Which one of the following gases dissolves in water to form an acidic solution due to the formation of an acid in aqueous medium?

Introduction / Context:
This question belongs to the topic of acid base chemistry and environmental science. It asks which gas, when dissolved in water, produces an acidic solution. Gases can form acids or bases in water depending on their chemical behaviour. Understanding which gases cause acidification of water bodies is important for interpreting phenomena such as acid rain and ocean acidification, as well as for general knowledge about aqueous solutions.

Given Data / Assumptions:
Carbon dioxide can react with water to form carbonic acid, H2CO3, which is a weak acid.
Oxygen, nitrogen and hydrogen are relatively inert diatomic gases under normal conditions in water and do not form strong acids or bases simply by dissolving.
The question focuses on natural dissolution in water rather than special high energy or catalytic conditions.

Concept / Approach:
When carbon dioxide dissolves in water, it reacts to form carbonic acid according to the equilibrium CO2 + H2O reversible arrow H2CO3. Although carbonic acid is weak and only partially dissociates, its presence increases the hydrogen ion concentration and makes the solution slightly acidic. Oxygen dissolves in water mainly to support aquatic life but does not form an acid. Nitrogen and hydrogen also dissolve to a small extent but do not form acids or bases under normal environmental conditions. Therefore, the gas that clearly leads to an acidic solution is carbon dioxide.

Step-by-Step Solution:
Step 1: Consider carbon dioxide. When CO2 dissolves in water, it forms carbonic acid, which partially dissociates into hydrogen ions and bicarbonate ions, making the solution acidic. Step 2: Consider oxygen. O2 dissolves in water but remains as molecular oxygen and does not react significantly to form an acid or base in ordinary conditions, so the solution remains essentially neutral. Step 3: Consider nitrogen. N2 is even less reactive than oxygen under normal conditions and dissolves only slightly in water without forming an acid. Step 4: Consider hydrogen. H2 can dissolve in water but does not form an acid merely by dissolving; it requires specific conditions to react, such as in catalytic hydrogenation processes. Step 5: Therefore, among the options, only carbon dioxide clearly produces an acidic solution when dissolved in water due to carbonic acid formation.

Verification / Alternative check:
Environmental chemistry explains that ocean acidification and the slight acidity of natural rainwater are largely due to the dissolution of atmospheric carbon dioxide forming carbonic acid. Measured pH values of pure water in equilibrium with CO2 are slightly below 7, reflecting this weak acidity. In contrast, oxygen and nitrogen do not significantly affect the pH of water when they dissolve at normal atmospheric partial pressures. These observations confirm that carbon dioxide is the gas that creates an acidic solution.

Why Other Options Are Wrong:
Oxygen is wrong because it does not react substantially with water to form an acid under normal conditions and mainly serves as a dissolved gas for aquatic respiration.

Nitrogen is wrong because it is chemically inert in this context and does not form acids or bases simply by dissolving in water.

Hydrogen is wrong because, although reactive under certain controlled conditions, it does not form an acid with water under ordinary dissolution conditions and does not significantly change the pH.

Common Pitfalls:
Students may think that any gas that dissolves in water must change the pH, which is not true. Another pitfall is confusing the role of oxygen in oxidation reactions with acid formation. It is also possible to forget that carbon dioxide is acidic in water because it is often thought of as neutral in air. Remembering that CO2 forms carbonic acid in water and contributes to phenomena like ocean acidification helps maintain the correct association.

Final Answer:
The gas that dissolves in water to give an acidic solution is Carbon dioxide.