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Investigation Question 4:
Do Rocks Dissolve?

What To Do

• Setting the scene
• Presenting the Investigation Question
• Assessing What Your Students Already Know
• Exploring the Concept

Begin the investigation by reminding students of the abrasion activity they did in the last investigation.  Continue with the following questions:

1. What caused the rocks to break apart?
2. What other processes are responsible for breaking up rocks?  How do they work?

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.

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After the scene is set, introduce your students to the investigation question:

1. Do rocks dissolve?

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. 

2. How would you design an experiment that could be used to test the investigation question? 
3. What materials would be needed? 
4. What would you have to do?
5. What would be measured?
6. How long would the experiment take?

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.

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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:

1. What happens when you mix salt, sugar, or a drink mix in water?
2. If you mixed rock in water, could the same thing happen?  Why or why not?
3. What causes solids to dissolve in water?

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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1. Explain to students that rainwater is not the same as the water they drink.  Rainwater is a weak acid.  Tell them that they will be looking at the effects of rainwater on the weathering (breakdown) of rocks.  They will be putting chalk, which is similar in composition to limestone, into vinegar, which is a weak acid, much like rainwater.  They will observe the chalk over time to see whether or not the acid has any effect on the chalk.  They will also be putting chalk in water for comparison.
2. Break students into groups of 3 or 4.
3. Provide the materials to the groups.
   

Observation Sheet Word Document (30 KB)   Observation Sheet Adobe PDF (12 KB)

4. Instruct groups to cover their desktops with paper.
5. Provide the following instructions to groups:
1. Use the magnifying glass to examine each piece of chalk.  Draw or describe what each piece of chalk looks like on the observation sheet. 
2. Pour vinegar into one clear plastic cup until it is about 2/3 of the way full.  Use the masking tape to write “acid” and put the label on the cup.
3. Pour water into one clear plastic cup until it is about 2/3 of the way full.  Use the masking tape to write “water” and put the label on the cup.
4. Predict what will happen to the chalk after it sits in the vinegar for one hour.  Record your prediction on the observation sheet.
5. Place one piece of chalk in each plastic cup.
6. Store the cups in a safe place.
7. After 1 hour, examine the chalk with the magnifying glass. Draw or describe on the observation sheet what each piece of chalk looks like.
8. Predict what will happen to the chalk after it sits in the vinegar for a total of 24 hours.  Record your prediction on the observation sheet.
9. After 24 hours, examine the chalk with the magnifying glass.  Draw or describe on the observation sheet what each piece of chalk looks like.
10. On the observation sheet, write a conclusion that explains your observations.

6. Discuss students’ findings as a class.  Prompt them with the following questions:
1. Which piece of chalk changed the most, the one soaking in water or the one soaking in vinegar?  Why?
2. How did the chalk change after soaking in vinegar for 1 hour?  After 24 hours?  What may have caused these changes?
3. Was there more of a change after 24 hours?  Why?
4. How did your predictions compare with your results?
5. How does what you observed compare to what happens when rainwater falls on rocks?
6. How long does it take to dissolve rocks in nature?
7. Help students to understand that when rainwater mixes with carbon dioxide in the air or carbon dioxide in air pockets in soil, a weak acid called carbonic acid is produced.  When carbonic acid flows through the cracks of some rocks, it chemically reacts with the rock causing some of the rock to be dissolved.  Over many thousands of years, much rock can be dissolved. 

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