Solubility Rules and Classification of Ionic Compounds in Water

Solubility and Solubility Rules

Introduction to Solubility

Solubility is a chemical property that describes the ability of a solute to dissolve in a solvent, forming a homogeneous solution. In aqueous solutions, water acts as the solvent.

• Soluble: A substance that can dissolve in water to form an aqueous solution.

• Insoluble: A substance that does not dissolve appreciably in water.

Example: When aluminum bromide (AlBr3) is placed in water, it dissolves:

$\text{AlBr}_3 (s) \xrightarrow{\text{H}_2\text{O}} \text{Al}^{3+} (aq) + 3\text{Br}^- (aq)$

Silver bromide (AgBr), however, does not dissolve:

$\text{AgBr} (s) \xrightarrow{\text{H}_2\text{O}} \text{No reaction}$

Example Calculation: For sodium sulfate (Na2SO4):

$\text{Na}_2\text{SO}_4 \rightarrow 2\text{Na}^+ + \text{SO}_4^{2-}$ (3 ions)

Solubility Rules

Solubility rules are guidelines that help predict whether an ionic compound will dissolve in water. These rules are essential for understanding precipitation reactions and solution chemistry.

• Mnemonic Devices: GANA CASH is used for soluble ionic solutes, with exceptions that form insoluble compounds (precipitates). COPS is used for insoluble ionic solutes, with exceptions that make them soluble.

Classification of Ionic Compounds by Solubility

Soluble Compounds

The following groups of ionic compounds are generally soluble in water, with noted exceptions:

Group	Structures	Exceptions	Explanation
Group 1A	Li+, Na+, K+, etc.	None	Always soluble
Acetate	CH3COO-	None	Always soluble
Nitrate	NO3-	None	Always soluble
Ammonium	NH4+	None	Always soluble
Halides	Cl-, Br-, I-	Ag+, Pb2+, Hg22+	Insoluble with these cations
Sulfate	SO42-	Ca2+, Ba2+, Sr2+, Pb2+	Insoluble with these cations

Mnemonic: CBS HAPpy (Ca2+, Ba2+, Sr2+, Hg22+, Ag+, Pb2+) are exceptions for halides and sulfates.

Insoluble Compounds

The following groups of ionic compounds are generally insoluble in water, with noted exceptions:

Group	Structures	Exceptions	Explanation
Carbonate/Chromate	CO32-, CrO42-	Group 1A, NH4+	Soluble with these cations
Oxides/Hydroxides	O2-, OH-	Ca2+, Ba2+, Sr2+ (CBS)	Soluble with these cations
Phosphate	PO43-	Group 1A, NH4+	Soluble with these cations
Sulfide	S2-	Group 1A, NH4+, Ca2+, Ba2+, Sr2+	Soluble with these cations

Mnemonic: COPS (Carbonate, Oxide/hydroxide, Phosphate, Sulfide) are generally insoluble except with Group 1A, NH4+, and CBS for hydroxides and sulfides.

Examples and Applications

• Example 1: According to the solubility rules, BaSO4 is insoluble because sulfate is generally soluble, but Ba2+ is an exception.

• Example 2: AgCl is insoluble because halides are generally soluble, but Ag+ is an exception.

• Practice: Na2S is soluble (contains Group 1A cation), while CuS is insoluble (no exception applies).

Summary Table: Solubility Rules (Condensed)

Compound Type	Solubility	Exceptions
Group 1A, NH4+, Nitrate, Acetate	Soluble	None
Halides (Cl-, Br-, I-)	Soluble	Ag+, Pb2+, Hg22+
Sulfate (SO42-)	Soluble	Ca2+, Ba2+, Sr2+, Pb2+
Carbonate, Phosphate, Sulfide, Hydroxide	Insoluble	Group 1A, NH4+, (CBS for OH- and S2-)

Key Terms

• Solute: The substance being dissolved.

• Solvent: The substance doing the dissolving (water in aqueous solutions).

• Precipitate: An insoluble solid formed from a reaction in solution.

• Aqueous (aq): Dissolved in water.

Additional info:

• Solubility rules are empirical and may have rare exceptions not listed here.

• These rules are foundational for predicting the outcomes of double displacement (precipitation) reactions in aqueous solutions.