Which acidic gas from the air dissolves in rainwater to form c... | General Science MCQ | Examoogle

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Which acidic gas from the air dissolves in rainwater to form carbonic acid (H₂CO₃)?

Nitrogen ( $N_{2}$ )

Oxygen ( $O_{2}$ )

Carbon dioxide ( $CO_{2}$ )

Sulfur dioxide ( $SO_{2}$ )

None of the above

Correct Answer

🔬 SC • Science Concept & PrincipleGeneral ScienceChemistry

Option C

Carbon dioxide ( $CO_{2}$ )

Quick Summary:

Carbon dioxide ( $CO_{2}$ ) is a naturally occurring gas in the atmosphere that dissolves in rainwater to form carbonic acid. This process makes normal rainwater slightly acidic, typically with a pH around 5.6.

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🔬SCScience SolutionConcept & Principle

💡 Explanation

💡 Everyday Analogy (Real-World Intuition)

Think of carbonated soft drinks: when $CO_{2}$ gas is forced into water under pressure, it creates carbonic acid, which gives the drink its sharp, tangy taste.

🧠 Memory Mnemonic / Shortcut Aid

Remember 'C-C': Carbon dioxide + Carbonic acid.

🔢 Key Formulas

$CO_{2} + H_{2}O \rightleftharpoons H_{2}CO_{3}$ — Formation of carbonic acid from carbon dioxide and water.

$H_{2}CO_{3} \rightleftharpoons H^{+} + HCO_{3}^{-}$ — Dissociation of carbonic acid in water.

⚙️ Working Principle

When atmospheric carbon dioxide dissolves in liquid water (rain), it undergoes a reversible chemical reaction to produce carbonic acid. The reaction is: $CO_{2} + H_{2}O \rightleftharpoons H_{2}CO_{3}$ . Carbonic acid then partially dissociates into hydrogen ions ( $H^{+}$ ) and bicarbonate ions ( $HCO_{3}^{-}$ ), which lowers the pH of the water.

📌 Key Points

▸
Normal, unpolluted rainwater has a pH of approximately 5.6 due to the presence of dissolved $CO_{2}$ .
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Acid rain, which has a pH lower than 5.6, is typically caused by sulfur dioxide ( $SO_{2}$ ) and nitrogen oxides ( $NO_{x}$ ), not carbon dioxide.
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Carbonic acid is a weak acid, unlike the strong acids formed by industrial pollutants.

🛠️ Applications / Uses

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Formation of natural geological features like limestone caves (karst topography).
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Carbonation in food and beverage industries.

📄 Additional Information

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Option A ( $N_{2}$ ) is inert under these conditions.
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Option B ( $O_{2}$ ) does not form an acid when dissolved in water.
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Option D ( $SO_{2}$ ) reacts with water to form sulfurous acid ( $H_{2}SO_{3}$ ), which is a key contributor to severe acid rain, but the question specifically asks for the gas forming carbonic acid.

📊 Diagram / Illustration

✅

C is correct — Carbon dioxide ( $CO_{2}$ ) reacts with rainwater to produce carbonic acid ( $H_{2}CO_{3}$ ), resulting in the natural acidity of rain.

Core Concepts Used

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Atmospheric chemistry Acid-base reactions Chemical equilibrium

💡 EXAM TIP

Don't confuse natural rainfall acidity ( $pH \approx 5.6$ due to $CO_{2}$ ) with industrial 'Acid Rain' ( $pH < 5.0$ due to $SO_{2}$ and $NO_{x}$ ).

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