What causes unpolluted rain to have a pH around 5.6?

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Study for the Grade 9 Environmental Chemistry Test. Use a blend of multiple-choice questions and detailed explanations. Master key concepts and prepare effectively!

Multiple Choice

What causes unpolluted rain to have a pH around 5.6?

Explanation:
The main idea is that carbon dioxide in the atmosphere makes rain slightly acidic when it dissolves in the water. When rainwater encounters CO2, it reacts to form carbonic acid: CO2 + H2O ⇌ H2CO3. Carbonic acid is a weak acid, so only a portion of it dissociates to release hydrogen ions: H2CO3 ⇌ H+ + HCO3−. This small amount of extra hydrogen ions lowers the pH from neutral (about 7) to around 5.6 in unpolluted rain. The level of CO2 in the air sets this baseline acidity, and if there were more CO2, the rain would be even more acidic, while other substances like minerals or ozone don’t determine this natural pH in clean air.

The main idea is that carbon dioxide in the atmosphere makes rain slightly acidic when it dissolves in the water. When rainwater encounters CO2, it reacts to form carbonic acid: CO2 + H2O ⇌ H2CO3. Carbonic acid is a weak acid, so only a portion of it dissociates to release hydrogen ions: H2CO3 ⇌ H+ + HCO3−. This small amount of extra hydrogen ions lowers the pH from neutral (about 7) to around 5.6 in unpolluted rain. The level of CO2 in the air sets this baseline acidity, and if there were more CO2, the rain would be even more acidic, while other substances like minerals or ozone don’t determine this natural pH in clean air.

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