Commercial common salt becomes slightly damp on keeping.

Explanation

**Step-by-Step Solution:**

1. **Understanding Common Salt**: Common salt, chemically known as sodium chloride (NaCl), is widely used in cooking and food preservation.

2. **Observation**: When common salt is kept for some time, it becomes slightly damp. This observation is crucial to understanding the underlying reason.

3. **Identifying Impurities**: Commercial common salt is not pure sodium chloride. It often contains impurities, such as magnesium chloride (MgCl2) and calcium chloride (CaCl2).

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