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ChemWiki: The Dynamic Chemistry E-textbook > Physical Chemistry > Equilibria > Solubility > Solubility Rules

Solubility Rules

In order to predict if there will be a precipitate in a reaction, the solubility of the substances involved must be known. There are rules, or guidelines, strictly devoted to determining solubility of substances. If a substance involved is not soluble, the reaction will form a precipitate.

Introduction

When a substance is mixed with its solvent, there are a few different possible results. What makes the difference between what happens as a result of this mixture is the solubility of the substance, which is the upper limit of the concentration of the solute. The solubility rules help determine which substances are soluble, and to which extent, so that we can find out what the result of the mixture will be.

Solubility Effects on Reactions

Based on the solubility of a substance, there are a few different possible results in reactions. 1) If the solution has less solute than the maximum amount that it is able (its solubility limit) to dissolve, it is known as a dilute solution. 2) If the solution's amount of solute is exactly the same amount as its solubility limit, it is known as saturated. 3) If there is more solute than is able to be dissolved, the excess solute begins to separate from the solution. If this separation includes crystallization, it forms a precipitate. The function of precipitation is to lower the concentration of the solute to the saturated level in order to make the solution more stable.

Solubility Rules

The following are the Solubility Rules for common ionic solids. If there is a case in which two of these contradict each other, the preceding rule must be followed.

  1. NH4+ and salts of alkali metal (group 1) cations tend to be soluble. Exceptions include some salts that include Li+.
  2. Perchlorates as well as nitrates and acetates tend to be soluble.
  3. Mercury (I), silver, and lead salts tend to be insoluble.
  4. Iodides, bromides, and chlorides tend to be soluble.
  5. Sulfides, oxides, hydroxides, carbonates, and phosphates tend to be insoluble. Exceptions include sulfides formed from group 2 cations and hydroxides formed from Ca2+, Sr2+ and Ba2+. These exceptions are slightly soluble.
  6. Sulfates tend to be soluble. Exceptions to this rule include those of Ca, Sr and Ba.

Here is a catchy song about Solubility Rules that you can use to remember these more effectively!

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References

  1. Petrucci, Ralph H., F. Geoffrey Herring, Jeffrey D. Madura and Carey Bissonnette. General Chemistry: Principles and Modern Applications. 10th ed. Upper Saddle River, New Jersey: Pearson Education, 2011. Print.
  2. Nathan, Harold D., and Charles Henrickson. Chemistry. New York: Wiley, 2001. Print.

Problems

1. Is FeCO3 soluble?

Rule #5 tells us that carbonates tend to be insoluble. Therefore, FeCO3 is likely to form a precipitate.

2. Does ClO4- tend to form a precipitate?

This is perchlorate, which Rule #2 tells us is likely to be soluble. Therefore, it will not form a precipitate.

3. Which of these substances is likely to form a precipitate?

a) CaSO4    b) table salt    c) AgBr

Letters a and c are both likely to form precipitates.

In a) Ca SO4 although it is a sulfate and sulfates tend to be soluble, Rule #6 tells us that calcium sulfate is an important exception to this rule.

For b), Rule #1 tells us table salt (NaCl) is soluble because it is a salt of an alkali metal.

C) is an example of two rules contradicting each other. Rule #4 tells us that bromides are usually soluble, but Rule #3 tells us that salts of silver are insoluble. Since Rule #3 precedes Rule #4, the compound is insoluble and will form a precipitate.

4. Predict if a precipitate will form as a result of this reaction:

 2AgNO3 + Na2S -----> Ag2S + 2NaNO3

We need to look at the products of the reaction, and if either one of the substances that formed as a result of the reaction are insoluble, a precipitate will form.

Looking at NaNO3, Rule #3 tells us that nitrates tend to be soluble. This compound does not imply that a precipitate will form.

Now, let's look at Ag2S. Rule #5 tells us that sulfides tend to be insoluble. Therefore, this compound shows that a precipitate will form in this reaction.

5. Predict if a precipitate will form as a result of this reaction:

 2NaOH + K2CrO4 -----> KOH + Na2CrO4

We need to look at the products of the reaction, and if either one of the substances that formed as a result of the reaction are insoluble, a precipitate will form.

The first product, KOH, is an example of two rules contradicting each other. Although Rule #5 says that hydroxides tend to be insoluble, Rule #1 states that salts of alkali metal cations tend to be soluble, and Rule #1 precedes Rule #5. Therefore, this compound will not contribute to any precipitation being formed.

The second product, Na2CrO4, also adheres to Rule #1, which states that salts of alkali metals tend to be soluble.

Since both products are soluble, no precipitate form as a result of this reaction.

Contributors

  • Antoinette Mursa UCD

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Last Modified
00:59, 3 Mar 2014

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