Which of the following is not a water softener?

Options

A. calgon
B. permutit
C. `Na_(2)SO_(4)`
D. `Na_(2)CO_(3)`

Answer

To determine which of the following is not a water softener, we need to understand what water softeners do. Water softeners are substances that help remove calcium, magnesium, and certain other metal cations from hard water. Let's analyze the options provided:

1. **Calgon**: This is a well-known water softener. It works by binding calcium and magnesium ions, preventing them from causing hardness in water.

2. **Permutate**: This is also a water softener. It functions similarly to Calgon by removing hardness-causing ions from water.

3. **Na2SO4 (Sodium sulfate)**: This compound does not have any effect on the hardness of water. It does not remove calcium or magnesium ions, which are responsible for water hardness.

4. **Na2CO3 (Sodium carbonate)**: This is another water softener. It can precipitate calcium and magnesium ions from water, thus softening it.

Now, since we are looking for the substance that is not a water softener, we can conclude that:

**Na2SO4 (Sodium sulfate)** is not a water softener.

Therefore, the correct answer is **Na2SO4**.

### Summary of Steps:
1. Understand the function of water softeners.
2. Analyze each option to see if it removes hardness-causing ions.
3. Identify that Na2SO4 does not affect water hardness.
4. Conclude that Na2SO4 is not a water softener.