Which is not a cause of acid rain?

Options

A. `NO_(2)`
B. `CH_(4)`
C. `SO_(2)`
D. `CO_(2)`

Answer

To determine which substance is not a cause of acid rain, we need to analyze the components that contribute to acid rain formation.

### Step-by-Step Solution:

1. **Understanding Acid Rain:**
Acid rain is a type of precipitation that contains acidic components, primarily sulfuric acid (H2SO4) and nitric acid (HNO3). These acids form when sulfur dioxide (SO2) and nitrogen oxides (NOx) are released into the atmosphere.

2. **Identifying Common Causes:**
The main contributors to acid rain include:
- **Sulfur Dioxide (SO2):** Released from industrial processes and burning fossil fuels.
- **Nitrogen Oxides (NOx):** Emitted from vehicle exhaust and industrial activities.
- **Volatile Organic Compounds (VOCs):** These can also contribute to the formation of ozone and other secondary pollutants.

3. **Chemical Reactions:**
- Sulfur dioxide reacts with oxygen and water vapor to form sulfuric acid:
\[ SO2 + O2 + H2O \rightarrow H2SO4 \]
- Nitrogen oxides react with oxygen and water vapor to form nitric acid:
\[ NO2 + O2 + H2O \rightarrow HNO3 \]
- Carbon dioxide (CO2) can react with water to form carbonic acid (H2CO3), but it is not a major contributor to acid rain compared to sulfuric and nitric acids.

4. **Identifying the Incorrect Option:**
Among the substances mentioned, we need to identify which one does not contribute to acid rain. The options provided typically include:
- Sulfur oxides (SOx)
- Nitrogen oxides (NOx)
- Carbon dioxide (CO2)
- Methane (CH4)

Methane (CH4) is not involved in the formation of acid rain. It is a greenhouse gas but does not contribute to the acidity of rain.

5. **Conclusion:**
Therefore, the substance that is not a cause of acid rain is **methane (CH4)**.

### Final Answer:
The answer to the question "Which is not a cause of acid rain?" is **methane (CH4)**.

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