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>

• If the solute is a solid, its solubility will increase when it is subjected to increasing temperature.
• If the solute is a liquid, its solubility will not be affected by a change in temperature.
• If the solute is a gas, its solubility will increase when it is subjected to decreasing temperature.
• The solubility of a gas increases with increasing pressure.
• The solubility of liquids and solids is unaffected by changes in pressure.<ref>Wile, Dr. Jay L. Exploring Creation With Chemistry. Apologia Educational Ministries, Inc. 1998</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:

• Ionic compounds containing the Li+, Na+, K+, NH4+, NO3-, or C2H3O2- ions are soluble in water.
• Ionic compounds containing the Cl-, Br-, I-, or SO4-2 ions are soluble in water, excepting those containing the Ag+, Hg2+2, Sr+2, Ba+2, or Pb+2 ions as their positive ion.
• Ionic compounds containing the CO3-2, or PO4-3 ions are insoluble in water, except for those with a Group 1A positive ion or ammonium ion.
• Ionic compounds containing the S-2 or OH-1 ion are insoluble in water, except for those with a Group 1A positive ion, ammonium ion, calcium ion, strontium ion, or barium ion.

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