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What to do |
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Assessment |
In the last investigation, students began thinking about how rocks change through time. Students were introduced to the concept of weathering and looked at abrasion as one of several physical weathering processes. In this investigation, students explore how rocks weather chemically. Chemical weathering is the process that breaks down rock through chemical changes. The most common agents of chemical weathering include water, oxygen, carbon dioxide, and living organisms. Chemical weathering creates holes or soft spots in rock, so the rock breaks apart more easily. Chemical and mechanical weathering often go hand in hand; mechanical weathering breaks rock into pieces, exposing more surface area to chemical weathering. In this investigation, students examine the effects of naturally formed acids on the breakdown of rock. When water (e.g. rainwater) mixes with carbon dioxide gas in the air or in air pockets in soil, a weak acid solution, called carbonic acid, is produced. When carbonic acid flows through the cracks of some rocks, it chemically reacts with the rock causing some of it to dissolve. Carbonic acid is especially reactive with calcite, which is the main mineral that makes up limestone. Over many thousands of years, the dissolving action of carbonic acid on limestone sometimes produces underground caves.
In this investigation, students will simulate the effect of carbonic acid on limestone. Pieces of chalk represent limestone and vinegar represents carbonic acid. Students place the chalk into the acid and observe how it changes over time.
For each student group:
For instructional purposes:
This investigation is generally considered safe to do with students. Students should be reminded not to drink the water or vinegar. Also, review the investigation for your specific setting, materials, students, and conventional safety precautions. Be sure to remind students to wash their hands when they finish.
Begin the investigation by reminding students of the abrasion activity they did in the last investigation. Continue with the following questions:
Have your students discuss these questions, first in pairs, then groups and then as a whole class. Record their answers on a flipchart that you can refer to throughout the investigation.
After the scene is set, introduce your students to the investigation question:
Have your students discuss the question in pairs, then in groups, and then as a whole class. Record their answers on a flipchart.
Have your students brainstorm ideas about how this investigation question could be investigated.
Tell your students that they will be investigating this question and at the end of their study they will be able to provide reliable answers.
Students will have had some experiences with solids dissolving in liquid, e.g. mixing salt, sugar, or a drink mix in water. Students will probably not recognize that rocks can dissolve in water. They will also not know that rainwater is naturally slightly acidic.
Here are some initial questions that your students can discuss, in pairs, then in groups:
Have your students share their ideas with the class and record them as a list on a flipchart.
Have students think about what they would like to learn about how rocks dissolve. Record their ideas on the flipchart as a list called “Questions we have about how rocks dissolve.” This list will provide further insights into what your students know, and also what they would like to know. By the end of the investigation, some of these questions will probably be answered.
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Show students the Images of Chemical Weathering pictures. Have students answer the following questions related to the pictures:
Complete this investigation by asking your students the following:
As a result of this investigation, students should be able to state that certain rocks can dissolve when exposed to rainwater, which is slightly acidic. This exposure increases the breakdown of rocks into smaller and smaller pieces.
The following passage provides more detailed information related to this investigation that you may choose to explain to your students.
Chemical weathering is the decomposition of rocks due to chemical reactions occurring between the minerals in rocks and the environment. Following are examples of the chemical weathering processes.
Water, and many chemical compounds found in water, is the main agent of chemical weathering. Feldspar, one of the most abundant rock-forming minerals, chemically reacts with water and water-soluble compounds to form clay.
Water contains many weak acids, such as carbonic acid. This weak, but abundant, acid is formed when carbon dioxide gas from the atmosphere mixes with rainwater. Sulfur dioxide and nitrogen gases create other types of acids 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. Although relatively weak, this acid’s abundance and long term effects produce noticeable damage to vegetation, fabrics, paints and rocks.
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.