Which of the following chemical substances can be used for removing permanent hardness of water by precipitation reaction?

Options

A. Baking soda
B. Washing soda
C. Alum
D. Quick lime

Explanation

To determine which chemical substance can be used for removing permanent hardness of water by precipitation reaction, we need to understand the nature of permanent hardness and the substances that can address it.

### Step-by-Step Solution:

1. **Understand Permanent Hardness**:
- Permanent hardness of water is primarily caused by the presence of soluble salts, specifically chlorides and sulfates of calcium (Ca²⁺) and magnesium (Mg²⁺).

2. **Identify the Chemicals Used for Softening**:
- To remove permanent hardness, we can use certain chemicals that will react with the calcium and magnesium ions to form insoluble precipitates.

3. **Chemical Substance for Precipitation**:
- One common chemical used to remove permanent hardness is sodium carbonate (Na2CO3).
- When sodium carbonate is added to hard water, it reacts with calcium and magnesium ions to form insoluble carbonates (CaCO3 and MgCO3), which precipitate out of the solution.

4. **Reaction Example**:
- For example, the reaction can be represented as:
- Ca²⁺(aq) + Na2CO3(aq) → CaCO3(s) + 2Na⁺(aq)
- Here, calcium carbonate (CaCO3) precipitates out of the solution.

5. **Conclusion**:
- Therefore, sodium carbonate (Na2CO3) is the chemical substance that can be used for removing permanent hardness of water by precipitation reaction.