Which one of the following gases gives acidic solution on dissolving in water?
Carbon dioxide
Understanding Gases and Acidic Solutions in Water
When different gases dissolve in water, they can behave in various ways. Some gases simply dissolve without reacting, while others may react with water to form acidic or basic solutions. This question asks us to identify which of the given gases forms an acidic solution when dissolved in water.
Analyzing the Options
Let's examine each gas listed in the options:
- Hydrogen (\(\text{H}_2\)): Hydrogen gas is non-polar and does not react significantly with water to form an acid. It is only slightly soluble in water.
- Carbon dioxide (\(\text{CO}_2\)): Carbon dioxide is a non-metal oxide. Non-metal oxides often react with water to form acidic solutions. Carbon dioxide reacts reversibly with water to form carbonic acid (\(\text{H}_2\text{CO}_3\)).
- Nitrogen (\(\text{N}_2\)): Nitrogen gas is very stable due to the strong triple bond between the nitrogen atoms. It is largely unreactive under normal conditions and does not react with water to form an acid or a base. It has low solubility in water.
- Oxygen (\(\text{O}_2\)): Oxygen gas is essential for aquatic life because it dissolves in water, but it does not react with water to form an acid. It is only moderately soluble in water.
The Chemistry of Carbon Dioxide and Water
The reaction between carbon dioxide and water is the key to understanding which gas forms an acidic solution. When carbon dioxide dissolves in water, it reacts to form carbonic acid according to the following equilibrium reaction:
\[\text{CO}_2\text{(g)} + \text{H}_2\text{O(l)} \rightleftharpoons \text{H}_2\text{CO}_3\text{(aq)}\]
Carbonic acid (\(\text{H}_2\text{CO}_3\)) is a weak acid. Weak acids partially dissociate in water, releasing hydrogen ions (\(\text{H}^+\)) and thus lowering the pH of the solution, making it acidic.
\[\text{H}_2\text{CO}_3\text{(aq)} \rightleftharpoons \text{H}^+\text{(aq)} + \text{HCO}_3^-\text{(aq)}\]
This reaction is responsible for the slightly acidic nature of natural rainwater, as carbon dioxide from the atmosphere dissolves in water droplets.
Comparing the Gases
Based on their reactions with water:
| Gas | Reaction with Water | Solution Nature |
|---|---|---|
| Hydrogen (\(\text{H}_2\)) | Does not react significantly | Neutral (if pure water) |
| Carbon dioxide (\(\text{CO}_2\)) | Reacts to form \(\text{H}_2\text{CO}_3\) | Acidic |
| Nitrogen (\(\text{N}_2\)) | Does not react | Neutral (if pure water) |
| Oxygen (\(\text{O}_2\)) | Does not react | Neutral (if pure water) |
Therefore, carbon dioxide is the gas among the options that reacts with water to produce an acidic solution.
Conclusion on Acidic Gas Solution
Among the given options, carbon dioxide dissolves in water and reacts to form carbonic acid, which makes the solution acidic. The other gases listed do not react with water in a way that produces an acidic solution.
Revision Table: Gases and Water Solubility
| Gas | Solubility in Water | Chemical Reaction with Water | Nature of Solution |
|---|---|---|---|
| Hydrogen | Low | No significant reaction | Neutral |
| Carbon Dioxide | Moderate (reacts) | Forms Carbonic Acid (\(\text{H}_2\text{CO}_3\)) | Acidic |
| Nitrogen | Very Low | No reaction | Neutral |
| Oxygen | Moderate | No reaction | Neutral |
Additional Information: Acidic and Basic Oxides
The behavior of oxides when dissolved in water is a general concept related to this question. Oxides can be broadly classified as acidic, basic, or amphoteric.
- Acidic Oxides: These are typically oxides of non-metals. They react with water to form acids. Examples include \(\text{CO}_2\) (forms carbonic acid), \(\text{SO}_2\) and \(\text{SO}_3\) (form sulfurous and sulfuric acid), and \(\text{NO}_2\) (forms nitric and nitrous acid).
- Basic Oxides: These are typically oxides of metals, especially alkali and alkaline earth metals. They react with water to form bases (hydroxides). Examples include \(\text{Na}_2\text{O}\) (forms \(\text{NaOH}\)) and \(\text{CaO}\) (forms \(\text{Ca(OH)}_2\)).
- Amphoteric Oxides: These oxides can react with both acids and bases. Examples include \(\text{Al}_2\text{O}_3\) and \(\text{ZnO}\).
- Neutral Oxides: Some oxides do not react with acids or bases, such as \(\text{CO}\), \(\text{NO}\), and \(\text{N}_2\text{O}\).
In this question, carbon dioxide is an example of an acidic oxide gas.
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