Solubility is the ability of a substance to act as a solute in a solution, usually with water as a solvent, or the degree to which it will dissolve.<ref>http://school.eb.com/levels/high/article/solubility/473350</ref> The solubility of a substance depends on not only the substance's nature but also its state of matter, temperature, pressure applies to it, and the solvent. In general, substances in categories such as lipids and aromatic hydrocarbons will dissolve each other.<ref>Mead, James F. Lipids: chemistry, biochemistry, and nutrition. 1986.</ref> Much of the principles regarding solubility were discovered by Joel Hildebrand.<ref>http://school.eb.com/levels/high/article/Joel-H-Hildebrand/40442</ref>
The ability of water to dissolve a substance results from its polarity, which grants in an ability to dissolve ionic compounds by dissociating their ions. These substances are regarded as aqueous solutions.
Not all ionic compounds are soluble in water. The ability of an ionic compound to dissolve in water is dependent on the difference of electronegativities of the two ions. As for covalent compounds, their ability to dissolve in water is dependent on the difference in electronegativities of their elements; a greater difference implies a greater ability to dissolve. Many covalent compounds dissolved in water form acids.<ref>Blake, Bob. Solubility Rules: Three Suggestions for Improved Understanding. Department of Chemistry and Biochemistry, Texas Tech University. 2003.</ref>
There are certain patterns of ionic compounds that are soluble in water:
In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution.
The extent of the solubility of a substance in a specific solvent is generally measured as the concentration of the solute in a saturated solution, one in which no more solute can be dissolved. At this point, the two substances are said to be at the solubility equilibrium. For some solutes and solvents, there may be no such limit, in which case the two substances are said to be "miscible in all proportions" (or just "miscible").
The solute can be a solid, a liquid, or a gas, while the solvent is usually solid or liquid. Both may be pure substances, or may themselves be solutions. Gases are always miscible in all proportions, except in very extreme situations, and a solid or liquid can be "dissolved" in a gas only by passing into the gaseous state first.
The solubility mainly depends on the composition of solute and solvent (including their pH and the presence of other dissolved substances) as well as on temperature and pressure. The dependency can often be explained in terms of interactions between the particles (atoms, molecules, or ions) of the two substances, and of thermodynamic concepts such as enthalpy and entropy.
Under certain conditions, the concentration of the solute can exceed its usual solubility limit. The result is a supersaturated solution, which is metastable and will rapidly exclude the excess solute if a suitable nucleation site appears.
The concept of solubility does not apply when there is an irreversible chemical reaction between the two substances, such as the reaction of calcium hydroxide with hydrochloric acid; even though one might say, informally, that one "dissolved" the other. The solubility is also not the same as the rate of solution, which is how fast a solid solute dissolves in a liquid solvent. This property depends on many other variables, such as the physical form of the two substances and the manner and intensity of mixing.
The concept and measure of solubility are extremely important in many sciences besides chemistry, such as geology, biology, physics, and oceanography, as well as in engineering, medicine, agriculture, and even in non-technical activities like painting, cleaning, cooking, and brewing. Most chemical reactions of scientific, industrial, or practical interest only happen after the reagents have been dissolved in a suitable solvent. Water is by far the most common such solvent.
The term "soluble" is sometimes used for materials that can form colloidal suspensions of very fine solid particles in a liquid. The quantitative solubility of such substances is generally not well-defined, however.