What is the typical baseline pH of unpolluted rain and why?

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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 is the typical baseline pH of unpolluted rain and why?

Explanation:
Rain is not perfectly neutral; unpolluted rain is slightly acidic because carbon dioxide from the atmosphere dissolves in it and forms a weak acid called carbonic acid. When CO2 dissolves in water, it forms H2CO3, which only partly dissociates to give a small amount of H+ and HCO3−. That small amount of H+ lowers the pH just a bit, so the typical baseline for unpolluted rain is about 5.6. The other values would require neutral water (pH around 7), a strong acid (around 3), or a basic solution (around 9), which don’t describe clean rain. In polluted air, additional acids from sulfur and nitrogen oxides can lower the pH further, leading to acid rain.

Rain is not perfectly neutral; unpolluted rain is slightly acidic because carbon dioxide from the atmosphere dissolves in it and forms a weak acid called carbonic acid. When CO2 dissolves in water, it forms H2CO3, which only partly dissociates to give a small amount of H+ and HCO3−. That small amount of H+ lowers the pH just a bit, so the typical baseline for unpolluted rain is about 5.6. The other values would require neutral water (pH around 7), a strong acid (around 3), or a basic solution (around 9), which don’t describe clean rain. In polluted air, additional acids from sulfur and nitrogen oxides can lower the pH further, leading to acid rain.

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