Mistake Master
pH and solubility
A salt that barely dissolves in water can dissolve readily in acid — if its anion is basic. Lowering the pH quietly consumes that anion, and the dissolution equilibrium follows, dumping more solid into solution.
§1
pH can drive solubility.
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The pH of a solution can change a salt's solubility. It matters when the salt's anion is basic — that is, it reacts with H⁺.
Lowering the pH (adding acid) consumes that basic anion. By Le Chatelier, removing a product of the dissolution equilibrium shifts it forward, so more solid dissolves.
If the salt's anion is not basic (it does not react with H⁺, like the anion of a strong acid), the solubility is largely unaffected by pH.
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Predicting the pH effect.
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Ask whether the anion reacts with H⁺.
- Identify the salt's anion. What ion is released when the salt dissolves?
- Ask if the anion is basic. Does it react with H⁺ (is it the conjugate base of a weak acid)?
- If basic, lower pH increases solubility. Acid consumes the anion, shifting dissolution forward.
- If not basic, pH has little effect. Anions of strong acids do not react with H⁺.
§3
The pieces you'll meet.
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It hinges on the anion.
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Worked example: a salt with a basic anion.
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Salt. A metal fluoride MF₂ dissolves to give the basic anion F⁻ (the conjugate base of the weak acid HF).
Add acid. Lowering the pH provides H⁺, which reacts with F⁻ (forming HF), removing it from solution.
Shift. Removing F⁻ (a product of dissolution) shifts the dissolution equilibrium forward, so more MF₂ dissolves — it is more soluble in acid.
Contrast. A salt like a metal chloride, whose anion Cl⁻ is not basic (Cl⁻ is the conjugate base of the strong acid HCl), shows little pH-driven change in solubility.
§5
Mistakes that cost real points.
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"Lowering the pH increases the solubility of any salt."
Only salts with a basic anion (one that reacts with H⁺) become more soluble in acid. Salts whose anions are not basic — like the anions of strong acids — are largely unaffected by pH. The effect depends on the anion.
Fix. Check whether the anion is basic; only then does lowering pH increase solubility.
"pH has no effect on solubility at all."
pH strongly affects the solubility of salts with basic anions: lowering the pH consumes the anion and shifts dissolution forward, dissolving more solid. Ignoring the pH effect misses an important control on solubility.
Fix. Recognize that pH can substantially change solubility for salts with basic anions.
"Adding acid works by dissolving the metal cation."
The acid works by reacting with the basic anion, not the cation. Consuming the anion removes a dissolution product and shifts the equilibrium forward. Attributing the effect to the cation misreads the mechanism.
Fix. Attribute pH-driven solubility to the acid consuming the basic anion, shifting the dissolution equilibrium.
§6
Skill Check.
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Ten scenarios. Pick the chips that match your answer, then check. A scenario marks complete the first time every part is right. Progress saves on this device.