How to Determine Solubility
Learn about ionic compounds., Understand solubility., Study the rules of solubility., Compounds are soluble if they contain alkali metals, including Li+, Na+, K+, Rb+, and Cs+., Compounds of NO3-, C2H3O2-, NO2-, ClO3-, and ClO4- are soluble...
Step-by-Step Guide
-
Step 1: Learn about ionic compounds.
Each atom normally has a certain number of electrons, but sometimes they pick up an extra electron, or lose one.
The result is an ion, which has an electric charge.
When an ion with a negative charge (an extra electron) meets an ion with a positive charge (missing an electron), they bond together just like the negative and positive ends of two magnets.
The result is an ionic compound.
Ions with negative charges are called anions, while ions with positive charges are cations.
Normally, the number of electrons in an atom is equal to the number of protons, canceling out the electrical charges. -
Step 2: Understand solubility.
Water molecules (H2O) have an unusual structure, which makes them similar to a magnet: one end has a positive charge, while the other has a negative.
When you drop an ionic compound in water, these water "magnets" will gather around it, trying to pull the positive and negative ions apart.
Some ionic compounds aren't stuck together very well; these are soluble since the water will pull them apart and dissolve them.
Other compounds are bonded more strongly, and are insoluble since they can stick together despite the water molecules.Some compounds have internal bonds that are similar in strength to the pull of the water.
These are called slightly soluble, since a significant amount of compounds will be pulled apart, but the rest will stay together. , Because the interactions between atoms are quite complex, it's not always intuitive which compounds are soluble and which are insoluble.
Look up the first ion in the compound on the list below to find out how it usually behaves, then check the exceptions to make sure the second ion doesn't have an unusual interaction.
For example, to check Strontium Chloride (SrCl2), look for Sr or Cl in the bold steps below.
Cl is "usually soluble," so check underneath it for exceptions.
Sr is not listed as an exception, so SrCl2 must be soluble.
The most common exceptions to each rule are written beneath it.
There are other exceptions, but you are unlikely to encounter them in a typical chemistry class or laboratory. , These are also called the Group IA elements: lithium, sodium, potassium, rubidium, and cesium.
Almost every single compound that includes one of these ions is soluble.
Exception:
Li3PO4 is insoluble. , Respectively, these are the nitrate, acetate, nitrite, chlorate, and perchlorate ions.
Note that acetate is often abbreviated OAC.Exceptions:
Ag(OAc) (silver acetate) and Hg(OAc)2 (mercury acetate) are insoluble.
AgNO2- and KClO4- are only "slightly soluble."
The chloride, bromide, and iodide ions almost always make soluble compounds, called halogen salts.
Exception:
If any of these pair with the ions silver Ag+, mercury Hg22+, or lead Pb2+, the result is insoluble.
The same is true of less common compounds made from pairing with copper Cu+ and thallium Tl+. , The sulfate ion generally forms soluble compounds, but there are several exceptions.
Exceptions:
The sulfate ion forms insoluble compounds with the following ions: strontium Sr2+, barium Ba2+, lead Pb2+, silver Ag+, calcium Ca2+, radium Ra2+, and diatomic silver Ag22+.
Note that silver sulfate and calcium sulfate dissolve just enough that some people call them slightly soluble. , These are the hydroxide and sulfide ions, respectively.
Exceptions:
Remember the alkali metals (Group I-A) and how they love forming soluble compounds? Li+, Na+, K+, Rb+, and Cs+ all form soluble compounds with the hydroxide or sulfide ions.
In addition, hydroxide forms soluble salts with the alkali earth (Group II-A) ions: calcium Ca2+, strontium Sr2+, and barium Ba2+.
Note that the compounds resulting from hydroxide and an alkali earth do have just enough molecules that stay bonded to sometimes be considered "slightly soluble."
One last check for carbonate and phosphate ions, and you should know what to expect from your compound.
Exceptions:
These ions form soluble compounds with the usual suspects, the alkali metals Li+, Na+, K+, Rb+, and Cs+, as well as with ammonium NH4+. -
Step 3: Study the rules of solubility.
-
Step 4: Compounds are soluble if they contain alkali metals
-
Step 5: including Li+
-
Step 6: and Cs+.
-
Step 7: Compounds of NO3-
-
Step 8: C2H3O2-
-
Step 9: and ClO4- are soluble.
-
Step 10: Compounds of Cl-
-
Step 11: and I- are usually soluble.
-
Step 12: Compounds containing SO42- are usually soluble.
-
Step 13: Compounds containing OH- or S2- are insoluble.
-
Step 14: Compounds containing CO32- or PO43- are insoluble.
Detailed Guide
Each atom normally has a certain number of electrons, but sometimes they pick up an extra electron, or lose one.
The result is an ion, which has an electric charge.
When an ion with a negative charge (an extra electron) meets an ion with a positive charge (missing an electron), they bond together just like the negative and positive ends of two magnets.
The result is an ionic compound.
Ions with negative charges are called anions, while ions with positive charges are cations.
Normally, the number of electrons in an atom is equal to the number of protons, canceling out the electrical charges.
Water molecules (H2O) have an unusual structure, which makes them similar to a magnet: one end has a positive charge, while the other has a negative.
When you drop an ionic compound in water, these water "magnets" will gather around it, trying to pull the positive and negative ions apart.
Some ionic compounds aren't stuck together very well; these are soluble since the water will pull them apart and dissolve them.
Other compounds are bonded more strongly, and are insoluble since they can stick together despite the water molecules.Some compounds have internal bonds that are similar in strength to the pull of the water.
These are called slightly soluble, since a significant amount of compounds will be pulled apart, but the rest will stay together. , Because the interactions between atoms are quite complex, it's not always intuitive which compounds are soluble and which are insoluble.
Look up the first ion in the compound on the list below to find out how it usually behaves, then check the exceptions to make sure the second ion doesn't have an unusual interaction.
For example, to check Strontium Chloride (SrCl2), look for Sr or Cl in the bold steps below.
Cl is "usually soluble," so check underneath it for exceptions.
Sr is not listed as an exception, so SrCl2 must be soluble.
The most common exceptions to each rule are written beneath it.
There are other exceptions, but you are unlikely to encounter them in a typical chemistry class or laboratory. , These are also called the Group IA elements: lithium, sodium, potassium, rubidium, and cesium.
Almost every single compound that includes one of these ions is soluble.
Exception:
Li3PO4 is insoluble. , Respectively, these are the nitrate, acetate, nitrite, chlorate, and perchlorate ions.
Note that acetate is often abbreviated OAC.Exceptions:
Ag(OAc) (silver acetate) and Hg(OAc)2 (mercury acetate) are insoluble.
AgNO2- and KClO4- are only "slightly soluble."
The chloride, bromide, and iodide ions almost always make soluble compounds, called halogen salts.
Exception:
If any of these pair with the ions silver Ag+, mercury Hg22+, or lead Pb2+, the result is insoluble.
The same is true of less common compounds made from pairing with copper Cu+ and thallium Tl+. , The sulfate ion generally forms soluble compounds, but there are several exceptions.
Exceptions:
The sulfate ion forms insoluble compounds with the following ions: strontium Sr2+, barium Ba2+, lead Pb2+, silver Ag+, calcium Ca2+, radium Ra2+, and diatomic silver Ag22+.
Note that silver sulfate and calcium sulfate dissolve just enough that some people call them slightly soluble. , These are the hydroxide and sulfide ions, respectively.
Exceptions:
Remember the alkali metals (Group I-A) and how they love forming soluble compounds? Li+, Na+, K+, Rb+, and Cs+ all form soluble compounds with the hydroxide or sulfide ions.
In addition, hydroxide forms soluble salts with the alkali earth (Group II-A) ions: calcium Ca2+, strontium Sr2+, and barium Ba2+.
Note that the compounds resulting from hydroxide and an alkali earth do have just enough molecules that stay bonded to sometimes be considered "slightly soluble."
One last check for carbonate and phosphate ions, and you should know what to expect from your compound.
Exceptions:
These ions form soluble compounds with the usual suspects, the alkali metals Li+, Na+, K+, Rb+, and Cs+, as well as with ammonium NH4+.
About the Author
Jacob Stewart
Enthusiastic about teaching cooking techniques through clear, step-by-step guides.
Rate This Guide
How helpful was this guide? Click to rate: