Acid rain with pH 4.0 is 100 times more acidic than rain with pH 6.0 due to the logarithmic nature of the pH scale. This multiple-choice question tests understanding of acidity differences in environmental science. The correct answer is (C) 100 times.​

pH Scale Basics

The pH scale measures hydrogen ion concentration logarithmically, where each unit drop represents a tenfold increase in acidity. A solution with pH 4.0 has 10 times more H⁺ ions than pH 5.0, and 100 times more than pH 6.0. Normal rain typically falls around pH 5.6 from dissolved CO₂ forming weak carbonic acid.​

Correct Answer Explanation

Acid rain at pH 4.0 compared to pH 6.0 shows a difference of 2 pH units (6.0 – 4.0 = 2). Since each pH unit equals a factor of 10 in acidity, the total factor is 10² or 100 times more acidic. Acid rain forms from SO₂ and NOₓ reacting with water to produce strong sulfuric and nitric acids.​

Why Not Other Options?

• (A) 2 times: This ignores the logarithmic scale and assumes a linear difference, which applies to arithmetic scales, not pH.

• (B) 10 times: Correct for a 1-unit pH drop (e.g., pH 5.0 to 4.0), but here the gap is 2 units.​

• (D) 1000 times: Matches a 3-unit drop (10³), exceeding the actual 2-unit difference between pH 4.0 and 6.0.​

Environmental Impact

Lower pH in acid rain harms ecosystems by leaching soil nutrients and toxic metals into water bodies. Rain at pH 4.0 damages forests and aquatic life far more than near-neutral pH 6.0 precipitation.​