Exceptions to Solubility Rules
While general solubility rules provide a helpful guideline for predicting whether an ionic compound will dissolve in water, several important exceptions exist. These exceptions can influence the formation of precipitates in chemical reactions and are vital in analytical and inorganic chemistry.
Key Solubility Rules and Their Exceptions
1. Nitrates (NO₃⁻):
Rule: All nitrates are soluble.
Exception: No common exceptions.
Rule: All nitrates are soluble.
Exception: No common exceptions.
2. Acetates (CH₃COO⁻):
Rule: Most acetates are soluble.
Exception: Silver acetate (AgC₂H₃O₂) is only slightly soluble.
Rule: Most acetates are soluble.
Exception: Silver acetate (AgC₂H₃O₂) is only slightly soluble.
3. Halides (Cl⁻, Br⁻, I⁻):
Rule: Most halide salts are soluble.
Exceptions: Halides of Ag⁺, Pb²⁺, and Hg₂²⁺ are insoluble.
Rule: Most halide salts are soluble.
Exceptions: Halides of Ag⁺, Pb²⁺, and Hg₂²⁺ are insoluble.
4. Sulfates (SO₄²⁻):
Rule: Most sulfates are soluble.
Exceptions: Sulfates of Ba²⁺, Pb²⁺, Hg₂²⁺, Sr²⁺, and Ca²⁺ are insoluble or sparingly soluble.
Rule: Most sulfates are soluble.
Exceptions: Sulfates of Ba²⁺, Pb²⁺, Hg₂²⁺, Sr²⁺, and Ca²⁺ are insoluble or sparingly soluble.
5. Carbonates (CO₃²⁻), Phosphates (PO₄³⁻), Sulfides (S²⁻), and Hydroxides (OH⁻):
Rule: These are generally insoluble.
Exceptions: Salts of Group 1 elements (Li⁺, Na⁺, K⁺, etc.) and ammonium (NH₄⁺) are soluble.
Hydroxides of Ba²⁺, Sr²⁺, and Ca²⁺ are slightly soluble.
Rule: These are generally insoluble.
Exceptions: Salts of Group 1 elements (Li⁺, Na⁺, K⁺, etc.) and ammonium (NH₄⁺) are soluble.
Hydroxides of Ba²⁺, Sr²⁺, and Ca²⁺ are slightly soluble.
6. Chromates (CrO₄²⁻) and Dichromates (Cr₂O₇²⁻):
Rule: Typically insoluble.
Exceptions: Soluble with Group 1 elements and NH₄⁺.
Rule: Typically insoluble.
Exceptions: Soluble with Group 1 elements and NH₄⁺.
Why Exceptions Matter
- They determine whether a reaction forms a precipitate.
- They are crucial in qualitative analysis for identifying ions.
- They affect the solubility equilibria used in titrations and separation techniques.
Understanding these exceptions allows chemists to better predict chemical behavior in aqueous solutions and design experiments accordingly.