Question

Which of the following will be more soluble in acid solution than in pure water: (a) CuI, (b) \(\mathrm{Ag}_{2} \mathrm{SO}_{4}\), (c) \(\mathrm{Zn}(\mathrm{OH})_{2}\) (d) \(\mathrm{BaC}_{2} \mathrm{O}_{4}\) (e) \(\mathrm{Ca}_{3}\left(\mathrm{PO}_{4}\right)_{2} ?\)

Short answer

Zn(OH)2, BaC2O4, and Ca3(PO4)2 are more soluble in acid than in pure water.

Step-by-step solution

Understand the Concept

To determine which salts are more soluble in acid than in water, identify which can react with the acid. Salts of weak acids, for instance, dissolve better in acid due to the common ion effect.

Examine CuI

CuI does not dissolve in acid more than in water because iodine does not form a weak acid in solution. CuI's solubility is not significantly affected by the presence of acid.

Examine Ag2SO4

Ag2SO4 is less likely to increase its solubility in acid as sulfate (\( ext{SO}_4^{2-}\)) is not a base of a weak acid that significantly reacts with additional protons.

Examine Zn(OH)2

Zn(OH)2 dissolves more in acid because it reacts with \( ext{H}^+\) forming Zn2+ ions and water, thus increasing solubility: \(\mathrm{Zn(OH)}_2 + 2\mathrm{H}^+ \rightarrow \mathrm{Zn}^{2+} + 2\mathrm{H}_2\mathrm{O}\).

Examine BaC2O4

BaC2O4 is more soluble in acid as the oxalate anion (\( ext{C}_2 ext{O}_4^{2-}\)) forms oxalic acid, a weak acid, when reacting with \( ext{H}^+\), enhancing solubility.

Examine Ca3(PO4)2

Ca3(PO4)2 becomes more soluble in acid because phosphate (\( ext{PO}_4^{3-}\)) can combine with \( ext{H}^+\) to form phosphoric acid, hence increasing its solubility in acidic conditions.

Key concepts

Common Ion Effect

The "common ion effect" influences the solubility of salts through the introduction of an ion already present in the equilibrium. When a common ion is added, it shifts the equilibrium according to Le Châtelier's principle. This can often result in decreased solubility. For instance, when you add \( ext{H}^+ \) ions to a solution containing salts that liberate a weak acid, the increased concentration of the common ion (such as oxalic acid from the oxalate ion in \( ext{BaC}_2 ext{O}_4 \)) causes more dissociation of the salt into its ions.
In essence, the common ion effect impacts salts of weak acids, shifting the dissolution equilibrium towards the undissolved solid until the opposing ions in the acid consume or buffer these changes, thereby altering solubility dynamics.

Solubility of Salts

Solubility refers to the ability of a substance to dissolve in a solvent. Some salts are more soluble in acidic solutions compared to neutral water due to their chemical nature. For example, salts like \( ext{Zn(OH)}_2 \) and \( ext{Ca}_3( ext{PO}_4)_2 \) dissolve more readily in an acid.
This happens because the anions in these salts react with \( ext{H}^+ \) ions from the acid, increasing the overall dissolution of the compound. Such reactions often produce a weak acid or a completely different compound that's soluble in water, for example:

• \( ext{Zn(OH)}_2 \) reacts to form \( ext{Zn}^{2+} \) ions and water.
• \( ext{BaC}_2 ext{O}_4 \) reacts to form oxalic acid.

These processes highlight how specific conditions, like the presence of an acid, can enhance solubility through chemical reactions.

Weak Acids and Bases

Understanding weak acids and bases helps determine solubility changes. A weak acid does not completely dissociate in water, which means its conjugate base remains available to react with any added \( ext{H}^+ \) ions from acids.
For example, oxalates and phosphates are such weak bases that form by partial dissociation of conjugate bases in water. In acids, they react with and consume added \( ext{H}^+ \) ions. Their capacity to form additional bonds with protons makes them more soluble under acidic conditions.

• Oxalic acid, formed from the reaction of an oxalate ion in acid, is a weak acid.
• Phosphate turns into phosphoric acid under similar conditions.

In summary, the reaction of weak acid anions with hydrogen ions from an acid solution assists in solubilizing certain salts otherwise less soluble in water.

Acid-Base Reactions

Acid-base reactions are pivotal in increasing the solubility of some salts in acidic solutions. A typical acid-base reaction involves the donation of an \( ext{H}^+ \) ion (from an acid) to a base (often the anion of a salt). This mechanism is widespread in salts associated with weak acid anions.
Consider \( ext{Zn(OH)}_2 \). It reacts with \( ext{H}^+ \) ions from an acid to form \( ext{Zn}^{2+} \) ions and water, a process that effectively removes \( ext{OH}^- \) ions from the equilibrium, allowing more \( ext{Zn(OH)}_2 \) to dissolve. Similarly, salts like \( ext{Ca}_3( ext{PO}_4)_2 \) respond to acids by transforming the phosphate into phosphoric acid, heightening solubility.

• Acids provide an environment rich in \( ext{H}^+ \) ions for these transformations.
• This encourages the breakdown of insoluble salts, increasing their solubilization.

Ultimately, the principle behind these reactions is the neutralization of bases by \( ext{H}^+ \), facilitating a shift in solubility.