The question asks us to identify which of the given gases, when dissolved in water, produces an acidic solution. This involves understanding how different gases interact chemically with water.

When a gas dissolves in water, it can either simply mix with the water molecules (physical dissolution) or it can react chemically with the water molecules to form new substances. If the reaction produces an acid, the resulting solution will be acidic. If it produces a base, it will be basic. If there is no significant chemical reaction producing acids or bases, the solution will remain neutral (assuming the water was initially neutral).

Examining Gases Dissolving in Water

Let's consider each option:

• Carbon dioxide ($\text{CO}_2$): When carbon dioxide dissolves in water, a small portion of it reacts with water to form carbonic acid ($\text{H}_2\text{CO}_3$). The chemical equation for this reaction is:

  $\text{CO}_2\text{(g)} + \text{H}_2\text{O(l)} \rightleftharpoons \text{H}_2\text{CO}_3\text{(aq)}$

  Carbonic acid ($\text{H}_2\text{CO}_3$) is a weak acid. It can dissociate in water to release hydrogen ions ($\text{H}^+$), which makes the solution acidic. This is why carbonated drinks, which contain dissolved carbon dioxide, are slightly acidic.

• Oxygen ($\text{O}_2$): Oxygen gas dissolves in water (which is essential for aquatic life), but it does not react chemically with water to form an acid or a base under normal conditions. Dissolved oxygen remains as $\text{O}_2$ molecules in the water. Therefore, dissolving oxygen in neutral water does not significantly change its pH, and the solution remains neutral.
• Nitrogen ($\text{N}_2$): Nitrogen gas is the main component of air. It dissolves in water to a limited extent. Like oxygen, nitrogen is relatively unreactive and does not react chemically with water to form an acid or a base under normal conditions. Dissolved nitrogen remains as $\text{N}_2$ molecules. Dissolving nitrogen in neutral water results in a neutral solution.
• Hydrogen ($\text{H}_2$): Hydrogen gas is also relatively unreactive with water under normal conditions. It dissolves slightly in water but does not react to form an acid or a base. Dissolving hydrogen in neutral water yields a neutral solution.

Comparison of Gas Solubility and Acidity

Here's a quick comparison:

Based on the chemical interactions, only carbon dioxide reacts with water to produce an acidic compound (carbonic acid).

Conclusion on Acidic Solutions from Gases

The analysis shows that among the given options, carbon dioxide is the gas that reacts chemically with water to form carbonic acid, which then dissociates to produce hydrogen ions, making the solution acidic. The other gases (Oxygen, Nitrogen, and Hydrogen) do not undergo such a reaction with water under typical conditions and thus yield neutral solutions.

Revision Table: Gases and Water Solubility

Additional Information: Gases and Acid Rain

While carbon dioxide forms a weak acid in water, other gases, particularly oxides of sulfur (like $\text{SO}_2$, $\text{SO}_3$) and oxides of nitrogen (like $\text{NO}$, $\text{NO}_2$), can react with water in the atmosphere to form stronger acids (sulfuric acid and nitric acid). These acids contribute significantly to the phenomenon known as acid rain, which can have harmful effects on the environment.

Understanding which gases dissolve in water and how they affect pH is important in various fields, including environmental science, chemistry, and biology.