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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.
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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.
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