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10.26: Acid Rain

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    What would the artists think?

    Notre Dame Cathedral in Paris opened in 1345. Construction took almost 200 years. For about 500 years, the cathedral was pristine. Yet this gargoyle has pits in it. The pits are caused by acid rain. Acid rain forms from pollutants from fossil fuel burning.

    Acid Rain

    Acid rain is rain that has a pH less than 5 (Figure below). Acidity is measured on the pH scale. Lower numbers are more acidic, and higher numbers are less acidic (also called more alkaline). An acid has a pH of less than 7. The pH of normal rain is 5.6. It’s slightly acidic because carbon dioxide in the air dissolves in rain. This forms carbonic acid, a weak acid.

    The pH scale

    This pH scale includes both normal and acid rain. At what pH do fish have problems reproducing?

    How Acid Rain Forms

    Acid rain forms when nitrogen and sulfur oxides in air dissolve in rain (Figure below). This forms nitric and sulfuric acids. Both are strong acids. Acid rain with a pH as low as 4.0 is now common in many areas. Acid fog may be even more acidic than acid rain. Fog with a pH as low as 1.7 has been recorded. That’s the same pH as toilet bowl cleaner!

    Nitrogen and sulfur oxides combine with rain to form acid rain

    Nitrogen and sulfur oxides combine with rain to form acid rain.

    Effects of Acid Rain

    The image below shows some of the damage done by acid rain (Figure below). Acid rain ends up in soil and bodies of water. This can make them very acidic. The acid strips soil of its nutrients. These changes can kill trees, fish, and other living things. Acid rain also dissolves limestone and marble. This can damage buildings, monuments, and statues.

    Acid rain has killed trees in this forest in the Czech Republic

    Acid rain has killed trees in this forest in the Czech Republic.


    • Nitrogen and sulfur compounds emitted high into the atmosphere create acids. These acids may fall as acid rain.
    • Acidity is measured on a pH scale. Rain that is 5.0 or less on that scale is considered acid rain.
    • Acid rain weakens plants and animals and damages cultural treasures.


    1. Why does acid rain do a lot of damage far from where the smoke was released?
    2. What damage does acid rain do to organisms? What damage does it do to cultural structures?
    3. Describe the pH scale. What can you say about a liquid with a pH of 2.5? What about a liquid with a pH of 10.5?

    Explore More

    Use the resource below to answer the questions that follow.

    1. What causes acid rain?
    2. What two gases react with water to make acid rain?
    3. Where relative to where the gases are emitted does acid rain strike?
    4. What damage can acid rain cause?
    5. How can we reduce acid rain?
    6. What are scrubbers?
    7. What is slurry?
    8. How effective are scrubbers?

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