Environmental engineering — classify rainfall as “acid rain”: at what pH value or lower is precipitation considered acidic?

Introduction / Context:
Acid rain is precipitation with a hydrogen ion concentration higher than that of normal, unpolluted rain. In environmental engineering and atmospheric science, classifying rainfall as “acidic” helps assess impacts on water bodies, soils, buildings, and ecosystems, and it guides mitigation strategies for emissions of sulfur oxides (SOx) and nitrogen oxides (NOx).

Given Data / Assumptions:

• Normal clean rain equilibrated with atmospheric CO2 has a slightly acidic pH around 5.6.
• Question asks for a threshold pH at or below which rainfall may be classified as acidic in practical environmental monitoring.
• pH is defined as −log10[H+]; lower pH means greater acidity.

Concept / Approach:

While 5.6 is the theoretical pH of pure rainwater in equilibrium with CO2, regulatory and textbook MCQs often expect a stricter operational cutoff for “acid rain” classification. Values ≤ 5.0 clearly indicate anthropogenic acidification beyond natural carbonation. Hence, pH ≤ 5 is a commonly used simple criterion in many question banks to label precipitation as acidic beyond natural background effects.

Step-by-Step Solution:

Verification / Alternative check (if short method exists):

If an option at 5.6 were provided and explicitly tied to the CO2 equilibrium definition, one might argue for “≤ 5.6.” However, many standardized MCQs emphasize pH ≤ 5 for unequivocal acidification. Given the present options, 5 is the appropriate selection.

Why Other Options Are Wrong:

6 or 6.5 are too high (near neutral to mildly acidic). A pH of 7 is neutral and not acid rain by any definition.

Common Pitfalls (misconceptions, mistakes):

Confusing natural slightly acidic rain (≈5.6) with anthropogenically acidified precipitation; assuming only very low pH (e.g., ≤4) counts as “acid rain.”

Final Answer:

5