When does a rain is considered to be acid rain ?

Explanation

**Step-by-Step Solution:**

1. **Definition of Acid Rain**: Acid rain is defined as rain that has a lower pH than normal rainwater. Normal rainwater typically has a pH of around 5.6 due to the presence of dissolved carbon dioxide, which forms carbonic acid.

2. **Chemical Composition**: Acid rain is primarily caused by the presence of sulfur dioxide (SO2) and nitrogen oxides (NOx) in the atmosphere. These compounds are released into the air from various sources, including industrial emissions, vehicle exhaust, and burning fossil fuels.

3. **Chemical Reactions**: When sulfur dioxide and nitrogen oxides are released into the atmosphere, they can react with water (H2O) and oxygen (O2) to form sulfuric acid (H2SO4) and nitric acid (HNO3). The chemical reactions can be summarized as follows:
- SO2 + H2O + O2 → H2SO4
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