Why common salt `(NaCl)` sometimes appear yellow?

Explanation

### Step-by-Step Solution:

1. **Understanding the Composition of NaCl**:
- Sodium chloride (NaCl) is composed of sodium ions (Na⁺) and chloride ions (Cl⁻). In its pure form, NaCl appears white due to the regular arrangement of these ions in a crystal lattice.

2. **Heating NaCl in the Presence of Sodium Vapor**:
- When NaCl crystals are heated in the presence of sodium vapor, a chemical reaction occurs. The sodium vapor reacts with the chloride ions (Cl⁻) in the crystal lattice.

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