid-liquid extractionLiquid–liquid extraction, also known as solvent extraction and partitioning, is a method to separate compounds based on their relative solubilities in two different immiscible liquids, usually water and an organic solvent. It is an extraction of a substance from one liquid phase into another liquid...
.
Rate of dissolution
DissolutionSolvation, also sometimes called dissolution, is the process of attraction and association of molecules of a solvent with molecules or ions of a solute...
is not always an instantaneous process. It is fast when salt and sugar dissolve in water but much slower for a tablet of
aspirinAspirin , also known as acetylsalicylic acid , is a salicylate drug, often used as an analgesic to relieve minor aches and pains, as an antipyretic to reduce fever, and as an anti-inflammatory medication. It was discovered by Arthur Eichengrun, a chemist with the German company Bayer...
or a large crystal of hydrated
copper(II) sulfate. These observations are the consequence of two factors: the rate of solubilization is related to the solubility product and the surface area of the material. The speed at which a solid dissolves may depend on its crystallinity or lack thereof in the case of amorphous solids and the surface area (crystallite size) and the presence of
polymorphismPolymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...
. Many practical systems illustrate this effect, for example in designing methods for controlled
drug deliveryDrug delivery is the method or process of administering a pharmaceutical compound to achieve a therapeutic effect in humans or animals. Drug delivery technologies modify drug release profile, absorption, distribution and elimination for the benefit of improving product efficacy and safety, as well...
. Critically, the dissolution rate depends on the presence of mixing and other factors that determine the degree of undersaturation in the liquid solvent film immediately adjacent to the solid solute crystal. In some cases, solubility equilibria can take a long time to establish (hours, days, months, or many years; depending on the nature of the solute and other factors). In practice, it means that the amount of solute in a solution is not always determined by its thermodynamic solubility, but may depend on kinetics of dissolution (or precipitation).
The rate of dissolution and solubility should not be confused as they are different concepts, kinetic and thermodynamic, respectively. The solubilization kinetics, as well as apparent solubility can be improved after complexation of an active ingredient with cyclodextrin. This can be used in the case of drug with poor solubility.
Quantification of solubility
Solubility is commonly expressed as a concentration, either by mass (g of solute per kg of solvent,
g per dL (100mL) of solventIn astronomy or astrophysics mass concentration or mascon is a region of a planet or moon's crust that contains a large positive gravitational anomaly. In general, the word "mascon" can be used as a noun to describe an excess distribution of mass on or beneath the surface of a planet , such as Hawaii...
, molarity,
molality,
mole fraction, or other similar descriptions of
concentrationIn chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...
. The maximum equilibrium amount of solute that can dissolve per amount of solvent is the solubility of that solute in that solvent under the specified conditions. The advantage of expressing solubility in this manner is its simplicity, while the disadvantage is that it can strongly depend on the presence of other species in the solvent (for example, the common ion effect).
Solubility constants are used to describe saturated solutions of ionic compounds of relatively low solubility (see
solubility equilibriumSolubility equilibrium is a type of dynamic equilibrium. It exists when a chemical compound in the solid state is in chemical equilibrium with a solution of that compound. The solid may dissolve unchanged, with dissociation or with chemical reaction with another constituent of the solvent, such as...
). The solubility constant is a special case of an
equilibrium constant. It describes the balance between dissolved ions from the salt and undissolved salt. The solubility constant is also "applicable" (i.e., useful) to
precipitationPrecipitation is the formation of a solid in a solution or inside anothersolid during a chemical reaction or by diffusion in a solid. When the reaction occurs in a liquid, the solid formed is called the precipitate, or when compacted by a centrifuge, a pellet. The liquid remaining above the solid...
, the reverse of the dissolving reaction. As with other equilibrium constants,
temperatureTemperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
can affect the numerical value of solubility constant. The solubility constant is not as simple as solubility, however the value of this constant is generally independent of the presence of other species in the solvent.
The
Flory-Huggins solution theoryFlory-Huggins solution theory is a mathematical model of the thermodynamics of polymer solutions which takes account of the great dissimilarity in molecular sizes in adapting the usual expression for the entropy of mixing. The result is an equation for the Gibbs free energy change \Delta G_m for...
is a theoretical model describing the solubility of polymers. The
Hansen Solubility ParametersHansen Solubility Parameters were developed by Charles Hansen as a way of predicting if one material will dissolve in another and form a solution...
and the
Hildebrand solubility parameterThe Hildebrand solubility parameter provides a numerical estimate of the degree of interaction between materials, and can be a good indication of solubility, particularly for non polar materials such as many polymers...
s are empirical methods for the prediction of solubility. It is also possible to predict solubility from other physical constants such as the
enthalpy of fusionThe enthalpy of fusion is the change in enthalpy resulting from heating one mole of a substance to change its state from a solid to a liquid. The temperature at which this occurs is the melting point....
.
The
partition coefficientIn chemistry and the pharmaceutical sciences, a partition- or distribution coefficient is the ratio of concentrations of a compound in the two phases of a mixture of two immiscible solvents at equilibrium. The terms "gas/liquid partition coefficient" and "air/water partition coefficient" are...
(Log P) is a measure of differential solubility of a compound in a
hydrophobicIn chemistry, hydrophobicity is the physical property of a molecule that is repelled from a mass of water....
solvent (
octanolOctanol is a straight chain fatty alcohol with eight carbon atoms and the molecular formula CH37OH. Although the term octanol usually refers exclusively to the primary alcohol 1-octanol, there are other less common isomers of octanol such as the secondary alcohols 2-octanol, 3-octanol and...
) and a
hydrophilicA hydrophile, from the Greek "water" and φιλια "love," is a molecule or other molecular entity that is attracted to, and tends to be dissolved by water. A hydrophilic molecule or portion of a molecule is one that has a tendency to interact with or be dissolved by, water and other polar substances...
solvent (
waterWater is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
). The logarithm of these two values enables compounds to be ranked in terms of hydrophilicity (or hydrophobicity).
Applications
Solubility is of fundamental importance in a large number of scientific disciplines and practical applications, ranging from ore processing, to the use of medicines, and the transport of pollutants.
Solubility is often said to be one of the "characteristic properties of a substance," which means that solubility is commonly used to describe the substance, to indicate a substance's polarity, to help to distinguish it from other substances, and as a guide to applications of the substance. For example, indigo is described as "insoluble in water, alcohol, or ether but soluble in chloroform, nitrobenzene, or concentrated sulfuric acid".
Solubility of a substance is useful when separating mixtures. For example, a mixture of salt (
sodium chlorideSodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...
) and silica may be separated by dissolving the salt in water, and filtering off the undissolved silica. The synthesis of chemical compounds, by the milligram in a laboratory, or by the ton in industry, both make use of the relative solubilities of the desired product, as well as unreacted starting materials, byproducts, and side products to achieve separation.
Another example of this is the synthesis of
benzoic acidBenzoic acid , C7H6O2 , is a colorless crystalline solid and the simplest aromatic carboxylic acid. The name derived from gum benzoin, which was for a long time the only source for benzoic acid. Its salts are used as a food preservative and benzoic acid is an important precursor for the synthesis...
from
phenylmagnesium bromidePhenylmagnesium bromide, with the simplified formula , is a magnesium-containing organometallic compound. It is so commonly used that it is commercially available as a solution in diethyl ether or tetrahydrofuran . Phenylmagnesium bromide is a Grignard reagent...
and
dry iceDry ice, sometimes referred to as "Cardice" or as "card ice" , is the solid form of carbon dioxide. It is used primarily as a cooling agent. Its advantages include lower temperature than that of water ice and not leaving any residue...
. Benzoic acid is more soluble in an organic solvent such as
dichloromethaneDichloromethane is an organic compound with the formula CH2Cl2. This colorless, volatile liquid with a moderately sweet aroma is widely used as a solvent. Although it is not miscible with water, it is miscible with many organic solvents...
or
diethyl etherDiethyl ether, also known as ethyl ether, simply ether, or ethoxyethane, is an organic compound in the ether class with the formula . It is a colorless, highly volatile flammable liquid with a characteristic odor...
, and when shaken with this organic solvent in a
separatory funnelA separatory funnel, also known as separation funnel, separating funnel, or colloquially sep funnel, is a piece of laboratory glassware used in liquid-liquid extractions to separate the components of a mixture between two immiscible solvent phases of different densities Typically, one of the...
, will preferentially dissolve in the organic layer. The other reaction products, including the magnesium bromide, will remain in the aqueous layer, clearly showing that separation based on solubility is achieved. This process, known as
liquid-liquid extractionLiquid–liquid extraction, also known as solvent extraction and partitioning, is a method to separate compounds based on their relative solubilities in two different immiscible liquids, usually water and an organic solvent. It is an extraction of a substance from one liquid phase into another liquid...
, is an important technique in synthetic chemistry.
Solubility of ionic compounds in water
Some ionic compounds (salts) dissolve in water, which arises because of the attraction between positive and negative charges (see:
solvationSolvation, also sometimes called dissolution, is the process of attraction and association of molecules of a solvent with molecules or ions of a solute...
). For example, the salt's positive ions (e.g. Ag
+) attract the partially-negative oxygens in H
2O. Likewise, the salt's negative ions (e.g. Cl
−) attract the partially-positive hydrogens in H
2O. Note: oxygen is partially-negative because it is more
electronegativeElectronegativity, symbol χ , is a chemical property that describes the tendency of an atom or a functional group to attract electrons towards itself. An atom's electronegativity is affected by both its atomic number and the distance that its valence electrons reside from the charged nucleus...
than hydrogen, and vice-versa (see:
chemical polarityIn chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in...
).
- AgCl(s) Ag+(aq) + Cl−(aq)
However, there is a limit to how much salt can be dissolved in a given volume of water. This amount is given by the solubility product, K
sp. This value depends on the type of salt (AgCl vs. NaCl, for example), temperature, and the common ion effect.
One can calculate the amount of AgCl that will dissolve in 1 liter of water, some algebra is required.
- Ksp = [Ag+] × [Cl−] (definition of solubility product)
- Ksp = 1.8 × 10−10 (from a table of solubility products)
[Ag
+] = [Cl
−], in the absence of other silver or chloride salts,
- [Ag+]2 = 1.8 × 10−10
- [Ag+] = 1.34 × 10−5
The result: 1 liter of water can dissolve 1.34 × 10
−5 molesThe mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...
of AgCl
(s) at room temperature. Compared with other types of salts, AgCl is poorly soluble in water. In contrast, table salt (NaCl) has a higher K
sp and is, therefore, more soluble.
| Soluble | Insoluble |
|---|
Group IThe alkali metals are a series of chemical elements in the periodic table. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, along with hydrogen. The alkali metals are lithium , sodium , potassium , rubidium , caesium , and francium...
and NH4+The ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...
compounds | Carbonate In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....
s (Except Group IThe alkali metals are a series of chemical elements in the periodic table. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, along with hydrogen. The alkali metals are lithium , sodium , potassium , rubidium , caesium , and francium...
, NH4+The ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...
and uranylThe uranyl ion is an oxycation of uranium in the oxidation state +6, with the chemical formula [UO2]2+. It has a linear structure with short U-O bonds, indicative of the presence of multiple bonds between uranium and oxygen. Four or more ligands are bound to the uranyl ion in an equatorial plane...
compounds) |
NitrateThe nitrate ion is a polyatomic ion with the molecular formula NO and a molecular mass of 62.0049 g/mol. It is the conjugate base of nitric acid, consisting of one central nitrogen atom surrounded by three identically-bonded oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a...
s | Sulfite Sulfites are compounds that contain the sulfite ion SO. The sulfite ion is the conjugate base of bisulfite. Although the acid itself is elusive, its salts are widely used.-Structure:...
s (Except Group IThe alkali metals are a series of chemical elements in the periodic table. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, along with hydrogen. The alkali metals are lithium , sodium , potassium , rubidium , caesium , and francium...
and NH4+The ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...
compounds) |
| Acetate An acetate is a derivative of acetic acid. This term includes salts and esters, as well as the anion found in solution. Most of the approximately 5 billion kilograms of acetic acid produced annually in industry are used in the production of acetates, which usually take the form of polymers. In...
s (Ethanoates) (Except Ag+Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
compounds) | PhosphateA phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
s (Except Group IThe alkali metals are a series of chemical elements in the periodic table. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, along with hydrogen. The alkali metals are lithium , sodium , potassium , rubidium , caesium , and francium...
and NH4+The ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...
compounds) |
ChlorideThe chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
s (Chlorates and Perchlorates), bromideA bromide is a chemical compound containing bromide ion, that is bromine atom with effective charge of −1. The class name can include ionic compounds such as caesium bromide or covalent compounds such as sulfur dibromide.-Natural occurrence:...
s and iodideAn iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. This page is for the iodide ion and its salts. For information on organoiodides, see organohalides. In everyday life, iodide is most commonly encountered as a component of iodized salt,...
s (Except Ag+Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
, Pb2+Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
, Cu+Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
and Hg22+Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
) | Hydroxide Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...
s and oxideAn oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
s (Except Group IThe alkali metals are a series of chemical elements in the periodic table. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, along with hydrogen. The alkali metals are lithium , sodium , potassium , rubidium , caesium , and francium...
, NH4+The ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...
, Ba2+Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in Group 2, a soft silvery metallic alkaline earth metal. Barium is never found in nature in its pure form due to its reactivity with air. Its oxide is historically known as baryta but it reacts with...
, Sr2+Strontium is a chemical element with the symbol Sr and the atomic number 38. An alkaline earth metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically. The metal turns yellow when exposed to air. It occurs naturally in the minerals celestine and...
and Tl+Thallium is a chemical element with the symbol Tl and atomic number 81. This soft gray poor metal resembles tin but discolors when exposed to air. The two chemists William Crookes and Claude-Auguste Lamy discovered thallium independently in 1861 by the newly developed method of flame spectroscopy...
) |
SulfateIn inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...
s (Except Ag+Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
, Pb2+Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
, Ba2+Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in Group 2, a soft silvery metallic alkaline earth metal. Barium is never found in nature in its pure form due to its reactivity with air. Its oxide is historically known as baryta but it reacts with...
, Sr2+Strontium is a chemical element with the symbol Sr and the atomic number 38. An alkaline earth metal, strontium is a soft silver-white or yellowish metallic element that is highly reactive chemically. The metal turns yellow when exposed to air. It occurs naturally in the minerals celestine and...
and Ca2+Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
) | Sulfide A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
s (Except Group IThe alkali metals are a series of chemical elements in the periodic table. In the modern IUPAC nomenclature, the alkali metals comprise the group 1 elements, along with hydrogen. The alkali metals are lithium , sodium , potassium , rubidium , caesium , and francium...
, Group IIThe alkaline earth metals are a group in the periodic table. In the modern IUPAC nomenclature, the alkaline earth metals are called the group 2 elements. Previously, they were called the Group IIA elements . The alkaline earth metals contain beryllium , magnesium , calcium , strontium , barium and...
and NH4+The ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...
compounds) |
Solubility of organic compounds
The principle outlined above under polarity, that
like dissolves like, is the usual guide to solubility with organic systems. For example,
petroleum jellyPetroleum jelly, petrolatum, white petrolatum or soft paraffin, CAS number 8009-03-8, is a semi-solid mixture of hydrocarbons , originally promoted as a topical ointment for its healing properties...
will dissolve in
gasolineGasoline , or petrol , is a toxic, translucent, petroleum-derived liquid that is primarily used as a fuel in internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Some gasolines also contain...
because both petroleum jelly and gasoline are non-polar hydrocarbons. It will not, on the other hand, dissolve in ethyl alcohol or water, since the polarity of these solvents is too high. Sugar will not dissolve in gasoline, since sugar is too polar in comparison with gasoline. A mixture of gasoline and sugar can therefore be separated by
filtrationFiltration is commonly the mechanical or physical operation which is used for the separation of solids from fluids by interposing a medium through which only the fluid can pass...
, or extraction with water.
Solubility in non-aqueous solvents
Most publicly available solubility values are those for solubility in water. The reference also lists some for non-aqueous solvents. Solubility data for non-aqueous solvents is currently being collected via an
open notebook scienceOpen Notebook Science is the practice of making the entire primary record of a research project publicly available online as it is recorded. This involves placing the personal, or laboratory, notebook of the researcher online along with all raw and processed data, and any associated material, as...
crowdsourcingCrowdsourcing is the act of sourcing tasks traditionally performed by specific individuals to a group of people or community through an open call....
project.
Solid solution
This term is often used in the field of
metallurgyMetallurgy is a domain of materials science that studies the physical and chemical behavior of metallic elements, their intermetallic compounds, and their mixtures, which are called alloys. It is also the technology of metals: the way in which science is applied to their practical use...
to refer to the extent that an
alloyAn alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
ing element will dissolve into the
base metalIn chemistry, the term base metal is used informally to refer to a metal that oxidizes or corrodes relatively easily, and reacts variably with diluted hydrochloric acid to form hydrogen. Examples include iron, nickel, lead and zinc...
without forming a separate phase. The solubility line (or curve) is the line (or lines) on a
phase diagramA phase diagram in physical chemistry, engineering, mineralogy, and materials science is a type of chart used to show conditions at which thermodynamically distinct phases can occur at equilibrium...
that give the limits of solute addition. That is, the lines show the maximum amount of a component that can be added to another component and still be in
solid solutionA solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase...
. In the solid's crystalline structure, the 'solute' element can either take the place of the matrix within the lattice (a substitutional position, for example: chromium in iron) or take a place in a space between the lattice points (an interstitial position, for example: carbon in iron).
In microelectronic fabrication, solid solubility refers to the maximum concentration of impurities one can place into the substrate.
Incongruent dissolution
Many substances dissolve congruently; i.e., the composition of the solid and the dissolved solute stoichiometrically match. However, some substances may dissolve
incongruentlyIncongruent transition, in chemistry, is a mass transition between two phases which involves a change in composition. This is contrasted with congruent transition, for which the composition remains the same....
, whereby the composition of the solute in solution does not match that of the solid. This solubilization is accompanied by alteration of the "primary solid" and possibly formation of a secondary solid phase. However, in general, some primary solid also remains and a complex solubility equilibrium establishes. For example, dissolution of
albiteAlbite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. As such it represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi3O8. It is a tectosilicate. Its color is usually pure white, hence...
may result in formation of
gibbsiteGibbsite, Al3, is one of the mineral forms of aluminium hydroxide. It is often designated as γ-Al3 . It is also sometimes called hydrargillite ....
.
- NaAlSi3O8(s) + H+ + 7H2O = Na+ + Al(OH)3(s) + 3H4SiO4.
In this case, the solubility of albite is expected to depend on the solid-to-solvent ratio. This kind of solubility is of great importance in geology, where it results in formation of
metamorphic rockMetamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
s.
See also
- Biopharmaceutics Classification System
The Biopharmaceutics Classification System is a guide for predicting the intestinal drug absorption provided by the U.S. Food and Drug Administration . The fundamental basis for the BCS was established by Dr...
- Dühring's rule
Dühring's rule states that a linear relationship exists between the temperatures at which two solutions exert the same vapor pressure. The rule is often used to compare a pure liquid and a solution at a given concentration....
- Fajans-Paneth-Hahn Law
The Fajans-Paneth-Hahn Law , in chemistry, is a rule governing how a small amount of one substance is carried down to a precipitate of another substance present in much larger amount by co-precipitation or adsorption.The rule...
- Flexible SPC water model
The Flexible Simple Point Charge water model is a re-parametrization of the three-site SPC water model. The SPC model is rigid, whilst the flexible SPC model is flexible. In the model of Toukan and Rahman, the O-H stretching is made anharmonic and thus the dynamical behavior is well described...
- Hot water extraction
The Hot Water Extraction method, is a method used in chemistry for extraction and for "steam cleaning"...
- Hydrotrope
A hydrotrope is a compound that solubilises hydrophobic compounds in aqueous solutions. Typically, hydrotropes consist of a hydrophilic part and a hydrophobic part but the hydrophobic part is generally too small to cause spontaneous self-aggregation.Hydrotropes do not have a critical concentration...
- Raoult's law
- Simulations Plus
Simulations Plus, Inc. develops absorption, distribution, metabolism, excretion, and toxicity modeling and simulation software for the pharmaceutical and biotechnology industries...
- Solubility equilibrium
Solubility equilibrium is a type of dynamic equilibrium. It exists when a chemical compound in the solid state is in chemical equilibrium with a solution of that compound. The solid may dissolve unchanged, with dissociation or with chemical reaction with another constituent of the solvent, such as...
- Solubilization
Solubilization, according to an IUPAC definition, is a short form for micellar solubilization, a term used in colloidal and surface chemistry....
- Apparent molar property
External links
- VCClab.org, "ALOGPS" free interactive calculation of aqueous solubility of compounds at Virtual Computational Chemistry Laboratory using several algorithms.
- ACDlabs.com? ACD/Solubility DB aqueous solubility prediction
- Simulations-plus.com, S+Sw, an aqueous solubility prediction model.