Soft water obtained on ion-exchange treatment

Options

A. contains free `H^(+)` ions
B. contains free `OH^(-)` ions
C. contains resin molecules
D. is perfectly deionised.

Answer

To solve the question regarding soft water obtained on ion-exchange treatment, we will analyze the options provided and understand the ion-exchange process.

### Step-by-Step Solution:

1. **Understanding Hard Water**: Hard water contains dissolved minerals, primarily calcium (Ca²⁺) and magnesium (Mg²⁺) ions, which cause hardness.

2. **Ion-Exchange Treatment**: The ion-exchange method is a process used to remove these hardness-causing ions from water. In this method, a resin (or zeolite) is used, which is typically sodium aluminum silicate (NaZ).

3. **Mechanism of Ion Exchange**:
- The resin (NaZ) exchanges sodium ions (Na⁺) for hardness ions (Ca²⁺ and Mg²⁺) present in the water.
- The reaction can be represented as:
\[ \text{NaZ} + \text{Ca}^{2+} \rightarrow \text{CaZ} + 2\text{Na}^{+} \]
- Here, the calcium ions replace sodium ions on the resin, forming insoluble calcium salts (CaZ) and releasing sodium ions into the water.

4. **Resulting Composition of Soft Water**: After the ion-exchange process, the water is free from hardness ions (Ca²⁺ and Mg²⁺). The water does not contain free H⁺ or OH⁻ ions, nor does it contain resin molecules.

5. **Conclusion**: The soft water obtained from ion-exchange treatment is perfectly deionized, meaning it does not contain any hardness ions or the resin itself. Therefore, the correct answer is that soft water obtained on ion-exchange treatment is "perfectly deionized".

### Final Answer:
Soft water obtained on ion-exchange treatment is perfectly deionized.