Some pollutant gases in the atmosphere act as contributors to the formation of acid rain, a serious — Acids and Bases Chemistry Question

STEPS:

1. Understand the chemistry of acid rain: Acid rain refers to precipitation with a pH lower than that of natural rainwater (which is slightly acidic, around 5.6, due to dissolved carbon dioxide). It is formed when primary gaseous pollutants react with water, oxygen, and other chemicals in the atmosphere to form strong acids, such as nitric acid ($\text{HNO}_3$) and sulfuric acid ($\text{H}_2\text{SO}_4$).
2. Identify the primary atmospheric precursors to acid rain: The major contributors to acid rain are nitrogen oxides ($\text{NO}_x$, particularly nitrogen dioxide, $\text{NO}_2$) and sulfur oxides ($\text{SO}_x$, particularly sulfur dioxide, $\text{SO}_2$).
3. Examine the atmospheric reaction of nitrogen dioxide ($\text{NO}_2$): When $\text{NO}_2$ (produced primarily as a byproduct of high-temperature combustion in vehicles and power plants) is released into the atmosphere, it reacts with liquid water droplets in clouds to form nitric acid:
$$3\text{ NO}_2(g) + \text{H}_2\text{O}(l) \rightarrow 2\text{ HNO}_3(aq) + \text{NO}(g)$$
The highly soluble $\text{HNO}_3$ dissociates completely in water, yielding hydronium ions ($\text{H}_3\text{O}^+$) that drastically lower the pH of the precipitation.
4. Match with the choices: Among the choices provided, $\text{NO}_2$ (Option D) is a well-established pollutant gas that undergoes this reaction to produce acid rain.

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WHY_OTHERS_WRONG:

• A is incorrect: Nitrogen gas ($\text{N}_2$) is the most abundant natural component of Earth's atmosphere (approx. 78%). It is extremely stable and unreactive at normal atmospheric temperatures due to its strong nitrogen-nitrogen triple bond, meaning it does not react with water to form acid.
• B is incorrect: Oxygen gas ($\text{O}_2$) is a natural, non-polluting component of the atmosphere (approx. 21%) necessary for aerobic life and does not chemically react with water to lower its pH.
• C is incorrect: Water vapor ($\text{H}_2\text{O}$) is a natural part of the water cycle. While it acts as the solvent and reactant that *receives* the pollutants, pure water itself is neutral and does not cause acidification on its own.
• E is incorrect: Methane ($\text{CH}_4$) is a hydrocarbon that acts as a potent greenhouse gas contributing to climate change, but it is nonpolar, has negligible solubility in water, and does not undergo reactions that form acidic rain.