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How do Sinkholes Form? (9 Favorites)

LAB in Observations, Chemical Change, pH, Acid Base Reactions, Chemical Change. Last updated February 7, 2019.


Summary

In this lab students will learn how acidic groundwater reacts with limestone, causing it to erode. Sinkholes form when eroded limestone underneath the surface of the earth can no longer support the ground above it. Students will test how acid reacts with a variety of rocks and determine which rocks would be best to build a city on top of in order to reduce the chance of sinkholes forming.

Grade Level

Middle and High School

NGSS Alignment

This lab will help prepare your students to meet the performance expectations in the following standards:

  • MS-PS1-2: Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
  • MS-ETS1-3: Analyze data from tests to determine similarities and difference among several design solutions to identify the best characteristics of each that can be combined into a new solution to better meet the criteria for success.
  • Scientific and Engineering Practices:
    • Analyzing and Interpreting Data
    • Constructing Explanations and Designing Solutions
    • Engaging in Argument from Evidence
    • Obtaining, Evaluating, and Communicating Information

Objectives

By the end of this lab, students will be able to

  • Identify a pH value as either acidic, basic, or neutral.
  • Determine that acid reacts with limestone to cause a chemical reaction.
  • Explain how sinkholes form.
  • Infer the effects of acid rain on statues made of limestone and other carbonate rocks.

Chemistry Topics

This lab supports students’ understanding of

  • Chemical Reactions
  • Chemical Change
  • Chemical Weathering
  • Observations
  • Acids and Bases
  • pH
  • Acid-Base Reaction

Time

Teacher Preparation: 15 minutes

Lesson: 45 minutes

Materials

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • When working with acids, if any solution gets on students’ skin, they should immediately alert you and thoroughly flush their skin with water.
  • When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.
  • Students should wash their hands thoroughly before leaving the lab.

Teacher Notes

  • Groups of two or three students is ideal. If there are enough supplies, students may also work individually.
  • If needed, substitutions can be made for the granite and basalt. Make sure substitutes are not carbonate rocks (do not use dolostone, chalk, or marble).
  • Rocks may be purchased from sites such as Flinn Scientific.
    • Note that on this site many rocks are sold as a package of ten rocks.
  • Since the rocks are durable, they may be reused each year.
  • Small cups and dropper pipettes can be used if you do not have dropper bottles available.
  • The cool thing about this activity is that it can be adapted for several different units depending on what your students are learning and their abilities. For example, you could focus more on the acid/base reaction or challenge them to consider the chemical equations involved.

Background:

  • Physical Weathering occurs when rocks break up into smaller pieces, but their chemical makeup does not change.
  • Chemical Weathering occurs when chemical reactions cause the rock to disintegrate. Ions, such as Ca2+, that are freed from the rock can be carried away and dissolved in water.
  • Groundwater is naturally acidic because of dissolved carbon dioxide from the atmosphere or decaying organic matter. As this acidic groundwater passes over carbonate rocks, such as limestone, an acid/base reaction occurs and the ions that make up limestone disassociate.
    • Limestone (CaCO3) breaks apart into a calcium ion and a carbonate ion (CO32-). The carbonate ion combines with a hydrogen from a water molecule to form bicarbonate (HCO3-).
    • Chemically, this reaction looks like: H2O(l) + CO2(g) + CaCO3(s) → Ca2+(aq) + 2HCO3-(aq)
  • This website contains more information about the reaction between carbonate rocks and acids, and has a video showing what the reaction should look like.
  • This website is a very comprehensive source; it is long, yet easy to read or skim.

Lab Outline:

  1. Prepare two dropper bottles for each group of students and label them 1 for water and 2 for acid. Students will have to determine which bottle is the acid and infer what is in the other bottle.
  2. Make sure there are enough rocks for every group to have a sample of each type of rock.
  3. Before handing out the student activity, show the students photos or videos of sinkholes to spark their interest. Explain why sinkholes are relevant and how they can be dangerous.
  • These photos show numerous sinkholes from all over the world.
  • This video does a great job showing the sinkholes and explaining the problems they can cause. However, around the 2 minute mark, the video mentions limestone, which gives away the lab results. You can either skip that section or show half the video before the lab, and the rest when the lab is complete.
  • Remind students that should they ever see a sinkhole, it is best to keep their distance and not to approach it.
  1. Students should complete the pre-lab questions either as a class discussion or in groups.
  2. Instruct students to complete the lab in their groups.
  3. After the activity is completed and turned in, have a class discussion to go over results and discuss sinkholes in more detail.
    1. This would be a good time to introduce students to the information in the “Background” section. Seeing the chemical formulas may also help students to understand what they saw happen to the limestone.
    2. Maps of where sinkholes are prevalent in the U.S. can be used to discuss which places may be safer than others to live.
    3. Pictures and descriptions of karst topography can show students landscapes where sinkholes are prevalent and where much of the carbonate rock has been eroded away.
    4. Maps and diagrams from the USGS may be helpful visual aids.

Expected Results:

  • When acid is added to the limestone, the reaction produces many small, but visible bubbles.
  • When acid is added to basalt and granite, there should be no signs of bubbles, and therefore, no signs that a reaction has taken place.
  • None of the rocks should show a reaction when a drop of water is added.

For the Student

Lesson

Background

Sinkholes form when rock underneath the surface of the earth gets eroded by acidic water. When the rock is unable to support the ground above it, the ground collapses. Sinkholes are becoming increasingly common in states such as Florida and Kentucky; these sinkholes can appear in the road, outside shopping malls, or even in someone’s backyard!

Prelab Questions

  1. List some acidic compounds that you have heard of in your science classes, as well as some you encounter in everyday life.
  2. How can you test if a liquid is acidic or not?
  3. The pH of pure water is 7 (neutral). Do you think that water can become acidic? Explain your thoughts.

Objective

A big sinkhole has appeared in the city of Rockville! The city council is worried that more sinkholes might appear and has voted to move the entire city. You and your team have been hired by the mayor’s office to choose a new location for the city. Your job is to investigate which rock is most likely underneath the current city, and determine what type of rock the new city of Rockville should be built on.

Materials

  • Dropper bottles (2 per group)
  • 1 of each type of rock per group: limestone, granite, basalt.
  • pH paper

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • When working with acids and bases, if any solution gets on your skin immediately rinse the area with water.
  • Follow the teacher’s instructions for cleanup of materials and disposal of chemicals.
  • Wash your hands thoroughly before leaving the lab.

Procedure

  • Complete pre-lab questions in your group.
  • Collect pH paper, two dropper bottles (labeled “1” and “2”) and one of each type of rock from your teacher. Make sure you know the name of each rock.
  • You will need to determine what type of liquid is in each dropper bottle. To do this, squeeze one drop of liquid from dropper 1 onto a piece of pH paper. Compare the color change of the pH paper with the associated pH chart. Record your results in Table 1.
  • Repeat step 3 using the second dropper bottle and a new piece of pH paper.
  • Next, you will test how these two liquids react with different rocks. Select a rock and add a drop of liquid from each bottle to the surface of the rock. Record your observations (any indicators of chemical change) in Table 2.
  • Repeat step 5 for each rock that you have.

Results and Observations

Table 1: pH Test Results

Dropper pH Acidic, Basic, or Neutral?
1
2

Table 2: Rock Observations

Rock Type Observations
Dropper 1 Dropper 2
Basalt
Granite
Limestone

Analysis

  1. Which dropper bottle contained an acid? Explain how you know.
  2. What do you think was in the other bottle? Justify your answer.
  3. Which rock(s) reacted with the acid? Did the reaction indicate that a physical or chemical change occurred?
  4. Given what you have learned about how sinkholes form, which rock(s) likely caused the sinkholes in Rockville? Or in other words, which rock(s) do you think is underneath the city of Rockville? Be sure to explain your answer using evidence from your experiment.
  5. The city council of Rockville has found three possible locations for the new city. The first location would have granite underneath the city, the second location would have limestone underneath the city, and the third location would have basalt underneath the city. Which location(s) would be the safest for the city to move to? Explain your reasoning.
  6. Dolostone, which is another type of rock, is known to produce a small amount of fizz (gas) when it interacts with an acidic substance. The Rockledge Company is thinking of making an outdoor statue out of dolostone and has asked for your advice. Since you are an expert scientist, you know that the pH of rainwater is 5.5. Do you think this statue is a good idea? Why or why not?
  7. Based on the results of your testing and what you now know about dolostone, could there be a connection between dolostone and any of the rocks in this activity? If so, what do you think it is? (Things to consider: Do the rocks have anything in common? What may cause them to share physical or chemical properties?)
  8. Challenge: The chemical reaction that causes sinkhole formation is: H2O(l) + CO2(g) + CaCO3(s) → Ca2+(aq) + 2HCO3-(aq). A calcium ion and a bicarbonate ion are products of the reaction. What role do you think the carbon dioxide gas (CO2) plays in this reaction?

Conclusion

In your own words, explain how sinkholes form. Be sure to include why they form, how to tell if an area may be affected by sinkholes, and the role that acidic substances play.