AP Chemistry Solubility Rules

Alkali Metal Ions (Group 1 Cations) e.g., $NaCl(s) \rightarrow Na^+(aq) + Cl^-(aq)$

Always soluble in aqueous solution. Compounds containing Group 1 cations ($Li^+$, $Na^+$, $K^+$, $Rb^+$, $Cs^+$) dissociate completely in water and act as spectator ions in precipitation reactions.

$NH_4^+$ (Ammonium Ion) e.g., $NH_4Cl(s) \rightarrow NH_4^+(aq) + Cl^-(aq)$

Always soluble in aqueous solution without exceptions. Salts containing the ammonium ion dissociate completely in water and do not form precipitates.

$NO_3^-$ (Nitrate Ion) e.g., $AgNO_3(s) \rightarrow Ag^+(aq) + NO_3^-(aq)$

Always soluble in aqueous solution without exceptions. All nitrate salts fully dissociate in water, making $NO_3^-$ a standard spectator ion in AP Chemistry net ionic equations.

$ClO_3^-$ (Chlorate Ion) e.g., $KClO_3(s) \rightarrow K^+(aq) + ClO_3^-(aq)$

Always soluble in aqueous solution without exceptions.

$ClO_4^-$ (Perchlorate Ion) e.g., $NaClO_4(s) \rightarrow Na^+(aq) + ClO_4^-(aq)$

Always soluble in aqueous solution without exceptions.

$C_2H_3O_2^-$ (Acetate Ion) e.g., $NaC_2H_3O_2(s) \rightarrow Na^+(aq) + C_2H_3O_2^-(aq)$

Always soluble in aqueous solution without exceptions.

$Cl^-$ (Chloride Ion) e.g., $Ag^+(aq) + Cl^-(aq) \rightarrow AgCl(s)$

Soluble in water except when combined with silver ($Ag^+$), lead(II) ($Pb^{2+}$), or mercury(I) ($Hg_2^{2+}$) ions (AP/H exception rule), which form insoluble precipitates.

$Br^-$ (Bromide Ion) e.g., $Pb^{2+}(aq) + 2Br^-(aq) \rightarrow PbBr_2(s)$

Soluble in water except when combined with silver ($Ag^+$), lead(II) ($Pb^{2+}$), or mercury(I) ($Hg_2^{2+}$) ions (AP/H exception rule), which form insoluble precipitates.

$I^-$ (Iodide Ion) e.g., $Hg_2^{2+}(aq) + 2I^-(aq) \rightarrow Hg_2I_2(s)$

Soluble in water except when combined with silver ($Ag^+$), lead(II) ($Pb^{2+}$), or mercury(I) ($Hg_2^{2+}$) ions (AP/H exception rule), which form insoluble precipitates.

$F^-$ (Fluoride Ion) e.g., $Ca^{2+}(aq) + 2F^-(aq) \rightarrow CaF_2(s)$

Soluble in water except when combined with calcium ($Ca^{2+}$), strontium ($Sr^{2+}$), barium ($Ba^{2+}$), or lead(II) ($Pb^{2+}$) ions (CBS-PM exception rule), which form insoluble precipitates.

$SO_4^{2-}$ (Sulfate Ion) e.g., $Ba^{2+}(aq) + SO_4^{2-}(aq) \rightarrow BaSO_4(s)$

Soluble in water except when paired with calcium ($Ca^{2+}$), strontium ($Sr^{2+}$), barium ($Ba^{2+}$), or lead(II) ($Pb^{2+}$) ions (CBS/PBS exception rule), which form insoluble precipitates.

$O^{2-}$ (Oxide Ion) e.g., $CaO(s) + H_2O(l) \rightarrow Ca^{2+}(aq) + 2OH^-(aq)$

Insoluble in water except with alkali metal ions and ammonium ($NH_4^+$). Oxides of calcium ($Ca^{2+}$), strontium ($Sr^{2+}$), and barium ($Ba^{2+}$) react with water to form slightly soluble hydroxide solutions.

$OH^-$ (Hydroxide Ion) e.g., $Mg^{2+}(aq) + 2OH^-(aq) \rightarrow Mg(OH)_2(s)$

Insoluble in water except with alkali metal ions and ammonium ($NH_4^+$). Hydroxides of calcium ($Ca^{2+}$), strontium ($Sr^{2+}$), and barium ($Ba^{2+}$) are considered slightly soluble strong bases in AP Chemistry.

$CO_3^{2-}$ (Carbonate Ion) e.g., $Ca^{2+}(aq) + CO_3^{2-}(aq) \rightarrow CaCO_3(s)$

Insoluble in water except when combined with alkali metal cations or ammonium ($NH_4^+$).

$PO_4^{3-}$ (Phosphate Ion) e.g., $3Ca^{2+}(aq) + 2PO_4^{3-}(aq) \rightarrow Ca_3(PO_4)_2(s)$

Insoluble in water except when combined with alkali metal cations or ammonium ($NH_4^+$).

$S^{2-}$ (Sulfide Ion) e.g., $Cu^{2+}(aq) + S^{2-}(aq) \rightarrow CuS(s)$

Insoluble in water except when combined with alkali metal cations or ammonium ($NH_4^+$).

$SO_3^{2-}$ (Sulfite Ion) e.g., $Ba^{2+}(aq) + SO_3^{2-}(aq) \rightarrow BaSO_3(s)$

Insoluble in water except when combined with alkali metal cations or ammonium ($NH_4^+$).

$C_2O_4^{2-}$ (Oxalate Ion) e.g., $Ca^{2+}(aq) + C_2O_4^{2-}(aq) \rightarrow CaC_2O_4(s)$

Insoluble in water except when combined with alkali metal cations or ammonium ($NH_4^+$).

$CrO_4^{2-}$ (Chromate Ion) e.g., $2Ag^+(aq) + CrO_4^{2-}(aq) \rightarrow Ag_2CrO_4(s)$

Insoluble in water except when combined with alkali metal cations or ammonium ($NH_4^+$).