When lime water is exposed to air for a long time, very thin white coating is found floating on the surface. Give reasons.

Explanation

**Step-by-Step Solution:**

1. **Understanding Lime Water**: Lime water is a dilute solution of calcium hydroxide (Ca(OH)₂) in water. It is commonly used to test for the presence of carbon dioxide (CO₂).

2. **Exposure to Air**: When lime water is exposed to air, it comes into contact with the atmospheric gases, including carbon dioxide, which is present in very low concentrations (approximately 0.04%).

3. **Reaction with Carbon Dioxide**: The carbon dioxide in the air reacts with the calcium hydroxide in the lime water. The chemical reaction can be represented as:
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