Why a salt containing lead turns black in colour, when placed for a long time in laboratory?

Explanation

**Step-by-Step Solution:**

1. **Identify the Lead Salt**: The question refers to a salt containing lead. A common example of such a salt is lead(II) chloride (PbCl2).

2. **Understand the Environment**: In a laboratory setting, various gases can be present in the atmosphere. One such gas is hydrogen sulfide (H2S), which can be found in trace amounts in the air.

3. **Chemical Reaction**: When lead(II) chloride (PbCl2) is exposed to hydrogen sulfide (H2S) over time, a chemical reaction occurs. The reaction can be represented as follows:
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