Predicting Precipitates Using Solubility Rules

Learning Objectives

  • Use solubility rules to predict when a product will precipitate.

Will it rain?

Predicting if precipitation occurs is partly accomplished by examining a set of rules

Weather forecast. Courtesy of John Hart & Jeremy Grams of the Storm Prediction Center, US National Oceanic and Atmospheric Administration.

Predicting the weather is tricky business. A thorough examination of a large amount of data is needed to make the daily forecast. Wind patterns, historical data, barometric pressure – these and many other data are fed into computers that then use a set of rules to predict what will happen based on past history.

Predicting Precipitates Using Solubility Rules

Some combinations of aqueous reactants result in the formation of a solid precipitate as a product. However, some combinations will not produce such a product. If solutions of sodium nitrate and ammonium chloride are mixed, no reaction occurs. One could write a molecular equation showing a double-replacement reaction, but both products, sodium chloride and ammonium nitrate, are soluble and would remain in the solution as ions. Every ion is a spectator ion and there is not net ionic equation at all.

It is useful to be able to predict when a precipitate will occur in a reaction. To do so, you can use a set of guidelines called the solubility rules (see the table below).

Solubility Rules for Ionic Compounds in Water
Soluble Compounds containing the alkali metal ions (Li + , Na + , K + , Rb + , Cs + ) and ammonium ion (NH 4 + )
Soluble Compounds containing the nitrate ion (NO 3 − ), acetate ion (CH 3 COO − ), chlorate ion (ClO 3 − ), and bicarbonate ion (HCO 3 − )
Mostly soluble Compounds containing the chloride ion (Cl − ), bromide ion (Br − ), and iodide ion (I − ) — Exceptions are those of silver (Ag + ), mercury(I) (Hg 2 2+ ), and lead(II) (Pb 2+ )
Mostly soluble Compounds containing the sulfate ion (SO 4 2− ) — Exceptions are those of silver (Ag + ), calcium (Ca 2+ ), strontium (Sr 2+ ), barium (Ba 2+ ), mercury(I) (Hg 2 2+ ), and lead(II) (Pb 2+ )
Mostly insoluble Compounds containing the carbonate ion (CO 3 2− ), phosphate ion (PO 4 3− ), chromate ion (CrO 4 2− ), sulfide ion (S 2− ), and silicate ion (SiO 3 2− ) — Exceptions are those of the alkali metals and ammonium
Mostly insoluble Compounds containing the hydroxide ion (OH − ) — Exceptions are those of the alkali metals and the barium ion (Ba 2+ )

As an example on how to use the solubility rules, predict if a precipitate will form when solutions of cesium bromide and lead(II) nitrate are mixed.

text{Cs}^+(aq)+text{Br}^-(aq)+text{Pb}^{2+} (aq)+text{2NO}^-_3 (aq) rightarrow ?

The potential precipitates from a double-replacement reaction are cesium nitrate and lead(II) bromide. According to the solubility rules table, cesium nitrate is soluble because all compounds containing the nitrate ion, as well as all compounds containing the alkali metal ions, are soluble. Most compounds containing the bromide ion are soluble, but lead(II) is an exception. Therefore, the cesium and nitrate ions are spectator ions and the lead(II) bromide is a precipitate. The balanced net ionic reaction is:

text{Pb}^{2+} (aq)+2text{Br}^-(aq) rightarrow text{PbBr}_2(s)

Summary

  • Solubility rules allow prediction of what products will be insoluble in water.

Review

  1. Are all alkali metal salts soluble?
  2. What chloride salts are not soluble?
  3. Would you expect sodium silicate to dissolve?
  4. Is lead(II) sulfate soluble?

 Glossary

  • solubility rules: A set of guidelines for predicting insoluble products of reactions.