Understanding Acid Rain and Its Causes

Acid rain is a form of precipitation that is significantly more acidic than normal rain. This increased acidity is primarily caused by air pollutants dissolving in rainwater.

Normal rain is slightly acidic, with a pH of around 5.6, due to the presence of carbon dioxide (\(\text{CO}_2\)) in the atmosphere, which forms weak carbonic acid.

However, acid rain has a much lower pH, typically below 5.0. This higher acidity is caused by strong acids formed from pollutants like sulphur dioxide and nitrogen oxides.

Key Gases Contributing to Acid Rain

The two main atmospheric pollutants responsible for forming acid rain are:

• Sulphur Dioxide (\(\text{SO}_2\)): This gas is primarily released from the burning of fossil fuels, especially coal, in power plants and industrial processes.
• Nitrogen Oxides (\(\text{NO}_x\)): These gases, mainly nitrogen monoxide (\(\text{NO}\)) and nitrogen dioxide (\(\text{NO}_2\)), are largely produced from the combustion of fuels in vehicles, power plants, and industrial furnaces.

These gases react with water, oxygen, and other chemicals in the atmosphere to form sulfuric acid (\(\text{H}_2\text{SO}_4\)) and nitric acid (\(\text{HNO}_3\)).

For example, the reactions involving sulphur dioxide can be simplified as:

• \(\text{SO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_3\) (Sulfurous acid, a weak acid)
• \(\text{SO}_2 + \frac{1}{2}\text{O}_2 \xrightarrow{\text{catalysts}} \text{SO}_3\)
• \(\text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4\) (Sulfuric acid, a strong acid)

Similarly, nitrogen oxides undergo reactions like:

• \(\text{2NO} + \text{O}_2 \rightarrow \text{2NO}_2\)
• \(\text{2NO}_2 + \text{H}_2\text{O} \rightarrow \text{HNO}_2 + \text{HNO}_3\) (Nitrous acid and Nitric acid)
• \(\text{3NO}_2 + \text{H}_2\text{O} \rightarrow \text{2HNO}_3 + \text{NO}\)

These strong acids then fall to the Earth as acid rain, acid snow, acid fog, or even as dry particles (acid deposition).

Analyzing the Options

Let's examine the given options in the context of acid rain:

• Hydrogen: Hydrogen gas (\(\text{H}_2\)) is highly flammable but does not contribute to the acidity of rain. It is not involved in the formation of sulfuric or nitric acid.
• Oxygen: Oxygen (\(\text{O}_2\)) is essential for the reactions that convert \(\text{SO}_2\) and \(\text{NO}_x\) into sulfuric and nitric acids, but it is not the primary gas pollutant causing the acid rain itself. It is a reactant, not the pollutant source of acidity.
• Nitrogen oxides: As discussed, nitrogen oxides (\(\text{NO}_x\)) are major precursors to nitric acid, one of the primary components of acid rain. They are directly involved in causing acid rain alongside sulphur dioxide.
• Carbon dioxide: Carbon dioxide (\(\text{CO}_2\)) dissolves in water to form carbonic acid, which makes normal rain slightly acidic (pH ~5.6). While it contributes to natural acidity, it is not the cause of the significantly lower pH values associated with severe acid rain, which are driven by the stronger acids from \(\text{SO}_2\) and \(\text{NO}_x\). The question mentions gases that cause acid rain *along with* sulphur dioxide and carbon monoxide, implying pollutants contributing to *acidic* rain beyond the natural state or other effects.

Considering the major pollutants responsible for acid rain beyond natural acidity, sulphur dioxide and nitrogen oxides are the key culprits. The question specifically asks for a gas that causes acid rain alongside sulphur dioxide and carbon monoxide. While carbon monoxide is an air pollutant, it does not directly form acid rain in the same way as \(\text{SO}_2\) and \(\text{NO}_x\). The option that fits as a primary acid rain precursor along with sulphur dioxide is nitrogen oxides.

Conclusion

Among the given options, nitrogen oxides are the gases that significantly contribute to acid rain formation alongside sulphur dioxide.

Revision Table: Acid Rain Components

Additional Information: Impact of Acid Rain

Acid rain has numerous harmful effects on the environment and human-made structures:

• Aquatic Ecosystems: Acidification of lakes and streams can kill fish and other aquatic organisms.
• Forests: Acid rain can damage trees and soils, making trees more susceptible to disease and insects.
• Buildings and Materials: Acid rain can corrode metals, erode stone buildings and statues, and damage paint.
• Human Health: While not directly harmful to walk in, the pollutants that cause acid rain (\(\text{SO}_2\) and \(\text{NO}_x\)) can cause respiratory problems.

Efforts to reduce acid rain focus on reducing emissions of sulphur dioxide and nitrogen oxides from power plants, industries, and vehicles.