4.5: Solubility of Ionic Compounds

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Learning Objectives

Predict solubilities of ionic compounds in water using the solubility rules.

The extent to which a substance may be dissolved in water, or any solvent, is quantitatively expressed as its solubility, defined as the maximum concentration of a substance that can be achieved under specified conditions. Substances with relatively large solubilities are said to be soluble. A substance will precipitate when solution conditions are such that its concentration exceeds its solubility. Substances with relatively low solubilities are said to be insoluble, and these are the substances that readily precipitate from solution. More information on these important concepts is provided in the text chapter on solutions. For purposes of predicting the identities of solids formed by precipitation reactions, one may simply refer to patterns of solubility that have been observed for many ionic compounds (Table \(\PageIndex{1}\)).

Table \(\PageIndex{1}\): Solubilities of Common Ionic Compounds in Water
Soluble compounds contain	Exceptions to these solubility rules include
group 1 metal cations (Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺) and ammonium ion \(\left(\ce{NH4+}\right)\) the halide ions (Cl⁻, Br⁻, and I⁻) the acetate \(\ce{(C2H3O2- )}\), bicarbonate \(\ce{(HCO3- )}\), nitrate \(\ce{(NO3- )}\), and chlorate \(\ce{(ClO3- )}\) ions the sulfate \(\ce{(SO4- )}\) ion	halides of Ag⁺, \(\ce{Hg2^2+}\), and Pb²⁺ sulfates of Ag⁺, Ba²⁺, Ca²⁺, \(\ce{Hg2^2+}\), Pb²⁺, and Sr²⁺
Insoluble compounds contain	Exceptions to these insolubility rules include
carbonate \(\ce{(CO3^2- )}\), chromate \(\ce{(CrO4^2- )}\), phosphate \(\ce{(PO4^3- )}\), and sulfide (S²⁻) ions hydroxide ion (OH⁻)	compounds of these anions with group 1 metal cations and ammonium ion hydroxides of group 1 metal cations and Ba²⁺

Example \(\PageIndex{1}\): Predicting Precipitation Reactions

Use table \(\PageIndex{1}\) to predict the solubility of each of the following ionic compounds.

barium sulfate
silver chloride
lead(II) nitrate

Solution

(a) According to the solubility table the presence of SO₄²^- renders the compound soluble except when SO₄²^- is combined with Ag⁺, Ba²⁺, Ca²⁺, \(\ce{Hg2^2+}\), Pb²⁺, and Sr²⁺. Since BaSO₄ has Ba²⁺, it is insoluble.

(b) According to the solubility table the presence of Cl^- renders the compound soluble except when Cl^- is combined Ag⁺, \(\ce{Hg2^2+}\), and Pb²⁺. Since AgCl has Ag⁺, it is insoluble.

(c) According to the solubility table the presence of NO₃^- renders the compund soluble with no exception. Therefore, Pb(NO₃)₂ is soluble.

Exercise \(\PageIndex{1}\)

Use table \(\PageIndex{1}\) to predict the solubility of each of the following ionic compounds.

calcium hydroxide
ammonium carbonate
mercury(I) bromide

Answer: (a) insoluble; (b) soluble; (c) insoluble

Predicting the Solubility of Ionic Compounds: https://youtu.be/U3QNwnfmvGU

Summary

Solubility is the maximum amount of solute that can dissolve in specific amount of solvent. Not all ionic compounds are able soluble in water. We use the solubility rules to predict whether an ionic compound dissolves in water or not.

Glossary

Solubility: Solubility is the maximum amount of solute that can dissolve in specific amount of solvent