Why is unpolluted rainwater slightly acidic, with a pH of about 5.6?
AIt contains dissolved salt from the ocean
BAtmospheric carbon dioxide dissolves in it and forms carbonic acid
CLightning adds sulfuric acid to all rain
DWater molecules are acidic by nature
CO₂ dissolves in cloud droplets and forms carbonic acid (H₂CO₃), a weak acid. In equilibrium with the atmosphere this gives a pH near 5.6. Acid rain is defined relative to this natural value.
Question 2 Short Answer
Two lakes in the same region receive identical acid rain. Lake A sits on limestone bedrock, and Lake B sits on granite with thin soil. Which lake is more likely to become acidified, and why?
Think about your answer, then reveal below.
Model answer: Lake B. Limestone (calcium carbonate) reacts with and neutralizes the acid, so Lake A is buffered. Granite and thin soils provide little buffering capacity, so the acid stays in Lake B and its pH falls.
Sensitivity depends on buffering capacity. This is why regions with granitic bedrock, such as parts of Scandinavia and the Adirondack Mountains, showed some of the worst lake acidification.
Question 3 True / False
Rain with a pH of 4.6 is ten times more acidic than rain with a pH of 5.6.
TTrue
FFalse
Answer: True
pH is a base-10 logarithmic scale of hydrogen ion concentration. A drop of one pH unit means ten times more hydrogen ions.
Question 4 Multiple Choice
What has been the main cause of the large fall in acid deposition in the United States since about 1990?
ANatural changes in rainfall patterns
BCuts in sulfur dioxide and nitrogen oxide emissions, including a cap-and-trade program for power plant SO₂
CAdding lime to every lake in the country
DA rise in atmospheric carbon dioxide that neutralized the acids
The 1990 Clean Air Act Amendments created the Acid Rain Program, which capped total SO₂ emissions from power plants and let companies trade allowances. Scrubbers, low-sulfur coal, and fuel switching cut emissions, and the acidity of rain fell in response.