Acid rain is precipitation or deposition that becomes unusually acidic because air pollutants react with water and oxygen in the atmosphere. This cheat sheet helps students connect human activities, atmospheric chemistry, and environmental damage in one clear reference. It is useful for reviewing pollution sources, key reactions, pH interpretation, and major effects on ecosystems and buildings.
Key Facts
- Normal rain is slightly acidic, with a pH of about 5.6, because carbon dioxide forms weak carbonic acid: CO2 + H2O -> H2CO3.
- Acid rain usually has a pH below 5.6 because sulfur dioxide and nitrogen oxides form stronger acids in the atmosphere.
- Sulfur dioxide can form sulfuric acid through reactions such as SO2 + O2 -> SO3 and SO3 + H2O -> H2SO4.
- Nitrogen dioxide can form nitric acid through the reaction 3NO2 + H2O -> 2HNO3 + NO.
- The pH scale is logarithmic, so a solution with pH 4 is 10 times more acidic than a solution with pH 5.
- Wet deposition includes acidic rain, snow, fog, and mist, while dry deposition includes acidic gases and particles that settle on surfaces.
- Major human sources of acid rain include coal-burning power plants, vehicle exhaust, industrial boilers, and metal smelting.
- Acid rain can leach nutrients such as Ca2+ and Mg2+ from soil and can release toxic Al3+ ions that harm plant roots and aquatic life.
Vocabulary
- Acid rain
- Precipitation or deposition with unusually low pH caused mainly by sulfur dioxide and nitrogen oxides reacting in the atmosphere.
- Sulfur dioxide
- A gas with the formula SO2 that is released by burning sulfur-containing fuels and can form sulfuric acid.
- Nitrogen oxides
- A group of gases often written as NOx that form during high-temperature combustion and can produce nitric acid.
- Wet deposition
- The removal of acidic pollutants from the atmosphere through rain, snow, fog, mist, or cloud water.
- Dry deposition
- The settling of acidic gases and particles onto land, water, plants, and buildings without precipitation.
- Buffering capacity
- The ability of soil or water to resist pH change by neutralizing added acids.
Common Mistakes to Avoid
- Calling all acidic precipitation acid rain only, because acid deposition also includes snow, fog, mist, and dry particles.
- Thinking pH changes are linear, because each one-unit decrease in pH means a 10 times increase in acidity.
- Blaming only rain clouds for acid rain, because the key pollutants usually come from combustion sources such as power plants, factories, and vehicles.
- Assuming acid rain damages only water bodies, because it also harms forests, soils, statues, metals, and building materials.
- Forgetting that normal rain is already slightly acidic, because dissolved CO2 naturally forms carbonic acid with a pH near 5.6.
Practice Questions
- 1 A lake has a pH of 4.5 and normal rain has a pH of 5.6. About how many times more acidic is the lake water than normal rain?
- 2 Write the balanced atmospheric reaction showing sulfur trioxide reacting with water to form sulfuric acid.
- 3 A rainfall sample has pH 4.2. Is it considered acid rain compared with normal rain at pH 5.6, and why?
- 4 Explain why two lakes receiving the same amount of acid deposition might show different pH changes over time.