Acid Deposition

You’ll be able to

Explain how sulfur and nitrogen oxides produce acid deposition
Use the pH scale to compare the acidity of rainwater samples
Describe the ecological and structural effects of acid rain and why it is a regional problem

Where acid deposition comes from

Acid deposition ("acid rain") forms when sulfur dioxide (SO₂) and nitrogen oxides (NOₓ) — released mainly by burning coal and by vehicles — react with water and oxygen in the atmosphere to form sulfuric acid (H₂SO₄) and nitric acid (HNO₃). These acids fall back to Earth as wet deposition (acidic rain, snow, or fog) or dry deposition (acidic particles). Normal rain is slightly acidic (pH ≈ 5.6) from dissolved CO₂; acid rain is defined as precipitation with a pH below about 5.

The pH scale

The pH scale (0–14) measures how acidic or basic a solution is: 7 is neutral, below 7 is acidic, above 7 is basic. It is logarithmic, so each whole-number step is a tenfold change in hydrogen-ion (H⁺) concentration. A solution at pH 4 is ten times more acidic than pH 5 and one hundred times more acidic than pH 6. This is why a seemingly small drop in rainfall pH represents a dramatic increase in acidity — the key quantitative idea for acid-rain problems.

Effects and the downwind problem

Acid deposition acidifies lakes and streams, killing fish and other aquatic life (especially in poorly buffered waters with little limestone to neutralize acid); leaches nutrients like calcium from soils while freeing toxic aluminum; damages forests, notably at high elevations; and corrodes buildings, statues, and limestone monuments. Because the pollutants travel on prevailing winds, acid rain is a regional, transboundary problem — emissions from Midwest power plants fall on the forests and lakes of the Northeast and Canada, far downwind of the source.

pH is logarithmic

each 1-unit drop in pH = 10× more acidic (10× higher H⁺ concentration)

A difference of n pH units equals a factor of 10ⁿ in acidity. pH 3 vs pH 6 is a difference of 3 units = 10³ = 1,000 times more acidic.

Worked example

Normal rainwater has a pH of about 5.6, but rain in an industrial region measures pH 3.6. How many times more acidic is the polluted rain than normal rain?

1.Find the difference in pH units: 5.6 − 3.6 = 2.0 units.
2.Recall that each pH unit is a tenfold change in acidity (H⁺ concentration).
3.A 2-unit difference means a factor of 10² = 100.
4.So the pH-3.6 rain is 100 times more acidic than the pH-5.6 rain.

Answer: The polluted rain is 100 times more acidic. Because the scale is logarithmic, that 2-unit pH drop represents a hundredfold jump in hydrogen-ion concentration.

Checkpoint

A lake acidified by acid rain has a pH of 4, while a healthy lake has a pH of 6. How much more acidic is the acidified lake?

Watch out

Never treat pH differences as linear. A drop from pH 6 to pH 4 is not "2 times" more acidic — it is 10² = 100 times. Count the number of whole pH units of difference, then raise 10 to that power. This logarithmic step trips up more students than any other acid-rain detail.

Checkpoint

Coal-burning power plants in the U.S. Midwest are blamed for acidifying lakes in the Northeast and eastern Canada, hundreds of miles away. What best explains this pattern?

On the exam

Two acid-rain must-knows: the chemistry (SO₂ → sulfuric acid, NOₓ → nitric acid) and the math (each pH unit = 10× acidity, so an n-unit drop = 10ⁿ). Lakes with limestone bedrock are buffered and resist acidification; granite-based lakes are not.

Answer the 2 checkpoints as you read.