What type of water forms scum with soap?

Explanation

### Step-by-Step Solution:

1. **Understanding Water Types**:
- Water can be classified into two main types: soft water and hard water.
- Soft water is free from dissolved minerals, while hard water contains significant amounts of dissolved minerals, particularly calcium and magnesium.

2. **Identifying Hard Water**:
- Hard water contains impurities such as calcium (Ca²⁺) and magnesium (Mg²⁺) ions.
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