pH and Solubility
Introduced| English |
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| solubility/ˌsɒljuːˈbɪlɪti/ |
Acid that dissolves the insoluble
- Some "insoluble" salts vanish when you add acid.
- Chalk fizzes and disappears in vinegar.
- The acid quietly removes one of the salt's ions.
- That removal pulls more solid into solution.
Salts with basic anions
- Salts whose anion is a weak base dissolve more in acid.
- Carbonates, hydroxides, and fluorides are examples.
- Their anion reacts with the added $\text{H}^+$.
Which type of salt dissolves more as the pH is lowered?
A basic anion reacts with added acid, so more dissolves.
Salts of weak-acid anions, such as carbonates and ____, dissolve more in acid.
Hydroxide and carbonate anions are basic, so acid dissolves them.
Removing an ion shifts the balance
- Acid consumes the dissolved anion.
- That drops $Q$ below $K_{sp}$, so more solid dissolves.
- It is Le Chatelier applied to solubility 溶解度.
Adding acid removes the anion, which drops $Q$ below $K_{sp}$, so the solid...
With $Q < K_{sp}$, more solid dissolves to restore equilibrium.
The pH effect on solubility is an example of Le Chatelier's principle.
Removing an ion shifts the dissolving equilibrium forward.
Not every salt cares
- Salts of strong-acid anions (like $\text{Cl}^-$) are unaffected by pH.
- Their anion is not a base, so acid does nothing.
- Only basic anions respond to pH.
The solubility of $\text{AgCl}$ changes strongly with pH.
$\text{Cl}^-$ is not a base, so pH has little effect.
Does $\text{CaCO}_3$ dissolve more in acid or in neutral water?
- Acid reacts with the carbonate ion.
- So $\text{CaCO}_3$ dissolves more in acid.
Does pH change solubility?
Sort each salt by whether lowering the pH makes it more soluble.
$\text{CaCO}_3$ dissolves more in...
Acid reacts with the basic carbonate ion, dissolving more solid.
Only salts with a basic anion (from a weak acid, like carbonate or hydroxide) dissolve more in acid -- salts of strong-acid anions like chloride do not respond. Lowering the pH removes the anion, pulling more solid in. This is just Le Chatelier acting on the dissolving equilibrium.
Salts with a basic anion (carbonates, hydroxides) dissolve more in acid: the added $\text{H}^+$ removes the anion, dropping $Q$ below $K_{sp}$ so more solid dissolves. Salts of strong-acid anions like chloride are unaffected -- it is Le Chatelier acting on solubility.