What do soda and the oceans have in common?

Activity Sections

Overview

Introduction

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.

Credits

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

Grade Level

This activity is suitable for students in grades 9 through 12.

Time Required

2 class periods (approximately 100 minutes)

Student 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.

Lesson Format

hands-on activity with background reading

Science Education Standards Addressed

Next Generation Science Standards

NGSS DCI HS-PS1 Matter and Its Interactions
NGSS Science and Engineering Practices: Developing and using models
NGSS Science and Engineering Practices: Engaging in arguement from evidence
NGSS Science and Engineering Practices: Obtailing, evaluating and communicating information
NGSS Crosscutting Concepts: Cause and effect, mechanism and explanation
NGSS Crosscutting Concepts: Stability and change

Instructions

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

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?
Students observe bottle of soda and bubbles coming out and discuss impact of pressure.
1. 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.
2. Students write down observations in 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?
Students read about Henry's Law. Discuss the main points:
- Gases can dissolve in liquid and is dependent on 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 solution.
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.)
Have the students make hypotheses about the state parameters of each of the three temperature sodas.
Ask the students to observe the differences between the three samples. Which is the most flat? Allow students to take sips of the soda to make observations using their senses if alergies 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.
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, therefor the warm soda should be “flat” and the cold soda should still have some effervescence.
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 affects 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 & Extensions

Background

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

Carbon dioxide molecule
Credit: Jacek FH, Licensed under CC BY-SA 3.0 via Commons

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 gain 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 are of the carbon dioxide molecule allowing it to go into solution. This process transfers the molecule from the a gas into a liquid.

Water molecule
Credit: Dbc334; Jynto, Public domain, via Commons

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 CO₂in the atmosphere could creates 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.

Credit: 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.

Student

Learn about Henry's Law

As formulated in 1803 by William Henry, Henry’s law states "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." In other words, at a given temperature a gas will dissolve into a liquid to a degree that is determined by the balance between the undissolved gas and the and the dissolved gas in the liquid.

Let’s take a look at Henry’s law: 

Where k is a constant for each gas that can be looked up on a table, though when the temperature of a system changes so does the constant value of k. Pgas is the partial pressure of the gas in the parcel, and Cx is the concentration of the gas in the liquid solution. The concentration of the the gas in solution (solute) is directly proportional to the partial pressure of the gas above the liquid.

Carbon dioxide molecule

Image: Wikipedia

Water molecule

Credit: Wikipedia

Molecular Process
Let’s first consider how the process happens on a chemical level. Carbon dioxide (CO₂) is made up of one oxygen atom and two carbon dioxide atoms. The atoms share the electron, but unevenly; the oxygen side of the molecule has a slight negative charge. CO₂ (Figure 1) is a linear, symmetrical, molecule with a slight negative charge on either end, which is why it is a (weak) non-polar molecule.

Water, H₂O, is made up of two hydrogen atoms and one oxygen atom (Figure 2). 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.

The positive charge, or electron rich, area of the water molecule attracts the negative charge, or electron deficient are of the carbon dioxide molecule allowing it to go into solution.

To dissolve in the water, the CO₂ molecule must pass through the air-water surface, where the CO₂ molecule gain an outer shell of the H₂O molecule. This process transfers the molecule from the gaseous state to an aqueous solution.

Pressure
Any parcel of gas, or air for example, 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.

Watch these videos from Khan Academy to learn about:
- Partial Pressure
- Henry’s Law
- CO2 and O2 Solubility

Temperature
Gas solubility is temperature dependent, gas can dissolve more readily in colder liquids. If you leave an opened bottle of soda on the counter it soon goes “flat”, or all of the CO₂ leaves the solution, however if you leave your opened bottle of soda in the refrigerator, it will not go flat as quickly. This is because, even at the same pressure, the colder temperature allow for the gas to stay in the solution longer.

This is also true for ocean temperature and the solubility of gases such as CO₂. As discussed earlier, the colder the temperature is the more CO2 can be dissolved, or colder ocean water can dissolve more CO₂ than warm ocean water. So in theory, colder water towards the poles can take up more CO₂ than warmer equatorial waters. (Figure in Folder)

Scientists are concerned that increased CO₂ in the atmosphere could creates 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.

Video: Southern Ocean Carbon Sink (New Zealand’s National Institute for Water and Atmospheric Research)

Assessment

A. Choose the correct words to fill in the blank:
In Henry’s law, “at a constant __________, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the __________ of that gas in equilibrium with that liquid."

1. pressure, solubility

2. partial pressure, temperature

3. temperature, partial pressure

4. solubility, concentration  

B. True or False:  Gases are more soluble in cold water.
   1.   True
   2.   False

C. True or False:  A gas such as CO₂ can molecularly dissolved in water because both the solute and solvent have polar bonds.
   1.   True
   2.   False

D. Why is better understanding of the role of the Southern Ocean so critical?
   1.   Because it drives warm waters to the surface that able to take up more CO₂
   2.   Because it sets the overturning circulation for the global oceans, and determining the partitioning of heat and dissolved gases between the atmosphere and deep ocean.
   3.   Because much of the Earth’s heat is stored in the ocean and the Southern Ocean releases it into the atmosphere
   4.   Because anthropogenic CO₂ is not able to be taken up by oceans, but biogenic CO₂ is, particularly in the Southern Ocean.

E. How do you think climate change and warming of Earth’s systems will affect the Southern Ocean? Check all that apply.
   1.   Warmer temperatures will decrease and slow the amount of gas that is dissolved into the ocean
   2.   There will be a positive feedback loop which will release more CO₂ into the atmosphere, potentially worsening global warming
   3.   There are still many scientific discoveries to be made enabling further research
   4.   Through global ocean circulation, heat can be transported to all levels of the ocean affecting ecosystems far and wide.