(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 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.
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 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.
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.