Now You See It, Now You Don't! How Acidic Waters Make Rocks Disappear
AbstractIf you have ever prepared a cup of coffee or tea with sugar, you have probably seen that the grains of sugar quickly dissolve and completely disappear in hot water. But sugar is not the only type of solid that can readily dissolve in a liquid. In fact, there are some types of rocks that can be dissolved by common liquids. It might be hard to imagine large, hard rocks being eaten away by some ordinary fluids, right? But it actually happens all the time! In this geology science project, you will investigate how one type of rock—specifically, limestone—can be dissolved by acidic water. How much acid will be needed to dissolve limestone? How quickly will the rocks dissolve? Get ready to try out some geochemistry in this science project to find out!
Teisha Rowland, PhD, Science Buddies
ObjectiveInvestigate how acidic water can dissolve limestone rocks.
Why can some substances dissolve, or disappear, in liquids while others cannot? It has to do with the solubility of the substance. The term solubility refers to how well a substance is dissolved by something else. Solubility depends on the chemical composition of a substance. Rocks, just like everything else, are made out of chemicals and have certain chemical compositions, which affects their solubility. The study of the chemistry of rocks is called geochemistry.
There are three basic types of rocks: igneous rocks, sedimentary rocks, and metamorphic rocks. This project focuses on sedimentary rocks. Sedimentary rocks are formed from particles of rocks, minerals, and organic matter (such as dead animals, leaf litter, feces, etc.) that collect and become compacted together on Earth's surface over time, sometimes forming distinct layers. For example, see the sedimentary rock layers in Figure 1.
Figure 1. The top part of this picture shows sedimentary rock layers on a hill in Death Valley National Park, at Zabriskie Point.
Some types of sedimentary rocks can chemically react with acids. Acids are substances that have a pH that is less than 7, such as lemon juice or battery acid. pH is a scale that measures how acidic or basic something is. A base has a pH that is above 7, such as baking soda or bleach, while a neutral substance has a pH of 7, such as drinking water. For a refresher, see the Science Buddies page on Acids, Bases, & the pH Scale. The types of sedimentary rocks that react with acids contain carbonate compounds. For example, the most common of these rocks is limestone, which is made of calcium carbonate (CaCO3). As shown in Equation 1, when calcium carbonate is exposed to an acid (demonstrated using sulfuric acid, H2SO4, which is the primary acid in acid rain), the carbonate compound (CO32-) reacts to create carbon dioxide (CO2) gas and water (H2O).
When limestone rock is dissolved by acidic waters, it can form interesting features on the landscape, and these formations are collectively called karst topography. The most common and readily recognizable of these is probably sinkholes. Sinkholes are holes, or depressions, in the ground that are usually caused by an overlying surface collapsing inward (imagine the roof of a cave collapsing). A sinkhole in the Arab state of Oman is shown in Figure 2.
Figure 2. This is the Bimmah sinkhole in the Arab state of Oman.
Sinkholes can take many years to form (potentially thousands or millions of years), or may be created suddenly. In fact, a sinkhole can be made so quickly that they can rapidly destroy roads or buildings; to see how this happens, watch the video on sinkholes below. As shown in the video, sinkholes can also vary greatly in size. Often, acidic groundwater is the culprit; it can dissolve supportive calcium carbonate in a layer of limestone rock, which gets washed away through the process of erosion. When the supportive limestone layer is gone, the ground above it falls down, creating a hole.
The pH of groundwater is generally about 5.6, and if it is below this then it is considered to be acidic groundwater. Acidic groundwater has several potential causes. For example, acidic groundwater can be created if groundwater is contaminated with acid rain, which has a pH of about 4.0. Groundwater can also become acidic if there are high amounts of iron that it can react with. Increasing atmospheric carbon dioxide levels can also contribute to the acidification of water. If acidic groundwater comes into contact with carbonates, like calcium carbonate in limestone, this can help to neutralize the groundwater.
In this geology science project, you will investigate how acidic water can dissolve limestone rocks. You will do this by soaking several limestone rocks in acidic solutions and measuring the mass of the rocks over time. You will make your acidic solutions using vinegar, which contains acetic acid and is a common acid with a pH of about 2. Some of the acidic solutions you will make are more acidic than acidic groundwater typically is; this is so that you can see results more quickly, and are not stuck spending years completing your science project! You will also investigate how the pHs of the solutions change over time. Do you think they will become more acidic or more basic? Get ready to do some geochemistry to find out!
Terms and Concepts
- Chemical composition
- Sedimentary rocks
- Calcium carbonate
- Acid rain
- Karst topography
- Acidic groundwater
- What are the three basic different types of rocks? How are they different?
- What is produced when limestone reacts with an acid?
- Karst topography includes sinkholes, but what other features can be made by the dissolving of limestone?
- How do you think the pH of an acidic solution will be affected when limestone is dissolved in it?
To find out more about the three basic types of rocks, check out this webpage:
- Strickler, M. (n.d.). Ask GeoMan... What are the 3 basic types of rocks? University of Oregon. Retrieved January 14, 2014.
To find out more about acid rain and water acidification, visit these webpages:
- Casiday, R., and Frey, R. (n.d.). Acid Rain: Inorganic Reactions Experiment. Washington University: Department of Chemistry. Retrieved January 14, 2014.
- Government of Western Australia. (2009, January). Introduction to acidic saline groundwater in the WA Wheatbelt – characteristics, distribution, risks and management. Department of Water. Retrieved January 14, 2014.
- # Link Name="Geo_p047.5" Value="HtmlAnchor" HtmlText="Oceanus Magazine" #]. (2010). Ocean acidification: a risky shell game. Woods Hole Oceanographic Institution. Vol. 48, No. 1. Retrieved January 14, 2014.
You can watch an exciting video about sinkholes here:
- DiscoveryNews.com. (2013, March 2). How Scary Sinkholes are Formed. Retrieved January 15, 2014.
Materials and Equipment
- Limestone rocks (16). The rocks should be less than 50 g each.
- Scale, accurate to 0.1 g
- Clean glass jars (17). The jars should be wider than the rocks, at least 2–3 cm taller than the rocks, and hold at least 200 mL. Jars that are around 15 oz should work well for this. Lids are not required. Jars that meet these criteria are available as 12-packs from Carolina Biological Supply Company.
- Graduated cylinder, 100 mL
- pH test strips or pH test paper, with a range of at least pH 1–6 (at least 80 test strips or one roll of test paper)
You will also need to gather these items:
- Sticky notes (16) or small pieces of paper and tape
- White vinegar, distilled or not distilled (at least 1.4 L). This is available at most grocery stores.
- Distilled water (at least 2 L). This can be purchased at most grocery stores.
- Optional: Camera
- Disposable gloves. Can be purchased at a local drug store or pharmacy, or through an online supplier like Carolina Biological Supply Company. If you are allergic to latex, use vinyl or polyethylene gloves.
- Rags (2). One should be soft, without ragged or torn edges that could catch when drying the rocks. The other should be large enough to put all 16 rocks on it, spaced apart.
- Lab notebook
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Testing the Rocks
- In your lab notebook, make a data table like Table 1, and a second data table like Table 2. You will record your results and observations in these data tables.
|Jar Number||Test Solution||Day 0||Day 1||Day 2||Day 3||Day 4|
|Mass Measurements (in grams [g])|
|Rock Number||Test Solution||Day 0||Day 1||Day 2||Day 3||Day 4|
- Arrange the 16 limestone rocks in four groups based on their size. Specifically, one group should have the four largest rocks, one group should have the four next largest rocks, one group should have the next four largest rocks after that, and one group should have the four smallest rocks. Note: Use the scale to help you group the rocks this way.
- Rearrange the rocks into four new groups, where each new group has one rock from each of the current groups.
- In other words, each new group should have a large rock, a small rock, and a rock from each of the two in-between sizes, as shown in Figure 3.
- Once you have made the four new groups, carefully set them aside for now. You will use them in step 7.
Figure 3. Rearrange the groups of rocks you made in step 2 so that each group now has one rock from each of the original groups (in other words, one large rock, one small rock, and two mid-sized rocks), as shown here.
- Now take 16 of the glass jars and label them 1–16.
- You can do this using sticky notes, small pieces of paper attached with tape, or a permanent marker.
- Use the 100 mL graduated cylinder to fill each of the jars with 200 milliliters (mL) of their test solution using the following approach:
- Fill jars 1–4 each with 200 mL of distilled water. These will be your water-only controls.
- Fill jars 5–8 each with 50 mL of 100% vinegar and 150 mL of distilled water. This will make 25% vinegar solutions.
- Fill jars 9–12 each with 100 mL of 100% vinegar and 100 mL of distilled water. This will make 50% vinegar solutions.
- Fill jars 13–16 each with 200 mL of 100% vinegar.
- Note: 200 mL should be enough solution to completely cover one rock in the jar, and then have at least 2–3 centimeters (cm) of solution above the rock.
- To check this, do not put a rock in any of the jars yet, but place a rock next to the jar and look to see how much solution will likely cover it, as shown in Figure 4.
- If it looks like the jars need more solution in them, then add some, being sure to have the same total amount of solution in each jar (and only ever adding the same test solution to the same jar). You can make up more of the solution in the remaining empty glass jar you have (by repeating steps 5.a.–5.d., filling the jar with one solution at a time), use the graduated cylinder to measure out an amount to add to the jar with the solution, and then add it.
Figure 4. Do a quick visual check to make sure the amount of solution in each jar is enough to easily cover one of the limestone rocks (leaving at least around 2–3 cm of solution above it).
- Use the pH test strips (or test paper) to determine the pH of the solution in each of the 16 jars. Record your results in the data table in your lab notebook that is similar to Table 1. Specifically, record each pH value in the row for the correct jar number, in the "Day 0" column.
- Follow the instructions on the pH test strip packaging. You will need to match the color that each test strip changes to (after being dipped in the solution) to determine the solution's pH.
- Now take the remaining empty glass jar, fill it with at least 200 mL of distilled water, and then take one of the rock groups you made in step 3, and do the following:
- If there is a small, circular tag on any of the rocks, try to carefully remove as much of it as possible. If some tag is remaining on the rock, it is okay.
- Briefly dunk one of the rocks in this group in the glass jar you just filled with distilled water. Using a soft rag, gently blot the water off of the rock's surface. You do not want the rock to be dripping wet, but also do not want to wipe it so much that pieces of the rock break off. Note: You are getting the rocks wet in this step so that you can more accurately compare their damp mass now to their damp mass on the following days.
- Zero out the scale, if needed, and then weigh the rock on the scale. Record the rock's mass (in grams [g]) in the data table in your lab notebook that is similar to Table 2. Specifically, record this in the row for rock number 1, in the "Day 0" column.
- Put this rock in jar 1.
- What happens when you put the rock in the solution? Record any observations in your lab notebook.
- Note: If you want, you can take pictures of each of the rocks (with labels) right before you put them in the jar. This way you can compare how they looked at the beginning of the experiment to how they will look at the end of it. Pictures can go on Science Fair Project Display Boards.
- Repeat steps 7.a.–7.d. with the three other rocks in this group.
- For example, for the second rock you pick, record its mass in the row for rock number 2 and put it in jar 2.
- Repeat step 7 three more times so that all 16 rocks have been weighed and placed in the correct jar.
- For example, in the second group of rocks, the first rock you pick would be rock 5, and should be placed in jar 5.
- You should end up with one rock in each jar.
- Be sure to record any observations you make in your lab notebook when you add each rock to its jar.
- Record the time in your lab notebook when you are done with this step.
- When you are done placing each rock in its jar, watch the jars for a few more minutes. What do you see happening? Record any additional observations you make about each jar in your lab notebook. If you want, you could make an additional data table just to record your observations in.
- Tip: If you are not sure why you see what you do, try re-reading the Introduction, paying particular attention to the carbonate reaction shown in Equation 1.
- After about 24 hours, do the following:
- Check the pH of the solution in each jar and record the pH value in the data table in your lab notebook. Record it in the "Day 1" column.
- Weigh each rock by doing the following:
- Put on a pair of disposable gloves.
- Carefully remove rock 1 from its jar.
- Using a soft rag, gently blot water off of the rock's surface. You do not want the rock to be dripping wet, but also do not want to wipe it so much that pieces of the rock break off.
- Weigh the rock on the scale and record its mass in your data table. Record it in the "Day 2" column.
- Gently place the rock back in its jar.
- Repeat steps 10b.ii.–10.b.v. for each of the other 15 rocks. When going between different test solutions (such as weighing rock 4 and then weighing rock 5), briefly rinse the gloves with water and then dry them.
- Repeat step 10 at about 48 hours ("Day 2"), 72 hours ("Day 3"), and 96 hours ("Day 4") after starting the experiment. Record all of your data in the data tables in your lab notebook for the appropriate day. Also be sure to write any observations you make in your lab notebook.
- Note: If you want, you can take pictures of each of the rocks (with labels) on Day 4. If you also took pictures at the beginning of the experiment, you can compare how the rocks looked then compared to how they look now. Pictures can go on Science Fair Project Display Boards.
Analyzing Your Results
- Make a line graph of your data from Table 1. Put the time (in days or hours) on the x-axis and the pH on the y-axis. Make a line for each of the 16 jars.
- If you want, you can make a separate graph for each group. For example, you would have one graph for the 100% vinegar jars, one graph for the 50% vinegar jars, etc.
Make a second line graph of your data from Table 2. Put the time on the x-axis (in days or hours) and the mass (in grams) on the y-axis. Make a line for each of the 16 rocks.
- Again, if you want, you can make a separate graph for each group.
- Now, in your lab notebook, create a data table like Table 3. Do some calculations to determine what percentage of mass each rock was, over time, compared to the initial mass of that rock (making the initial mass, on Day 0, equal to 100%). Fill out your data table based on these calculations.
- For example, if the mass of a rock on day 0 was 35.5 g, and on day 1 it had a mass of 30.6 g, and then on day 2 it had a mass of 29.8 g, its percentage of the initial mass for day 1 would be 86.2%, and for day 2 it would be 83.9%. This is because 30.6 g divided by 35.5 g equals 0.862, which is the same as 86.2%, and 29.8 g divided by 35.5 g equals 0.839, which equals 83.9%.
|Percentage of Initial Mass|
|Rock Number||Test Solution||Day 0||Day 1||Day 2||Day 3||Day 4|
Table 3. In your lab notebook, make a data table like this one to record the percentage of the initial mass that each rock was over time. The "Initial Percentage" column has already been filled out, as it should be set to 100% for each rock.
- Repeat step 2 to make additional graphs, but this time use the data from Table 3. Put the percentage of the "Day 0" mass on the y-axis.
- Analyze your results and try to draw some conclusions. If you find it helpful, you can make additional graphs to help look for relationships between different variables, but this may not be necessary.
- How did the pH change in the jars over time? Did it go up, down, or stay at the initial pH? Did it change differently for the different test solutions? Can you explain your results?
- How did the mass of the rocks change over time? Did the masses change differently for the different test solutions? If so, how? Can you explain your results?
- How did the percentage of the vinegar solution (e.g., 100% vinegar vs. 25% vinegar) correlate with how the pH and/or mass of the rocks changed over time? Did one group lose more mass than the others? Do your results make sense to you?
- Compare the mass change over time of the smallest rocks in each test solution group to that of the largest rocks in the same group. Does the size of the rock appear to correlate with how it changed mass over time?
- Based on these comparisons, do you think you should average your data for each test solution group? Why or why not?
- Do you see any correlations between how the pH changed and how the mass of the rocks changed?
- Did any of the observations you made correlate with pH and mass data changes over time? Can you explain your results?
- Overall, what do your results tell you about how acidic groundwater affects limestone rocks and the surrounding environment? Hint: If you are stumped, try rereading the Introduction. You may also want to consider the pH of the solutions you tested compared to what acidic groundwater may typically be.
Ask an Expert
- In this science project, you tried 100% vinegar, 50% vinegar, and 25% vinegar solutions, but you could try a wider range of vinegar concentrations. How do different concentrations affect how the limestone rocks dissolve?
- Do different types of acids affect how quickly, or how well, the limestone rocks are dissolved? To find out, try this experiment again but compare some different types of acids, such as lemon juice, orange juice, tomato juice, and milk. Does the strength, or the pH, of the acid affect how quickly the rock dissolves? For more ideas, check out the Science Buddies resource Acids, Bases, & the pH Scale. Always be sure to follow the proper safety precautions when using different chemicals.
- In this science project, you may not have seen the rocks dissolve completely, but could this happen if they are left in the acid solution long enough? You could do this experiment for a longer amount of time to try and find out!
- There are other types of sedimentary rocks that contain carbonate compounds, such as dolostone, or dolomite, which is made of calcium magnesium carbonate. See if you can find a source of other rocks containing carbonate compounds like this and then try repeating this experiment. How well do other carbonate rocks dissolve compared to limestone? Be sure to do your background research so you understand the chemical reactions taking place and make sure it is safe to test them!
- Are other types of rocks—ones that do not contain carbonate compounds—dissolved by acidic water, or even typical, neutral water? If so, do they dissolve more than carbonate rocks? Research this and then test it out!
- Sinkholes are a karst topography formation, as discussed in the Introduction, but there are other features formed by karst topography. How are they different, or similar to, sinkholes? Can you devise a way to model sinkholes or other types of karst topography?
- In this science project, the 100% vinegar solution tested should have had a pH of about 2. However, acidic groundwater often has a pH that is not quite so acidic; it only has to be 5.5 or less to be considered acidic groundwater, and acid rain (which can lower the groundwater's pH) only has a pH of 4. Some of the solutions you tested may have had slightly higher pH values, such as around 3 to 4. Try making a solution with a pH around 4–5.5 and repeat this science project with it. How does this slightly higher pH solution (which is still acidic) affect the dissolution of the limestone rocks? How does it compare to the more acidic solutions?
- How do much larger, or much smaller, limestone rocks compare when being dissolved in acidic water? To try this, you could take some rocks and break them into smaller pieces, or use bigger containers to test larger rocks. Just be sure you do not make them so small that their mass is too little for your scale to measure!
- Increasing atmospheric carbon dioxide (CO2) levels can make waters more acidic. Devise a safe way to test this out. How much additional CO2 is needed to lower the pH of water by a certain amount? How does this compare to how CO2 levels are actually increasing in the atmosphere? What are the implications for substances with carbonate compounds that are in the water, such as limestone, but also shells and corals, which also have calcium carbonate in them?
- Water can react with high amounts of iron to create acidic water. Just how much iron is needed to lower the pH of water by a certain amount? How much iron is needed to make the water acidic enough to dissolve significant amounts of limestone? Devise a way to investigate this.
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