Carbon dioxide CO 2 combines with water as raindrops fall through the atmosphere. This makes a weak acid, called carbonic acid. Carbonic acid is a very common in nature where it works to dissolve rock. Pollutants, such as sulfur and nitrogen, from fossil fuel burning, create sulfuric and nitric acid. Sulfuric and nitric acids are the two main components of acid rain, which accelerate chemical weathering figure 7.
Acid rain is discussed in the Human Actions and the Atmosphere chapter. Figure 8. When iron rich minerals oxidize, they produce the familiar red color found in rust. Oxidation is a chemical reaction that takes place when oxygen reacts with another element. Oxygen is very strongly chemically reactive. The most familiar type of oxidation is when iron reacts with oxygen to create rust figure 8. Minerals that are rich in iron break down as the iron oxidizes and forms new compounds.
Iron oxide produces the red color in soils. Now that you know what chemical weathering is, can you think of some other ways chemical weathering might occur? Chemical weathering can also be contributed to by plants and animals.
As plant roots take in soluble ions as nutrients, certain elements are exchanged. Plant roots and bacterial decay use carbon dioxide in the process of respiration. A special type of oxidation takes place in areas where the rocks have elevated levels of sulphide minerals, especially pyrite FeS 2.
Pyrite reacts with water and oxygen to form sulphuric acid, as follows:. Some of the worst examples of ARD are at metal mine sites, especially where pyrite-bearing rock and waste material have been mined from deep underground and then piled up and left exposed to water and oxygen. One example of that is the Mt. At many ARD sites, the pH of the runoff water is less than 4 very acidic.
Under these conditions, metals such as copper, zinc, and lead are quite soluble, which can lead to toxicity for aquatic and other organisms. For many years, the river downstream from the Mt. Washington Mine had so much dissolved copper in it that it was toxic to salmon. Remediation work has since been carried out at the mine and the situation has improved. The hydrolysis of feldspar and other silicate minerals and the oxidation of iron in ferromagnesian silicates all serve to create rocks that are softer and weaker than they were to begin with, and thus more susceptible to mechanical weathering.
Some weathering processes involve the complete dissolution of a mineral. Calcite, for example, will dissolve in weak acid, to produce calcium and bicarbonate ions.
The equation is as follows:. Limestone also dissolves at relatively shallow depths underground, forming limestone caves. This is discussed in more detail in Chapter 14, where we look at groundwater. The main processes of chemical weathering are hydrolysis , oxidation , and dissolution.
Complete the following table by indicating which process is primarily responsible for each of the described chemical weathering changes:. Sulfur dioxide and nitrogen gases create other types of acid rain that act as chemical weathering agents. Some sources of sulfur dioxide are power plants that burn coal; as well as volcanoes and coastal marshes.
Sulfur gases react with oxygen and rainwater to form sulfuric acid. Oxidation is another kind of chemical weathering that occurs when oxygen combines with another substance and creates compounds called oxides. Rust, for example, is iron oxide. When rocks, particularly those with iron in them, are exposed to air and water, the iron undergoes oxidation, which can weaken the rocks and make them crumble. Skip to main content.
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