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9.6: Solubility Rules

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    Learning Objectives
    • Given a table of solubility rules and the name or formula for an ionic compound, determine whether the ionic compound is soluble in water.

    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 no 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 (Tables \(\PageIndex{1}\) and \(\PageIndex{2}\)).

    Table \(\PageIndex{1}\): Solubility Rules for Soluble Substances
    Soluble in Water Important Exceptions (Insoluble)
    All Group IA and NH4+ salts none
    All nitrates, chlorates, perchlorates and acetates none
    All sulfates CaSO4, BaSO4, SrSO4, PbSO4
    All chlorides, bromides, and iodides AgX, Hg2X2, PbX2 (X= Cl, Br, or I)
    All fluorides MgF2, CaF2, SrF2, BaF2, PbF2,
    Table \(\PageIndex{2}\): Solubility Rules for Sparingly Soluble Substances
    Sparingly Soluble in Water Important Exceptions (Soluble)
    All carbonates, phosphates, and chromates Group IA and NH4+ salts
    All oxides and hydroxides Group IA and NH4+ salts; Ba2+, Sr2+, Ca2+ sparingly soluble
    All sulfides Group IA, IIA and NH4+ salts; MgS, CaS, BaS sparingly soluble
    All oxalates Group IA and NH4+ salts

    As an example on how to use the solubility rules, let's predict if cesium nitrate and lead(II) bromide are soluble in water. 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, lead(II) bromide is insoluble in water.

    Example \(\PageIndex{1}\): Solubility

    Classify each compound as soluble or insoluble

    1. Zn(NO3)2
    2. PbBr2
    3. Sr3(PO4)2

    Solution

    1. All nitrates are soluble in water, so Zn(NO3)2 is soluble.
    2. All bromides are soluble in water, except those combined with Pb2+, so PbBr2 is insoluble.
    3. All phosphates are insoluble, so Sr3(PO4)2 is insoluble.
    Exercise \(\PageIndex{1}\): Solubility

    Classify each compound as soluble or insoluble.

    1. Mg(OH)2
    2. KBr
    3. Pb(NO3)2
    Answer a
    insoluble
    Answer b
    soluble
    Answer c
    soluble

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