At which of the following pH, rain is called as acid rain?

Options

A. 4
B. 6
C. 6.8
D. All of these

Answer

To determine at which pH rain is classified as acid rain, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Definition of Acid Rain**: Acid rain is defined as precipitation that has a pH lower than that of normal rainwater. Normal rainwater typically has a pH of around 5.5 to 6.5.

2. **Identify the Causes of Acid Rain**: Acid rain is primarily caused by the presence of sulfur oxides (SOx) and nitrogen oxides (NOx) in the atmosphere. These compounds can originate from automobile exhaust, industrial emissions, and other sources.

3. **Chemical Reactions Involved**:
- Sulfur dioxide (SO2) reacts with oxygen (O2) to form sulfur trioxide (SO3), which then reacts with water (H2O) to produce sulfuric acid (H2SO4).
- Nitrogen dioxide (NO2) reacts with oxygen and water to form nitric acid (HNO3).
- Carbon dioxide (CO2) can also react with water to form carbonic acid (H2CO3).

4. **Effect on pH**: The formation of these acids increases the concentration of hydrogen ions (H+) in the rainwater, which decreases the pH. The relationship between hydrogen ion concentration and pH is inverse; as H+ concentration increases, pH decreases.

5. **Determine the pH of Acid Rain**: Acid rain is typically identified as having a pH of less than 5.5. In many cases, the pH of acid rain can drop to around 4 or even lower, depending on the concentration of acids present.

6. **Conclusion**: Based on the information provided, rain is classified as acid rain when its pH is lower than 5.5. A common threshold used is a pH of 4.

### Final Answer:
Rain is called acid rain when its pH is less than 5.5, with a common threshold being around pH 4.

---