What do soda and the oceans have in common?

Students will use soda to explore how carbon dioxide is able to dissolve into liquid. They will learn about Henry's law, which describes how the solubility of gas into liquids is dependent on temperature, and develop hypotheses about how the amount of carbon dioxide in the atmosphere, a greenhouse gas, is affected by rising atmospheric and oceanic temperatures.

Learning Objectives

• Students will learn how atmospheric carbon dioxide gas (CO₂) dissolves in water and that temperature is a key factor in the amount of gas that is dissolved in water (Henry's law)
• Students will relate this gas law to Earth Systems science and climate change by considering how rising atmospheric and oceanic temperatures could impact ocean uptake of CO₂.

Materials

• Computer
• Cold bottle of soda
• Warm bottle of soda
• Small paper cups
• Stir sticks
• Student reading (under the "student" tab of this activity)
• Student notebook or journal

Directions

1. Introduce the activity to students (you may want to frame it as CO₂ the GHG or how a gas gets into a liquid, or ocean acidification, or something else). Tell students that the oceans play a dominant role in the uptake of anthropogenic carbon yet this process is poorly represented by models and its future trajectory remains highly uncertain. Ask students if they have heard that the oceans are carbon sinks. Ask students if they have wondered how atmospheric carbon dioxide (CO₂) is actually absorbed by the ocean?
2. Students observe bottle of soda and bubbles coming out and discuss the impact of pressure.
   • Give each student group a room temperature bottle of soda with the cap still screws on tightly. Ask the students to consider a topic that is familiar to them, a bottle of soda.
   • Students write down observations in a notebook using the following questions as a guide:
     • What happens when you open a bottle of soda?
     • What happens if you slowly unscrew the top, wait a few seconds and then slowly screw the top back on?
     • What makes the bubbles?
     • Where was the CO₂ before the bottle was opened?
     • Why do you think bubbles rush out of the soda bottle when you open it?
3. Students read about Henry's Law. Discuss the main points:
   • Gases can dissolve in liquid and is dependent on the chemical nature of the gas (solute) and the liquid (solvent), temperature, the partial pressure of the gas above the liquid.
   • The scientific law that explains this is Henry's law.
   • The dissolved carbon dioxide stays in the soda solution in the closed soda bottle, where the partial pressure in the space above the soda but below the cap was initially set high during bottling.
   • High partial pressure means more carbon dioxide will go into solution in the liquid.
   • When the bottle is opened the partial pressure above the liquid decreases allowing the carbon dioxide to come out of the solution.
4. Provide the students with two equal-sized samples of soda in cups, one warmed to around 100 degrees F and the other cooled to around 40 degrees F. (The cooled soda can be kept in the refrigerator. The warmed soda can be kept in a warm water bath.)
5. Have the students make hypotheses about the state parameters of each of the three temperature sodas.
6. Ask the students to observe the differences between the three samples. Which is the flattest? Allow students to take sips of the soda to make observations using their senses if allergies are not a concern. Have students put a finger into the soda if sipping the soda is not allowed. Students should first pour a small sample into their own individual paper cup for sipping and stirring. Visual observations of the bubbles can be made, they should see that the cold water has more bubbles or soluble gas and the warm carbonated water has few bubbles or soluble gasses. By sipping the carbonated water, students can feel the differences between the amount of dissolved gasses in the warm and cold carbonated water.
7. Students should come to the conclusion that the warm soda has few bubbles because as Henry’s Law indicates, as temperature increases, solubility decreases, and conversely, as temperature decreases solubility increases. The warmer the liquid, the less gas it can retain, therefore the warm soda should be “flat” and the cold soda should still have some effervescence.
8. Discussion: have students consider how climate warming will affect the process of CO₂ uptake in the ocean. (Less CO₂ able to be taken up by the ocean). Students can discuss these in small groups and write responses in notebook or assessment for understanding, or as a class discussion.
   • How will the increase of anthropogenic carbon dioxide change oceans all over the world?
   • The Southern Ocean links several very deep ocean currents, what are the implications to bringing heat and higher levels of CO₂ to those depths?
   • Identify others ocean ecosystems that are affected by increases levels of CO₂.
   • Warmer temperatures may slow the uptake of CO₂ and other gases by the ocean, how do you think that will affect atmospheric concentrations of CO₂ and other gases?
   • What effects will that have on terrestrial ecosystems and human life? What are ways that anthropogenic CO₂ can be reduced?

Assessment

Assessment of student participation in this activity including hypothesis generation, data collection/observation, and discussion addresses understanding of science and engineering practices.

Answers to questions at the end of the student reading will assess understanding of Henry's law. (Answers: A=3; B=1; C=2; D=2; E= all)

Background

(Note: Additional background science information and videos are included on the student section of this activity.)

In this activity, students explore how atmospheric carbon dioxide (CO₂) gas gets into and out of the ocean. This is important because the ocean currently absorbs much of the carbon dioxide that we add to the atmosphere. This is not enough to mitigate all of the anthropogenic carbon dioxide, but it has reduced the amount currently in the atmosphere. However this uptake of CO₂ by the oceans effects the pH level, which among other things causes coral bleaching. The amount of the gas that is able to be taken up by the ocean is regulated by Henry's law.

• Henry’s law: At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.

Within a carbon dioxide molecule (one oxygen atom and two carbon atoms), the atoms share electrons unevenly. This means that the oxygen side of the molecule has a slight negative charge, making it is a (weak) non-polar molecule.

Water (H₂O) is made up of two hydrogen atoms and one oxygen atom. Again, the atoms share electron, but not evenly. This uneven sharing of electrons give water molecules a slight positive charge near the two hydrogen atoms and a slight negative charge near the oxygen atom, which is called a polar molecule.

To dissolve in the water, the CO₂ molecule must pass through the air-water surface, where the CO₂ molecule gains an outer shell of the H₂O molecule. The positive charge, or electron-rich, area of the water molecule attracts the negative charge, or electron-deficient, is of the carbon dioxide molecule allowing it to go into solution. This process transfers the molecule from a gas into a liquid.

• Impact of Pressure: Air contains a mixture of gases, each of which has a partial pressure that contributes to the total sum of the pressure. The higher the concentration of a gas outside of the liquid, or partial pressure, the more gas of that gas will be able to go into solution.
• Impact of Temperature: Gas solubility is temperature-dependent. Gas can dissolve more readily in colder liquids. At the same pressure, the colder temperature allows a gas to stay in the solution longer.

How is this applied to the ocean and carbon dioxide?
The ocean temperature impacts the solubility of gases such as carbon dioxide. Colder ocean water can dissolve more CO₂ than warm ocean water. So in theory, colder water in the polar regions can take up more CO₂ than warmer equatorial waters.

Scientists are concerned that increased CO₂ in the atmosphere could create a positive feedback loop, a cycle in which the effects of a change in a system increase the magnitude of the change. For example, if ocean temperatures warm, CO₂ could be released from solution, increasing the atmospheric concentrations leading to further heat-trapping mechanisms, or at least slowing the rate of uptake by the oceans. The cycle continues to warm the atmosphere and the ocean.

Solubility, X_1, of CO2 in water. Handbook of Chemistry & Physics, 34th ed., 1953, Solubility of Gases in Water, p. 1532. The curve is the best–fit, fifth-order by the author.

How does soda relate to Henry's law and carbon dioxide uptake by the ocean?
After learning about Henry's law, students should be prepared to develop a hypothesis about the state parameters of each of the three temperature sodas, hopefully recognizing that the warm soda will retain the least amount of fizz and that the cold bottle will retain the most fizz (and thus, the most CO₂). Students should come to the conclusion that the warm soda has few bubbles because as Henry’s Law indicates, as temperature increases, solubility decreases, and conversely, as temperature decreases solubility increases.

Credits

• This activity was developed by Alison Rockwell of the NCAR Earth Observing Lab in support of the 2016 ORCAS field project.