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Cartoon leaf holding an umbrella over baby plant

Cartoon leaf holding an umbrella over baby plant (cotyledons, iStockphoto)

STEM in Context

What is Acid Rain?

Laura Hill & Let's Talk Science

Summary

Acid rain is any precipitation that has an unusually low pH. It can be rain, snow, fog, etc. But what is a low pH and why is this a problem?

These days, if you click on a news website, chances are you’ll see an article on climate change. But a couple of decades ago, the headlines were different. People were very worried about something called acid rain. Acid rain is any precipitation that has an unusually low, or acidic, pH. Precipitation can include rain, snow and fog.

Did you know?

Although acid rain was first observed in England as early as the 1850s, the Canadian Government didn't officially recognize its existence until 1979.

What is pH? 

A substance’s pH is the measurement of its dissolved positively charged hydrogen ions (H+) and negatively charged hydroxide ions (OH-). The pH of a substance is measured on a scale from 0 to 14. 

Substances that are closer to 0 on the pH scale are considered acidic. They have a very high concentration of dissolved H+ ions and a very low concentration of OH- ions. Substances that are closer to 14 on the pH scale are considered basic, or alkaline.They have a very low concentration of dissolved H+ ions and a very high concentration of OH- ions. 

A substance that sits at 7 on the pH scale is considered neutral. It is neither acidic nor basic. 

Every level on the pH scale is ten times greater than the one before. For instance, a substance that has a pH of 5 is ten times more acidic than a substance with a pH of 6! This kind of scale is called a logarithmic scale. 

What is acid precipitation?

Pure water is considered a neutral substance. It has a pH of 7. Clean, or unpolluted, rain is naturally slightly acidic with a pH of 5.6. Acid rain is any precipitation that has a pH lower than 5.6. Typically, the pH of acid rain is between 2 and 4.5. 

Have you ever put vinegar (aqueous acetic acid) on your french fries? If so, you consumed an acid (vinegar) with a pH of about 2 - 2.5. Fries with vinegar are pretty delicious. So if acid rain and vinegar both have low pH, then acid rain can’t be all that bad, right? Wrong! Acid rain can disturb forest growth, kill fish and aquatic plants, and harm buildings, too. 

pH scale with examples
pH scale with examples (Let’s Talk Science using an image by blueringmedia via iStockphoto)).

Why does acid precipitation exist?

Many human activities involve burning fossil fuels. For example, coal plants burn fossil fuels. But gasoline cars burn fossil fuels, too. Why are fossil fuels a problem? Well, burning fossil fuels releases many gases. Two of these gases are sulphur dioxide (SO2) and nitric oxides (NOx). Natural sources of these gases exist too, such as volcanoes.

When these gases combine with water and oxygen in the atmosphere, they produce sulphuric, nitric and nitrous acids. 

Chemical reaction of sulfur dioxide with water and oxygen to form sulfuric acid
Chemical reaction of sulphur dioxide with water and oxygen to form sulphuric acid (©2019 Let’s Talk Science).
Chemical reaction of nitrogen monoxide with water and oxygen to form nitric acid and nitrous acid
Chemical reaction of nitrogen monoxide with water and oxygen to form nitric acid and nitrous acid (©2019 Let’s Talk Science).

This can have two results. These acids can have a wet deposition. This means they fall as precipitation, in the form of rain or snow. They can also have a dry deposition. This means they can come down from the atmosphere in the form of gases or dust.

Acid rain can cause a lot of damage! It can ruin the paint job on a car. It can ruin the stone and metal of famous monuments.

How does acid rain ruin statues and monuments?

Statues and monuments are often made out of substances that corrode (break down) when exposed to acid rain. Examples of these substances include: 

  • Limestone

  • Sandstone

  • Marble

  • Granite

When sulphur dioxide (SO2) in the air mixes with water, it creates sulphuric acid (H2SO3). When rain falls from the sky onto a limestone (CaCO3) statue, a neutralization reaction occurs between sulphuric acid and calcium carbonate. This reaction creates calcium sulfate (CaSO4). Calcium sulfate is soluble in water, which eventually causes the statue to crumble away. 

Sulphuric acid reacts with calcium carbonate in a neutralization reaction:

Calcium carbonate + sulphuric acid → calcium sulfate + water + carbon dioxide

Chemical reaction of calcium carbonate with sulfuric acid to form calcium sulfate, water and carbon dioxide
Chemical reaction of calcium carbonate with sulphuric acid to form calcium sulfate, water and carbon dioxide (©2019 Let’s Talk Science).

 

Acid damage to a limestone gargoyle, on the left, and natural limestone, on the right
Acid damage to a limestone gargoyle, on the left, and natural limestone, on the right (Nino Barbieri [CC BY-SA 3.0] via Wikimedia Commons and Bernard Spragg from Christchurch, New Zealand [CC0] via Wikimedia Commons).

Did you know?

Countries have to work together to fight air pollution issues. This is especially important for countries that are neighbours! For example, Canada and the United States signed an Air Quality Agreement in 1991.

Acid Rain (2014) by FuseSchool (5:36 min.).

Scientists are working on the problem of acid rain. But to fully stop it, people need to stop depending on fossil fuels so much. Scientists at the University of Illinois collected data from 1984 to 2009. They found that environmental policy changes helped reduce acid rain. 

Another study suggests that acid rain can actually inhibit wetland bacteria from producing methane. Methane is another harmful form of greenhouse gas. If one greenhouse gas prevents the production of another, they will neutralize one another. This isn’t an ideal situation. But it shows that acid rain isn’t all bad. 

 

STARTING POINTS

Connecting and Relating

  • Have you noticed any changes in your local environment that may have been caused by acid rain? Explain.

  • What are some things that you and your family can do to reduce acid rain? Explain.

Connecting and Relating

  • Have you noticed any changes in your local environment that may have been caused by acid rain? Explain.

  • What are some things that you and your family can do to reduce acid rain? Explain.

Relating Science and Technology to Society and the Environment

  • Should the government place an environmental tax on gasoline to raise funds to address issues such as acid rain? Why or why not?

  • Who should play the greatest role in reducing the pollutants that cause acid rain? Government? Automobile manufacturers? Individuals? Explain.

  • Due to air currents, acid rain created in one country may fall to the surface in a neighbouring country. Who should be financially responsible for damages incurred? Explain.

Relating Science and Technology to Society and the Environment

  • Should the government place an environmental tax on gasoline to raise funds to address issues such as acid rain? Why or why not?

  • Who should play the greatest role in reducing the pollutants that cause acid rain? Government? Automobile manufacturers? Individuals? Explain.

  • Due to air currents, acid rain created in one country may fall to the surface in a neighbouring country. Who should be financially responsible for damages incurred? Explain.

Exploring Concepts

  • Why is “acid precipitation” a better term than “acid rain”? Explain.

  • Describe how acid precipitation forms in the atmosphere.

  • Describe the chemical processes that cause limestone and marble statues to corrode. 

  • The emissions from gasoline-burning vehicles are one of the leading causes of acid rain. Research alternative fuels that can be used to power vehicles.

  • Research the effects of acid rain on aquatic life, terrestrial ecosystems and human health.

  • Balance the chemical equation for: CaCO3 + H2SO4 --> CaSO4 + H2O + CO2

Exploring Concepts

  • Why is “acid precipitation” a better term than “acid rain”? Explain.

  • Describe how acid precipitation forms in the atmosphere.

  • Describe the chemical processes that cause limestone and marble statues to corrode. 

  • The emissions from gasoline-burning vehicles are one of the leading causes of acid rain. Research alternative fuels that can be used to power vehicles.

  • Research the effects of acid rain on aquatic life, terrestrial ecosystems and human health.

  • Balance the chemical equation for: CaCO3 + H2SO4 --> CaSO4 + H2O + CO2

Nature of Science/Nature of Technology

  • Explain how the study of acid rain demonstrates that science is tentative. In other words, science is subject to change in light of new evidence and new ways of thinking.

  • Science is based on empirical evidence: qualitative and quantitative data. What empirical evidence can we gather to demonstrate the effects of acid rain? Explain.

  • In science, discoveries often lead to new questions for future investigation. With the knowledge you have gained in this article about acid rain, propose two questions that scientists could ask about climate change.

  • How can our understanding of the chemistry of acids impact our understanding of issues such as acid rain? Explain.

Nature of Science/Nature of Technology

  • Explain how the study of acid rain demonstrates that science is tentative. In other words, science is subject to change in light of new evidence and new ways of thinking.

  • Science is based on empirical evidence: qualitative and quantitative data. What empirical evidence can we gather to demonstrate the effects of acid rain? Explain.

  • In science, discoveries often lead to new questions for future investigation. With the knowledge you have gained in this article about acid rain, propose two questions that scientists could ask about climate change.

  • How can our understanding of the chemistry of acids impact our understanding of issues such as acid rain? Explain.

Media Literacy

  • Have you heard about acid rain in the media lately? Why do you think that is?

  • How can social media be used to lobby industry and government to reduce the amount of fossil fuels that are being burned? Explain.

Media Literacy

  • Have you heard about acid rain in the media lately? Why do you think that is?

  • How can social media be used to lobby industry and government to reduce the amount of fossil fuels that are being burned? Explain.

Teaching Suggestions

  • This article can be used to support teaching and learning of Chemistry, Environmental Science, Earth Science, Weather, Pollution and Climate Change related to acid precipitation, the nitrogen cycle, precipitation and acids & bases. Concepts introduced include acid rain, precipitation, pH, hydrogen ions (H+), hydroxide ions (OH-), acidic, basic, alkaline, neutral, vinegar, fossil fuels, sulfur dioxide, nitric oxides, wet deposition, dry deposition, corrode, neutralization and soluble. .  

  • Before having students read the article, conduct a Vocabulary Preview learning strategy to familiarize students with key terminology. The ready-to-use Vocabulary Preview reproducible can be downloaded in [Google doc] or [PDF] formats.

  • After reading the article, teachers could have students use a Key Ideas Round Robin learning strategy to negotiate agreement of the key ideas in the article. The ready-to-use Key Ideas Round Robin reproducible can be downloaded in [Google doc] or [PDF] formats.

Teaching Suggestions

  • This article can be used to support teaching and learning of Chemistry, Environmental Science, Earth Science, Weather, Pollution and Climate Change related to acid precipitation, the nitrogen cycle, precipitation and acids & bases. Concepts introduced include acid rain, precipitation, pH, hydrogen ions (H+), hydroxide ions (OH-), acidic, basic, alkaline, neutral, vinegar, fossil fuels, sulfur dioxide, nitric oxides, wet deposition, dry deposition, corrode, neutralization and soluble. .  

  • Before having students read the article, conduct a Vocabulary Preview learning strategy to familiarize students with key terminology. The ready-to-use Vocabulary Preview reproducible can be downloaded in [Google doc] or [PDF] formats.

  • After reading the article, teachers could have students use a Key Ideas Round Robin learning strategy to negotiate agreement of the key ideas in the article. The ready-to-use Key Ideas Round Robin reproducible can be downloaded in [Google doc] or [PDF] formats.

Learn more

Acids, Alkalis, and the pH Scale 

Article from Compound Interest with an infographic outlining the acidic and alkaline range of the pH scale, the logarithmic nature of the scale, and examples of substances at each specific pH. 

What is Acid Rain? (2018) 

This National Geographic Video (1:58 min.) introduces acid rain and explains how it forms and travels, as well as some of the impacts it can have.

 

References

LibreTexts. (2019, April 27). Acid rain.

Ophardt, C. E. (2003). pH scale. Elmhurst College.

Ophardt, C. E. (2003). Acid rain effects on buildings. Elmhurst College.

United States Environmental Protection Agency. (n.d.). What Causes Acid Rain?

United States Environmental Protection Agency. (2019, April 18). What is acid rain?