Which of the following ion forms a hydroxide highly soluble in water ?

Options

A. `K^+ `
B. `Zn^(2+ )`
C. `Al^(3+ )`
D. `Ca^(2+ )`

Answer

To determine which ion forms a hydroxide that is highly soluble in water, we need to analyze the ions provided: potassium (K), zinc (Zn), aluminum (Al), and calcium (Ca).

### Step-by-Step Solution:

1. **Identify the Hydroxides Formed by Each Ion**:
- Potassium (K) forms potassium hydroxide (KOH).
- Zinc (Zn) forms zinc hydroxide (Zn(OH)₂).
- Aluminum (Al) forms aluminum hydroxide (Al(OH)₃).
- Calcium (Ca) forms calcium hydroxide (Ca(OH)₂).

2. **Understand the Concept of Hydration Energy**:
- Hydration energy is the energy released when an ion is surrounded by water molecules. It is a key factor that influences the solubility of ionic compounds in water.

3. **Evaluate the Solubility of Each Hydroxide**:
- KOH (potassium hydroxide) is highly soluble in water.
- Zn(OH)₂ (zinc hydroxide) is only slightly soluble in water.
- Al(OH)₃ (aluminum hydroxide) is also only slightly soluble in water.
- Ca(OH)₂ (calcium hydroxide) has moderate solubility in water.

4. **Determine Which Hydroxide Has the Highest Solubility**:
- Among the hydroxides formed, KOH has the highest solubility in water due to the high hydration energy associated with potassium ions. The hydration energy is significant because potassium is an alkali metal, which tends to form highly soluble hydroxides.

5. **Conclusion**:
- Therefore, the ion that forms a hydroxide highly soluble in water is potassium (K), as it forms potassium hydroxide (KOH).

### Final Answer:
Potassium ion (K) forms a hydroxide (KOH) that is highly soluble in water.