Solubility

Solubility of Substances in Solution

By Shawn P. Shields, Ph.D.

This work is licensed by Shawn P. Shields-Maxwell under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

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Solubility

“Solubility” is defined as the amount of a given substance that can be dissolved in a certain amount of solvent.

A “soluble substance”

An “insoluble substance”

Solubility

Substances fall into three main categories of solubility:

Generally, a substance is considered “soluble” if it dissolves into solution completely.

“Sparingly (or slightly) soluble” substances are partially soluble. A limited amount of substance dissolves.

“Insoluble” substances do not dissolve at all (or a very tiny amount dissolves.)

Salts

Salts are ionic compounds.

Recall that ionic compounds are composed of a metal and a nonmetal.

Metals form cations, and nonmetals form anions.

“Salt” is another name for an ionic compound.

Salts can be soluble, sparingly soluble, and insoluble, depending on the identity of the cation and anion.

Rules for Predicting Soluble Salts

Contains these Ions Soluble Exceptions

Li+, Na+, K+, Rb+, Cs+,

NH4+

Most salts (ionic compounds) containing alkali metals (Group 1) and ammonium are soluble.

none

NO3 Most salts containing nitrate are

soluble

Cl, Br, I

Most salts containing halogen anions (Main Group 7) are soluble. (Chloride, bromide, and iodide salts)

Salts containing Cu+, Ag+, Pb2+, and Hg2

2+

SO42 Many salts containing sulfate are

soluble.Salts containing Sr2+, Ba2+, Pb2+, and Hg2

2+

Rules for Predicting Insoluble SaltsContains these

Ions Insoluble Exceptions

OH and S2 Most salts containing hydroxide or sulfide anions are insoluble.

Salts containing alkali metals (Li+, Na+, K+, Rb+,

and Cs+), and the “heavy” alkaline earth metals (Ca2+,

Sr2+, and Ba2+)

CO32, PO4

3 Most salts containing carbonate or phosphate anions are insoluble.

Salts containing alkali metals (Group 1) and

ammonium are soluble. (Li+, Na+, K+, Rb+, Cs+, NH4

+)

SO42

Many salts containing sulfate and cations with charges greater than or equal to 2 are insoluble, including salts containing Sr2+, Ba2+, Pb2+, and Hg2

2+.

Salts containing alkali metals (Group 1) and

ammonium are soluble. (Li+, Na+, K+, Rb+, Cs+, NH4

+), and Mg2+

Is it Soluble?

Example: Determine whether each salt given below is soluble or insoluble, and rationalize your answer.

NaCl

BaSO4

NH4Cl

Is it Soluble?


NaCl contains two ions that are generally found in soluble compounds, Na+ and Cl. We predict the salt is soluble.

BaSO4

NH4Cl

Is it Soluble?



BaSO4 contains sulfate, which can be either. Barium sulfate is composed of the Ba2+ and SO4

2 ions. Salts containing cations with 2+ charges tend to be insoluble, so we predict this salt to be insoluble.

NH4Cl

Is it Soluble?



BaSO4 contains sulfate, which can be either. Barium sulfate is composed of the Ba2+ and SO4

2 ions. Salts containing cations with 2+ charges tend to be insoluble, so we predict this salt to be insoluble.

NH4Cl also contains two ions that are generally found in soluble compounds, NH4

+ and Cl. We predict the salt is soluble.

Precipitation Reactions

Precipitation reactions happen when

Two soluble salts are mixed

An exchange reaction occurs

And one of the salts formed is insoluble

Precipitation Reactions

form a solid through an exchange reaction. The ions that do not form an insoluble solid remain dissolved in solution.

An “insoluble substance” is formed

Soluble salts dissolve into ions and react to form precipitate

2 soluble salts are mixed.

Example Precipitation Reaction

When a solution of potassium chloride and silver nitrate are mixed, silver chloride solid is formed.

So what is going on here?

Remember that the two solutions are ions dissolved in solution. Written as a chemical equation showing all of the ions, we have

Example Precipitation Reaction

The reaction is an “exchange reaction”, since the two salts “exchange” partners.

After mixing two solutions of soluble salts, exchange the ions, then check to see if one of the products is insoluble.

If so, a precipitate will form.

Spectator Ions

Notice that certain aqueous (aq) ions show up on both sides of the equation.

These are called spectator ions.

We can cancel these out to give the “net ionic equation.”

𝐊+¿ (𝐚𝐪 )+𝐂𝐥− (𝐚𝐪 )+𝐀𝐠+¿ (𝐚𝐪 ) +𝐍𝐎𝟑− (𝐚𝐪 )→𝐊+ ¿ (𝐚𝐪 )+ 𝐍𝐎

𝟑− (𝐚𝐪 )+ 𝐀𝐠𝐂𝐥( 𝐬) ¿

¿ ¿

𝐂𝐥− (𝐚𝐪 )+𝐀𝐠+¿ (𝐚𝐪 )→𝐀𝐠𝐂𝐥 (𝐬)¿

Mini Quiz

Determine whether the following reaction is a precipitation reaction. If so, write the net ionic equation for the reaction. Identify the spectator ions.

Mix CaBr2 (aq) and Na2CO3(aq)

Mini Quiz Solution



First, let’s remind ourselves that we have mixed two soluble salts using the solubility table.

Salts containing Br are soluble, so CaBr2 is soluble.

Salts containing carbonate (CO32) and an alkali metal

cation (Na+) are soluble.

Mini Quiz Solution



Now, exchange the ions and check to see if an insoluble salt is formed.

Is either insoluble? (Check your table!)

Mini Quiz Solution



Now, exchange the ions and check to see if an insoluble salt is formed.

Is either insoluble? (Check your table!)

YES, CaCO3 is insoluble.

Mini Quiz Solution



Now, write out the ionic equation and cancel the spectators.

𝐂𝐚𝟐+¿ (𝐚𝐪 )+𝟐𝐁𝐫− (𝐚𝐪 )+𝟐𝐍𝐚 +¿ (𝐚𝐪)+𝐂𝐎𝟑𝟐 − (𝐚𝐪)→𝟐𝐍𝐚+ ¿ (𝐚𝐪 )+𝟐𝐁𝐫− (𝐚𝐪 )+ 𝐂𝐚𝐂𝐎

𝟑(𝐬) ¿

¿ ¿

Mini Quiz Solution



Now, write out the ionic equation and cancel the spectators.

The net ionic equation is

The spectator ions are Na+ and Br.

𝐂𝐎𝟑𝟐− (𝐚𝐪 )+𝐂𝐚𝟐+¿ (𝐚𝐪 )→𝐂𝐚𝐂𝐎𝟑 (𝐬)¿

What You Should Be Able to Do

Identify soluble and insoluble compounds using the solubility rules provided.

Predict whether there will be a precipitate when two solutions of soluble salts are mixed.

Identify the precipitate formed and the spectator ions when the two solutions are mixed (if applicable).

Write the net ionic equation for a precipitation reaction.

Education

Solubility