Acid Oceans: Part 2

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Objective

SWBAT explain what happens to the pH of water when it mixes with CO2 and predict how this may impact ocean acidification.

Big Idea

Once your students have an understanding of acids and bases you can use this lesson to demonstrate how CO2 decreases the pH of water and relate it to effects of ocean acidification.

Getting Started

Focus question: 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 

 

In this lesson, students continue to explore ocean acidification by first visualizing how blowing CO2 into water impacts water. 

  • 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 rocks containing carbonates, such as limestone. This is how groundwater has formed underground caverns.
  • 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. 

This lesson is adapted from Schlumberger Excellence in Education Development (SEED)

Engage

10 minutes

To review the properties of acids and bases, have students create a pH foldable.

Using interactive graphic organizers helps to encourage students engagement in the material being studied, promotes science literacy skills, and retention of science information. There are several types of foldable organizers available in the marketplace. A Google search of “foldables” should provide you with a number of examples.

I've modeled how to create the foldable in these slides: 

Materials:

  1. Acid Base Foldable student handout (x2) 
  2. Scissors
  3. Glue or tape
  4. Colored markers

Directions:

  1. Fold a sheet of paper like a hot dog, leaving a ½ inch gap on one side.
  2. With the paper horizontal and the fold of the hot dog up, fold the right side towards the center, trying to cover one half of the paper.
  3. Fold the left side over the right side to make a book with three folds.
  4. Open the folded book and cut up the short folds on the shorter side only. This will from three tabs.
  5. Title each of the tabs from left to right, Acid, Neutral, Base.
  6. Paste a colored tab under each tab. Red:Acid, Purple:Neutral, Yellow:Base
  7. Write the following on the labels
    1. 0,1,2,3,4,5,6 on the red label
    2. 7 on the purple label
    3. 8,9,0,11,12,13,14 on the tallow label
    4. Open each tab.
    5. Use this to organize the terms. Cut out each item and glue them onto the inside of the tabs. Use both the whole length of the tab. 

Explore

30 minutes

In this activity, students use the pH indicator from the first lesson to explore what happens to the pH when they breathe CO2 into the solution. 

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. This gives 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.

  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 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.)

Ask students 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.)

Some additional ideas for investigation:

  • 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.

 

In the image below you will see an example of student responses to some summary questions for this lab.

Explain

15 minutes

When we blew gently into the cabbage water for several minutes, we observed that the color of the cabbage water changed from its original purple to a slightly lighter color. This indicates that the water is slightly more acidic.

What’s happening here?

When carbon dioxide is bubbled through water, some of it dissolves into the water. That is, some of the carbon dioxide goes into the spaces between water molecules. A small proportion of this dissolved carbon dioxide creates carbonic acid, a weak acid. The slight change of color of the cabbage water caused by the carbonic acid indicates that some of the carbon dioxide is being stored in the water.

 

Extend

25 minutes

A really great extension to this lesson comes from Stanford & Gothenburg Universities and their Virtual Urchin website. You could opt to complete the entire set of lesson, but I use the opening set of slides with my students to review and build the connection to ocean acidification. 

You will need computers with internet access. I group two students to a computer and ask them to use the slides to answer the Ocean Acidification Part 2 Extend Questions