Which of the following produces relatively few ions in solution?

Analysis

The question asks us to identify which of the given substances is a weak electrolyte (produces relatively few ions in solution). To solve this, we must distinguish between strong bases (which dissociate completely) and weak bases (which dissociate only partially).

Concept Explanation

• Strong Bases: These are compounds that dissociate completely into their constituent ions when dissolved in water. Examples include Group 1 hydroxides (like NaOH and KOH) and some Group 2 hydroxides. Because they dissociate fully, they produce a high concentration of ions, making them strong electrolytes.
• Weak Bases: These are compounds that do not dissociate completely in water. They exist in an equilibrium where only a small fraction of the molecules break apart into ions. Consequently, they produce relatively few ions in solution. Aluminum hydroxide, $\text{Al(OH)}_3$, is a classic example of an insoluble or very weakly dissociating base.

Step-by-Step Solution

1. Evaluate NaOH (Option A): Sodium hydroxide is a Group 1 hydroxide. It is a strong base and dissociates completely: $\text{NaOH} \rightarrow \text{Na}^+ + \text{OH}^-$. It produces many ions.
2. Evaluate Ca(OH)$_2$ (Option B): Calcium hydroxide is a strong base (alkali). Although its solubility is limited, the portion that does dissolve dissociates completely into ions.
3. Evaluate KOH (Option C): Potassium hydroxide is also a Group 1 hydroxide. Like NaOH, it is a strong base that dissociates completely: $\text{KOH} \rightarrow \text{K}^+ + \text{OH}^-$.
4. Evaluate Al(OH)$_3$ (Option D): Aluminum hydroxide is an amphoteric hydroxide. It is practically insoluble in water and does not dissociate significantly into ions. Therefore, it produces very few ions in solution compared to the strong bases listed.

Conclusion

Since $\text{Al(OH)}_3$ is a weak, poorly soluble base, it produces the fewest ions among the options provided.

Exam Tip

Remember the "Group 1 Rule": Hydroxides of Group 1 elements (Li, Na, K, Rb, Cs) are generally strong bases. If you see them in an exam question asking for "strong electrolytes," they are usually your best bet. Conversely, transition metal hydroxides (like $\text{Al(OH)}_3$, $\text{Fe(OH)}_3$, or $\text{Cu(OH)}_2$) are typically weak or insoluble.