๐งช Topic 15: pH & Solubility – How to Always Choose the Right Effect
๐ What Is It About?
This topic explains how the pH of a solution affects the solubility of a salt.
- pH can make some salts more soluble or less soluble
- The key idea is how ions in the salt react with H₃O⁺ or OH⁻
๐ How It Works (Simple Rules)
- ✔️ If the salt has a basic ion, it will react with H₃O⁺ → more soluble in acid
- ✔️ If the salt has an acidic ion, it will react with OH⁻ → more soluble in base
- ๐ซ If the ion is negligible, pH has no effect
- ๐ก Use Le Chatelier’s Principle: if an ion reacts and is “removed”, equilibrium shifts → more dissolves!
๐งช Examples You MUST Know
Example 1: Fe(OH)₃Fe(OH)₃ (s) ⇌ Fe³⁺ + 3OH⁻
➤ Lower pH (add H₃O⁺): Removes OH⁻ → Shifts right → ⬆ Solubility
➤ Higher pH (add OH⁻): Adds OH⁻ → Common ion → Shifts left → ⬇ Solubility
Example 2: Al(NO₃)₃
Al(NO₃)₃ ⇌ Al³⁺ + 3NO₃⁻
➤ NO₃⁻ = Negligible
➤ Add OH⁻: Al³⁺ reacts → Al(OH)₃ → Shifts right → ⬆ Solubility
Example 3: BaF₂
BaF₂ (s) ⇌ Ba²⁺ + 2F⁻
➤ F⁻ is basic → reacts with H₃O⁺ → ⬆ Solubility in acid
➤ Ba²⁺ = negligible → no change in base
Example 4: PbC₂O₄
PbC₂O₄ (s) ⇌ Pb²⁺ + C₂O₄²⁻
➤ C₂O₄²⁻ reacts with H₃O⁺ in steps:
C₂O₄²⁻ + H₃O⁺ ⇌ HC₂O₄⁻ + H₂O
HC₂O₄⁻ + H₃O⁺ ⇌ H₂C₂O₄ + H₂O
➤ Result: Removes C₂O₄²⁻ → Shift right → More dissolves ✅
✅ How to Get Every Question Right
- ๐งช Ask: “Can the ion react with H₃O⁺ or OH⁻?”
- ✅ Yes → pH will affect solubility
- ๐ง No → pH does nothing
- ๐ Use Le Chatelier’s Rule to predict direction
- ๐ฏ Always write the equilibrium + the reaction with H₃O⁺ or OH⁻
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