In the Clark's method for softening compound _________ is used.

Answer

To answer the question regarding Clark's method for softening compounds, we can break down the solution into clear steps:

### Step-by-Step Solution:

1. **Understanding Clark's Method**:
- Clark's method is a technique used to soften hard water. Hard water contains high levels of calcium and magnesium ions, which can cause scaling and reduce the effectiveness of soaps and detergents.

2. **Identifying the Chemical Used**:
- In Clark's method, the compound used for softening hard water is sodium carbonate, commonly known as washing soda (Na2CO3).

3. **Mechanism of Action**:
- When sodium carbonate is added to hard water, it reacts with the calcium (Ca²⁺) and magnesium (Mg²⁺) ions present in the water.
- The sodium carbonate precipitates these ions out of the solution, forming insoluble compounds (calcium carbonate and magnesium carbonate), which can then be removed from the water.

4. **Resulting Effect**:
- The addition of sodium carbonate effectively reduces the concentration of calcium and magnesium ions in the water, thus softening it. This process makes the water more suitable for washing and other domestic uses.

5. **Alternative Methods**:
- Besides Clark's method, water can also be softened using ion exchange methods, where hard ions are exchanged for sodium ions, further enhancing the softening process.

### Final Answer:
In Clark's method for softening compounds, **sodium carbonate (washing soda)** is used.

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