Solubility
Solubility is the amount of a solute that will dissolve in a specific solvent under given conditions. The dissolved substance is called the
solute and the dissolving fluid (usually present in excess) is called the
solvent, which together form a
solution. The process of dissolving is called
solvation, or
hydration if the solvent is
water.
A solution at
equilibrium that cannot hold any more solute is said to be
saturated. The equilibrium of a solution is mainly dependent on temperature. The maximum equilibrium amount of solute which can normally dissolve per amount of solvent is the
solubility of that solute in that solvent. It is often expressed as a maximum
concentration of a saturated solution. The solubility of one substance dissolving in another is determined by the
intermolecular forces between the solvent and solute,
temperature, the
entropy change that accompanies the solvation, the presence and amount of other substances, and sometimes pressure or
partial pressure of a solute gas.
Solubility constants are used to describe saturated solutions of ionic compounds of relatively low solubility (see
solubility equilibrium). For
salts, solubility in
aqueous solutions or the maximum amount of salt that can be dissolved is the solubility constant. The solubility constant is a special case of an
equilibrium constant. It describes the balance between dissolved salt and undissolved salt. The solubility constant is also "applicable" (i.e. useful) to
precipitation, the reverse of the dissolving reaction. As with other equilibrium constants,
temperature can affect the numerical value of solubility constant.
While solutions are typically thought of as solids being mixed into liquids, any two states of matter can be mixed and be called a solution.
Carbonated water is a solution of a gas in a liquid,
hydrogen (a gas) can dissolve in
palladium (a solid), and
stainless steel is a solution of a solid in a solid (called an
alloy).
Phthalates dissolve in plastics and act as
plasticizer.
Solutions may, under special conditions, hold more solute than the solvent can normally dissolve. This is called
supersaturation.
Solvents are normally characterized as
polar or
nonpolar. The general rule of thumb is "Like Dissolves Like." This means that polar solvents will dissolve ionic compounds and covalent compounds which ionize, while nonpolar solvents will dissolve nonpolar covalent compounds. For example, ordinary
table salt, an
ionic compound, will dissolve in
water, but not in
ethanol.
Water and nonpolar solvents are
immiscible; they do not form
homogeneous mixtures but separate into two distinct phases or form milky
emulsions.
This table gives some indication how the
chemical bonding type relates to solubility in water.
| Bonding type | Solubility in water | Example |
|---|
| ionic | most soluble | See below |
|---|
| metallic | insoluble | Fe |
|---|
| unless they react with water | K |
| polar covalent | soluble if it has H bonds | glucose |
|---|
| soluble by reaction | HCl |
| insoluble otherwise | ether |
| non-polar covalent | most insoluble | benzene |
|---|
| some slightly soluble | O2 |
| covalent lattice | insoluble | diamond |
|---|
This table presents an overview of solubility of salts in water.
| Soluble | Insoluble |
|---|
| Group 1 and NH4+ compounds | carbonates (except Group 1 and NH4+ compounds) |
| nitrates | sulfites (except Group 1 and NH4+ compounds) |
| acetates (ethanoates) | phosphates (except Group 1 and NH4+ compounds) |
| chlorides, bromides and iodides (except Ag+, Pb2+, Cu+ and Hg22+) | hydroxides and oxides (except Group 1, NH4+, Ba2+, Sr2+, Ca2+ and Tl+) |
| sulfates (except Ag+, Pb2+, Ba2+, Sr2+ and Ca2+) | sulfides (except Group 1, Group 2 and NH4+ compounds) |
For a more comprehensive chart on solubility data, visit
solubility chart.
*
Biopharmaceutics Classification System*
Concentration*
Differential solubility (LogP)
*
Miscible*
Solvent*
Solubility Table Solubility data on ionic compounds
*
Hansen Solubility Parameters - an aproach to predicting when two substances are soluble
*
Flory-Huggins solution theory - a model of when polymers will dissolve into solutions
ALOGPS interactive calculation of aqueous solubility of compounds at
Virtual Computational Chemistry Laboratory
