The solubility rules in chemistry are a set of guidelines for identifying inorganic compounds that are soluble in water near room temperature.
What Is Solubility
A soluble chemical freely dissolves in a solvent in any ratio. For example, ethanol is soluble in water. In insoluble chemical does not dissolve in the solvent. But, solubility is not an all-or-nothing process. Many chemicals are slightly soluble, meaning they don’t completely dissolve, but they partially dissociate into their ions. Many “insoluble” chemicals are still very slightly soluble in a solvent, so a tiny fraction of the substance dissolves.
What Are the Solubility Rules?
The solubility rules are a set of guidelines predicting solubility of inorganic compounds in water at or near room temperature. Soluble compounds form aqueous solutions.
|Soluble Compounds||Exceptions (are insoluble)|
|Alkali metal compounds (Li+, Na+, K+, Rb+, Cs+)|
|ammonium ion compounds (NH4+)|
|Nitrates (NO3–), bicarbonates (HCO3–), chlorates (ClO3–)|
|Halides (Cl–, Br–, I–)||Halides of Ag+, Hg2+, Pb2+ (except AgF, which is soluble)|
|Sulfates (SO42-)||Sulfates of Ag+, Ca2+, Sr2+, Ba2+, Hg2+, Pb2+|
|Insoluble Compounds||Exceptions (are soluble)|
|Carbonates (CO32-), phosphates (PO42-), chromates (CrO42-)||Alkali metal compounds (Li+, Na+, K+) and those containing the ammonium ion (NH4+)|
|Hydroxides (OH–), sulfides (S2-)||Alkali metal compounds and those containing Ca2+, Sr2+, Ba2+|
Here is a solubility chart you can download or print. Either right-click and save the image or else download the PDF file.
How to Memorize Solubility Rules
The easiest way of memorizing solubility rules is using mnemonic devices. Four mnemonics that cover most compounds are NAG, SAG, PMS, and Castro Bear. NAG and SAG are always soluble, with PMS and Castro Bear being the exceptions.
- Nitrates (NO3–)
- Acetates (C2H3O2–)
- Group 1 (the alkali metals: Li+, Na+, K+, etc.)
- Sulfates (SO42-)
- Ammonium (NH4+)
- Group 17 (the halogens: F–, Cl–, Br–, etc.)
The exceptions are particular metal compounds.
- P: Pb2+, lead
- M: Mercury, Hg2+
- S: Silver, Ag+
Saying “castro bear” makes it easier to distinguish between these metals and other with similar names and symbols.
- Calcium (Ca2+)
- Strontium (Sr2+)
- Barium (Ba2+)
Factors That Affect Solubility
Several factors affect solubility:
- Temperature: If the dissolution reaction is endothermic, solubility tends to increase with temperature. If the dissolution is exothermic, solubility tends to decrease as temperature increases. Dissolving most solids and liquids is endothermic, so usually solubility increases with temperature. Solubility of organic compounds almost always increases with temperature (exception is cyclodextrin). Gas behavior is more complex and harder to predict.
- Phase: Solubility depends on phase. For example, the solubility of aragonite differs from that of calcite, even though both are forms of calcium carbonate (CaCO3).
- Presence of other species: Other species in a solution affect solubility. Factors include ligands, common ions, and ionic strength of solution.
- Pressure: Pressure plays a small role in the solubility of solids and liquids. While typically ignored in most applications, it is important in petroleum chemistry, where calcium sulfate fouling of oil wells occurs. Calcium sulfate solubility decreases with decreasing pressure.
- Particle shape and size: Increasing surface area tends to increase solubility, particularly approaching saturation. So, a fine powder is more soluble than a single chunk. Whether a substance is crystalline or amorphous matters. Typically, increasing order reduces solubility.
- Polarity: “Like dissolves like” means polar solvents dissolve polar compounds, while nonpolar solvents dissolve nonpolar compounds.
How to Use Solubility Rules
The solubility rules have multiple uses, including predicting whether a chemical will dissolve, predicting precipitate formation, and purifying samples. To use the solubility rules, check the anion (the negative part of the ion) and see whether it is soluble or insoluble. Pay attention to exceptions to the rules.
For example, predict whether FeCO3 is soluble.
From the solubility rules, carbonates (compounds containing CO32-) tend to be insoluble. So, FeCO3 likely is not soluble. As the product of a reaction, it forms a precipitate.
For example, predict whether a precipitate forms from this reaction:
2AgNO3 + Na2S → Ag2S + 2NaNO3
A precipitate form if either Ag2S or NaNO3 is insoluble. From the solubility rules, sulfides tend to be insoluble, so Ag2S likely forms a precipitate. NaNO3 is soluble and does not form a precipitate because most nitrates are soluble. Since Ag2S forms a precipitate, one does form in this reaction.
The solubility rules do not predict behavior under all circumstances. For example, they don’t necessarily work with organic compounds or at extremely high or low temperatures. The rules apply best to pure solutions of a single compound in water, so real behavior may deviate from predicted behavior in mixtures. While they are called “rules”, they are really “guidelines.”
- Hefter, G. T.; Tomkins, R. P. T (eds.) (2003). The Experimental Determination of Solubilities. Wiley-Blackwell. ISBN 978-0-471-49708-0.
- IUPAC (1997). “Solubility”. Compendium of Chemical Terminology (the “Gold Book”) (2nd ed.). Blackwell Scientific Publications. doi:10.1351/goldbook.S05740
- Jain, N.; Yalkowsky, S. H. (2001). “Estimation of the aqueous solubility I: application to organic nonelectrolytes”. Journal of Pharmaceutical Sciences. 90 (2): 234–252. doi:10.1002/1520-6017(200102)90:2<234::aid-jps14>3.0.co;2-v
- Petrucci, Ralph H.; et al. (2011). General Chemistry: Principles and Modern Applications (10th ed.). Upper Saddle River, New Jersey: Pearson Education. ISBN: 978-0132064521.
- Ran, Y.; N. Jain; S. H. Yalkowsky (2001). “Prediction of Aqueous Solubility of Organic Compounds by the General Solubility Equation (GSE)”. Journal of Chemical Information and Modeling. 41 (5): 1208–1217. doi:10.1021/ci010287z