AP® Environmental Science review sheet from Aim for Five (aimforfive.com/enviro/units/7/7-7)
Unit 7 · Topic 7.7
7.7 Acid Rain
Acid rain, or more broadly acid deposition, forms when sulfur dioxide and nitrogen oxides react in the air to make sulfuric and nitric acid. It lowers the pH of lakes, streams and soils, harms fish and forests, and dissolves limestone and marble, though some landscapes can neutralize it.
Key terms
- acid deposition
- sulfur dioxide
- nitrogen oxides
- pH
- buffering (limestone)
How acid deposition forms
Coal-burning power plants are the main source of sulfur dioxide (SO₂); vehicles and power plants are the main sources of nitrogen oxides (NOₓ). In the atmosphere these gases react with water, oxygen and other chemicals to form sulfuric acid (H₂SO₄) and nitric acid (HNO₃). The acids fall as rain, snow or fog (wet deposition) or settle out as acidic dry particles and gases (dry deposition). Together these are called acid deposition.
The pollution can travel hundreds of kilometers on the wind, so acid rain often falls far downwind of its source. Tall smokestacks reduced local pollution but spread acid rain farther, to places like the Adirondack Mountains and New England, downwind of Midwest coal plants.
pH is a logarithmic scale
pH measures acidity on a scale from 0 to 14. Below 7 is acidic, 7 is neutral, and above 7 is basic. The scale is logarithmic: each drop of 1 pH unit means the solution is 10 times more acidic. A drop of 2 units is 10 × 10 = 100 times more acidic.
Normal, unpolluted rain is already slightly acidic, around pH 5.6, because CO₂ dissolves in it to form weak carbonic acid. Rain below about 5.6 is called acid rain. Acid rain in polluted regions has measured around pH 4 or lower.
Effects
- Lakes and streams: lower pH harms or kills fish, amphibian eggs and insects. Acid also frees aluminum from soil, which washes into water and damages fish gills.
- Soil and forests: acid leaches nutrients like calcium and magnesium out of the soil and damages leaves and needles, weakening trees so they are more vulnerable to cold, drought and pests. High-elevation forests are often hit hardest.
- Buildings and statues: acid dissolves limestone and marble (both calcium carbonate), wearing away carvings and headstones, and corrodes metals.
Buffering and solutions
Some areas resist acid rain better than others. Limestone (calcium carbonate) neutralizes acid: CaCO₃ + 2H⁺ → Ca²⁺ + H₂O + CO₂. Lakes and soils on limestone bedrock have buffering capacity and change pH little. Lakes on granite bedrock, which can't neutralize acid, are the most vulnerable. Adding crushed limestone to a lake, called liming, is a short-term fix.
The long-term fix is cutting SO₂ and NOₓ emissions: scrubbers, low-sulfur coal, switching to natural gas or renewables, catalytic converters and fuel efficiency. The 1990 Clean Air Act Amendments created a cap-and-trade program for SO₂ from power plants. US power plant SO₂ emissions have since fallen by more than 90 percent, and many lakes and forests are slowly recovering.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Comparing acidity
Rain in a polluted region has a pH of 4.0. A nearby lake has a pH of 6.0. How many times more acidic is the rain than the lake?
Show the solutionHide the solution
- Step 1: Find the difference in pH: 6.0 − 4.0 = 2 units.
- Step 2: Each unit is a factor of 10, so the factor is 10² = 100.
- Step 3: The lower pH is the more acidic one, so the rain is the more acidic.
Answer: The rain is 100 times more acidic than the lake.
- Example 2
Trap: one pH unit is a big jump
A student says rain with pH 4.6 is 'only one unit lower' than normal rain at pH 5.6, so it is about 20% more acidic. Correct the student.
Show the solutionHide the solution
- Step 1: pH is logarithmic, not linear. A difference of 1 unit means a factor of 10 in acidity.
- Step 2: 5.6 − 4.6 = 1, so the pH 4.6 rain is 10¹ = 10 times more acidic than normal rain.
- Step 3: That is a 900% increase in acidity (hydrogen ion concentration), not 20%.
Answer: The pH 4.6 rain is 10 times more acidic than normal rain.
- Example 3
Why two lakes respond differently
Two lakes receive the same acid rain. Over 20 years Lake A's pH drops from 6.8 to 5.0, while Lake B's stays near 7.5. Propose an explanation.
Show the solutionHide the solution
- Step 1: Same input of acid, so the difference must be in the lakes' ability to neutralize it.
- Step 2: Lake B likely sits on limestone bedrock or limestone-rich soil. Calcium carbonate reacts with the acid and neutralizes it (buffering), keeping pH stable.
- Step 3: Lake A likely sits on granite or other rock with little calcium carbonate, so it has little buffering capacity and its pH falls.
Answer: Lake B is buffered by limestone (calcium carbonate) in its watershed; Lake A lacks buffering capacity, likely sitting on granite.
Common mistakes
- Saying CO₂ causes acid rain. CO₂ makes normal rain slightly acidic (about pH 5.6); acid rain comes from SO₂ and NOₓ.
- Treating pH as linear. Each whole unit is a tenfold change in acidity.
- Saying limestone areas are most damaged by acid rain. Limestone bedrock buffers lakes and soils; it is limestone buildings and statues that get dissolved.
On the exam
- pH comparison questions appear often. State the difference in units and convert it with powers of 10.
- If asked for a solution, name a device or policy that cuts SO₂ or NOₓ at the source, such as scrubbers or the SO₂ cap-and-trade program.
Connected topics
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Check yourself
4 questions on 7.7 Acid Rain. Pick an answer to see if you got it, and why.
| Region | Bedrock | Average pH of rain | Average pH of lakes |
|---|---|---|---|
| North | Granite | 4.4 | 5.0 |
| South | Limestone | 4.4 | 7.0 |
Hypothetical data from two regions downwind of the same coal-fired power plants
Which statement best explains the difference in lake pH between the two regions?
How many times greater is the hydrogen ion concentration (acidity) in the northern lakes than in the southern lakes?
Which pollutants released by the power plants are most responsible for the acid rain in both regions?
A marble statue in a city downwind of several coal-fired power plants has lost much of its fine detail over 80 years. Which process best explains this damage?
0 of 4 answered