Acid Rain: Effects, Causes & Quiz

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Margaret Cunningham

This lesson will focus on the environmental problem known as acid rain. It will also explore the causes and effects of acid rain, along with how it is measured and what is being done to reduce acid rain.

We also recommend watching Acid Deposition: Definition, Causes & Effects and Deforestation: Definition, Causes & Consequences


Most people think of rain as refreshing and beneficial to the environment, but not all types of rain are a good thing. Acid rain is one type of rain that is harmful to the environment. Although the name might give you the impression that this type of rain is pure acid falling from the sky, acid rain is actually created when certain gases are mixed with atmospheric moisture to create precipitation that is more acidic than normal.

Sulfur dioxide and nitrogen oxides are two of the gases involved in the creation of acid rain. When these gases are released into the environment and mixed with water, oxygen, and other chemicals, they create acidic compounds, such as sulfuric acid and nitric acid, which results in the formation of acid rain.

Although rain is most commonly thought of as wet, acid rain can come in both a wet or dry form based on how the acidic materials fall from the atmosphere. When the acidic materials fall from the atmosphere in a wet form, they fall as rain, sleet, snow, or fog. In the dry form, acid rain can fall from the atmosphere as gases or particles. Both the wet and dry forms of acid rain can be carried by the wind and travel a far distance before being deposited.

Acid Rain Cycle
acid rain diagram

Causes of Acid Rain

The causes of acid rain can be both natural and man-made. In terms of natural sources, both volcanoes and decaying vegetation release gases that result in the formation of acid rain. However, the majority of gases come from man-made sources such as fossil fuel combustion.

In the United States, around 2/3 of sulfur dioxide and 1/4 of nitrogen oxides in the atmosphere are released from electric power generation due to the burning of fossil fuels. The exhaust from vehicles also releases nitrogen oxides and sulfur dioxides into the air, and as the number of vehicles driven increases, so does the amount of gases released and the risk of acid rain.

Measurement of Acid Rain

We use the pH scale to measure acid rain. The pH scale is a scale for measuring how acidic a substance is. It runs from zero, most acidic, to 14, most basic, with a pH of 7 being neutral. Although pure water is known to have a pH of 7, normal rain water has a slightly more acidic pH of around 5.6. This pH level is due to the carbon dioxide in the atmosphere that dissolves into a weak carbonic acid. Acid rain has an average pH of 4.2-4.4, which is almost ten times more acidic than normal rain.

The level of acidity can be determined using high-tech devices or using litmus paper. Litmus paper is commonly used due to its ease and simplicity. When litmus paper is exposed to a substance it changes color depending on the acidity of the substance. If a substance is very acidic the paper will turn bright red and if the substance is very basic it will turn dark blue. The litmus paper will turn a variety of colors to indicate acidity levels in between extreme acids and bases.

pH Scale with Environmental Effects
pH scale

Effects of Acid Rain

Overall, the environment and its inhabitants are adapted to survive with a certain acidity level. Acid rain in both the dry and wet forms can travel far distances, and when acid rain falls it can dramatically alter the acidity level of the habitat and cause a great deal of damage to the living and non-living things in the habitat. Acid rain can negatively influence human health because it can create particles in the air that can cause respiratory problems or make breathing more difficult. Acid rain can also damage building materials and paints. Due to the increased acidity in the rain, building materials decay at a faster rate and paint peels more rapidly. The acidity also wears down stone statues, making then appear older and reducing their value.

Although acid rain can harm human health and buildings, the most drastic effects from acid rain can be seen in forests and waterways that are exposed to it. Acid rain does not kill trees directly, but instead it seeps into the soil and dissolves many of the essential nutrients the trees need. Acid rain also releases excess aluminum into the soil around trees, which makes it difficult for trees to take up the water they need. As a result of limited nutrients and water, the trees are more likely to be harmed by infections and insects and are less resistant to cold weather. In Germany, there is an evergreen forest known as the Black Forest, which received its name because acid rain caused the trees to drop their pine needles and the trees are now simply black trunks and branches.

Effects of Acid Rain: Statue Erosion (left) and Forest After Exposure (right)
effects of acid rain

Lakes and streams that have been exposed to acid rain can also be damaged. The acid rain increases the acidity level of the water and can be deadly to the aquatic animals that live there. Species such as rainbow trout, small mouth bass, frogs, spotted salamanders, and crayfish are adapted to a certain acidity level, and when the acidity increases due to acid rain they are no longer able to survive. Not only does the acid rain negatively affect the ecosystem where it falls, it also travels in the streams and rivers to influence ecosystems on a larger scale.

Reducing Acid Rain

Due to the vast impacts acid rain has had on our lives and the environment, the government has been working to reduce the prevalence of acid rain. The Environmental Protection Agency (EPA) started the Acid Rain Program, which was established to set limits on the amount of harmful gases a power plant could release into the air. The U.S. government is also investing resources in alternative energy sources and cleaner cars to reduce the air pollution that leads to acid rain. It is also recommended that as citizens we reduce our energy usage by using energy efficient machines, turning off appliances not in use, and by carpooling or taking public transportation. By being more informed about the issue and steps to help, it is possible for individuals to help reduce the risk of acid rain.

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