How does burning coal and fuel turn rain acidic, and what does that acid do downwind?
Topic 7.7 Acid Rain: explain how acid rain forms from sulfur and nitrogen oxides and describe its environmental impacts.
A focused answer to APES Topic 7.7, covering how acid deposition forms from sulfur dioxide and nitrogen oxides, the pH scale, the impacts on lakes, forests, soils and buildings, the transboundary nature of the problem, and how to reduce it, with a worked pH calculation.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this topic is asking
The College Board (Topic 7.7) wants you to explain how acid rain forms from sulfur and nitrogen oxides and describe its environmental impacts.
How acid rain forms
The pH scale and acidity
Impacts
Why this matters
Acid rain is the classic secondary-pollutant and transboundary problem of Unit 7, building on the primary pollutants of Topic 7.1 and solved by the controls of Topic 7.6. It links air pollution to the aquatic and soil impacts of Unit 8 and to fossil fuels (Unit 6), and the logarithmic pH point is a recurring AP exam calculation. A typical free-response question gives you data on lake pH or forest decline downwind of a power-plant region and asks you to identify the cause, explain the chemistry, and propose a solution. The transboundary angle is what makes acid rain politically hard: because the pollutants drift across state and national borders before falling, the region that suffers the damage is often not the one that emitted the pollutants, so controlling it requires cooperation between the source and the affected regions, much like the global agreements needed for ozone and climate.
Try this
Q1. Identify the two primary pollutants that cause acid rain. [1 point]
- Cue. Sulfur dioxide and nitrogen oxides.
Q2. Explain why acid rain is described as a transboundary problem. [2 points]
- Cue. The sulfur dioxide and nitrogen oxides are carried long distances by wind before they form and fall as acid, so the acid rain often lands in different regions or countries from where the pollutants were emitted.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2021 (style)4 marksSection II (FRQ). (a) Identify the two primary pollutants that cause acid rain and their main source. (b) Explain how these pollutants become acid rain. (c) Describe two environmental impacts of acid rain. (d) Explain why acid rain is often described as a transboundary problem.Show worked answer →
A 4-point FRQ on acid rain.
(a) Identify (1 point): sulfur dioxide (SO2) and nitrogen oxides (NOx), mainly from burning fossil fuels in power plants and vehicles.
(b) Explain (1 point): in the atmosphere these gases react with water and oxygen to form sulfuric and nitric acids, which fall as acidic rain, snow or dry particles.
(c) Describe (1 point): any two of acidifying lakes and killing fish, damaging forests and leaching nutrients from soil, mobilizing toxic aluminum, or corroding buildings and statues.
(d) Explain (1 point): the pollutants are carried long distances by wind, so the acid often falls in different regions or countries from where it was emitted.
Markers reward sulfur dioxide and nitrogen oxides from fossil fuels, the reaction with water to form sulfuric and nitric acids, two valid impacts, and the wind-transport explanation for the transboundary point.
AP 2018 (style)1 marksSection I (multiple choice). Acid rain forms primarily when which pollutants react with water in the atmosphere? (A) Carbon monoxide and ozone (B) Sulfur dioxide and nitrogen oxides (C) Radon and methane (D) Lead and particulates. Justify your choice.Show worked answer →
A 1-point MCQ on acid rain. The answer is (B).
Acid rain forms when sulfur dioxide and nitrogen oxides, mainly from burning fossil fuels, react with water and oxygen in the atmosphere to make sulfuric and nitric acids. Carbon monoxide and ozone (A), radon and methane (C), and lead and particulates (D) do not form acid rain. The trap is confusing acid rain (from sulfur and nitrogen oxides) with other pollution problems such as smog or the greenhouse effect.
Related dot points
- Topic 7.1 Introduction to Air Pollution: identify the major air pollutants and their sources and distinguish primary from secondary pollutants.
A focused answer to APES Topic 7.1, covering the major air pollutants, their natural and human sources, the criteria pollutants, and the distinction between primary and secondary pollutants, with a worked emissions calculation.
- Topic 7.6 Reduction of Air Pollutants: describe methods used to reduce air pollution, including regulation, scrubbers, catalytic converters and cleaner fuels.
A focused answer to APES Topic 7.6, covering methods to reduce air pollution including the Clean Air Act and regulation, scrubbers and electrostatic precipitators, catalytic converters, vapor recovery, cleaner fuels and renewable energy, with a worked scrubber efficiency calculation.
- Topic 7.4 Atmospheric CO2 and Particulates: describe the natural and human sources of atmospheric carbon dioxide and particulate matter and their effects.
A focused answer to APES Topic 7.4, covering the natural and human sources of atmospheric carbon dioxide and particulate matter, the difference between PM10 and PM2.5, why fine particles are most dangerous, the health and environmental effects, with a worked particulate exposure calculation.
- Topic 5.9 Impacts of Mining: compare surface and subsurface mining and explain their environmental consequences, including acid mine drainage and tailings.
A focused answer to APES Topic 5.9, covering surface mining (strip, open-pit, mountaintop removal) and subsurface mining, their environmental consequences, acid mine drainage, tailings, habitat destruction, and reclamation, with a worked overburden calculation.
- Topic 6.5 Fossil Fuels: explain how fossil fuels form and are used to generate electricity, and describe their environmental impacts, including cogeneration.
A focused answer to APES Topic 6.5, covering how fossil fuels form, how a fossil-fuel power plant generates electricity, fracking, cogeneration, and the environmental impacts of coal, oil and gas, with a worked power plant efficiency calculation.
Sources & how we know this
- AP Environmental Science Course and Exam Description — College Board (2020)