Solubility is a property referring to the ability for a given substance, the solute, to dissolve in a solvent.
It is measured in terms of the maximum amount of solute dissolved in a solvent at equilibrium.
The resulting solution is called a saturated solution.
Certain substances are soluble in all proportions with a given solvent, such as ethanol in water.
This property is known as miscibility.
Under various conditions, the equilibrium solubility can be exceeded to give a so-called supersaturated solution, which is metastable.
The solvent is often a solid, which can be a pure substance or a mixture.
The species that dissolves, the solute, can be a gas, another liquid, or a solid.
Solubilities range widely, from infinitely soluble such as ethanol in water, to poorly soluble, such as silver chloride in water.
The term insoluble is often applied to poorly soluble compounds, though strictly speaking there are very few cases where there is absolutely no material dissolved.
The process of dissolving, called dissolution, is relatively straightforward for covalent substances such as ethanol.
When ethanol dissolves in water, the ethanol molecules remain intact but form new hydrogen bonds with the water.
When, however, an ionic compound such as sodium chloride (NaCl) dissolves in water, the sodium chloride lattice dissociates into separate ions which are solvated (wrapped) with a coating of water molecules.
Nonetheless, NaCl is said to dissolve in water, because evaporation of the solvent returns crystalline NaCl.