Focus questions

• What happens when carbon dioxide gas is blown through water?

Essential materials

• two small transparent cups (about 100 ml or 3 ounces) or other similar containers
• drinking straw
• safety goggles
• sheet of paper or transparent plastic wrap
• paper towels
• pH indicator, such as cabbage water

Main ideas and background information

• Students may be surprised to learn that a gas can dissolve in water. However, they see evidence of this whenever they observe bubbles of carbon dioxide in a carbonated drink.
• When carbon dioxide is passed through water, some of it dissolves. A small fraction of the dissolved CO2 interacts with the water to become carbonic acid, H2CO3. Like other acids, this weak acid produces hydrogen ions. These ions react with other substances to produce the characteristic chemical behavior of acids.
• In this activity, the main idea is to look for a change in the pH toward the acidic range. This shift will take place as carbonic acid is created in the water. Drinking water usually has a pH of around 7.0 or so. Depending on local conditions, this could vary a little. Regardless of the initial drinking water pH value, creating carbonic acid by blowing bubbles into the water will shift the pH toward the acid range.
• Scientists have found that the top layers of our oceans are becoming more acidic due to carbon dioxide being absorbed from the atmosphere. This is changing the environment for the marine life living there.
• Sometimes surface water in a lake or stream becomes acidic by absorbing carbon dioxide from the atmosphere. This water can dissolve rock containing carbonates, such as limestone. This is how groundwater has formed underground caverns.
• For more details on the pH scale, see Digging Deeper.
• The calcium carbonate demonstration produces only a milky response because exhaled air contains only a small proportion (about 4%) of carbon dioxide. A much stronger response is produced by bubbling pure carbon dioxide through the limewater.

Procedural tips

• Provide two containers for the water. Students can blow bubbles through water in one container, and use the water in the other container for comparison purposes. The containers do not have to be identical, but they need to be transparent so that students can compare the color of the cabbage water in them.
• Cover the container with transparent plastic wrap prior to bubbling to prevent needless splashing.
• It is best for students to place the end of the straw as far into the water as possible and to blow gently into the straw. This will give the maximum exposure of the carbon dioxide bubbles to the water. It will also increase the rate of dissolving.
• Students should hold the container while bubbling for maximum stability and focus.
• Carbonic acid is very weak. It will take several minutes of blowing bubbles for the acidity of the water to change noticeably. Depending on the indicator the students are using, they probably will not be able to detect increasing acidity right away. In our experience, we thought that the color of the cabbage water changed slightly after about 10 minutes of bubbling.
• If possible, check the acidity of bubbled water with another type of indicator, such as pH test tablets.

Discussion

1. How do we know that bubbling carbon dioxide through water makes the water more acidic? (The indicator detects a pH level of less than the baseline reading, indicating a shift in the pH toward the acid range. If the baseline pH reading for the water was 7.0 (neutral), you would expect that the pH value after bubbling would be a value less than 7.0, which is in the acidic range. If the baseline pH reading for the water was 8.0 or more (alkaline), then the pH value after bubbling might not be in the acidic range. But you can expect it to be shifted toward the acidic range. Starting with pH 8.0 water, if you get a bubbling pH value of 7.3, the water is still alkaline; however, the pH value has shifted toward the acidic range.) 

2. How does the carbon dioxide make the water more acidic? (When carbon dioxide is bubbled through water, some of it dissolves in the water. A small proportion of the dissolved CO2 forms carbonic acid H2CO3. The hydrogen atoms in the carbonic acid behave like hydrogen ions and are available for chemical interactions with other substances. The hydrogen ions cause the acidic behavior.)

Assessment

Are students able to describe what happens when carbon dioxide gas is blown through water? (Some of the carbon dioxide dissolves in the water. A small proportion of the dissolved CO2 interacts with the water to form carbonic acid.)

Extensions and further investigations

• Have students measure the pH change for varying periods of time of blowing bubbles.

• Repeat the experiment with salt water, and then with sugar water.

Career connections

• Chemical engineering

• Geology

• Environmental engineering

Correlations with Standards

United States: This activity correlates with portions of NSES Content Standard A, Science as Inquiry, and Content Standard G, History and Nature of Science, Grades 5-8 and 9-12, and with the following additional standards:

Grades 5-8
Earth and Space Science: B1

Grades 9-12
Physical Science: B3
Earth and Space Science: D2

Glossary/vocabulary

acid
carbonic acid
pH

Resource links

Carbonated water, from Wikipedia, the free encyclopedia

Background information on carbonated water.

Rain, from Wikipedia, the free encyclopedia

Mentions how ordinary rain dissolves some of the atmospheric carbon dioxide.

Carbon Cycle, Virtual Chembook from Elmhurst College

Information about several cycles that include atmospheric carbon dioxide.

Precipitation and Redissolution of Calcium Carbonate

Additional details about the calcium carbonate demonstration.

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