The Dry-Ice-in-Water Cloud

Have you ever wondered where the cloud comes from when dry ice is placed in water? Consider the answer returned in my browser when I Googled the phrase “How does the dry ice cloud form”:

“Pellets of dry ice, solid carbon dioxide, are dumped into a basin of nearly boiling water. A dense white cloud of fog first rises above the basin. ... Fog forms when water vapor in the air condenses into tiny suspended droplets.”

Many scientists and science educators also think the dry-ice-in-water cloud originates from atmospheric water vapor. I recently tried to get a handle on how pervasive this misconception is via a poll on Twitter:

poll on dry ice cloud

It is easy to set up several simple experiments to provide evidence that atmospheric water vapor is not the source of the cloud produced when dry ice is placed in water. Some of these experiments are presented in the video below:

As you can see from the experiments in the video, there is good evidence that the cloud is formed from water that is in the very container into which the dry ice is placed. In fact, it appears that the cloud is formed within bubbles even before these bubbles reach the surface! How does this happen?

One possible mechanism for how the dry-ice-in-water cloud is produced has been described.¹ When dry ice is placed in water it sublimes, which is a phase change directly from the solid state to the gaseous state:

CO₂(s) --> CO₂(g)        Equation 1

The CO₂ gas thusly formed creates a continuous bubble of CO₂ around the dry ice. This bubble is devoid of any water. Therefore, water evaporates from the bulk into the CO₂ bubble:

H₂O (l, in the bulk water) <-- --> H₂O(g, in the CO₂ bubble)   Equation 2

 However, this gaseous bubble is extremely cold because it is very close to dry ice. This causes the water which evaporates into the bubble to condense into tiny liquid droplets of water:

H₂O (g, in the CO₂ bubble) --> H₂O (l droplets in CO₂ bubble)          Equation 3

The condensation of water in the bubble removes H₂O(g) within, causing Equation 2 to shift to the right by Le Chatelier’s principle. Through these processes enough water to form a thick cloud inside the bubble occurs. These processes must happen very fast, as evidenced by looking at slow motion video of the process: It is observed that each bubble is already filled with cloudy contents as the bubbles separate and rise from the dry ice. If you would like to learn more about this experiment and the proposed mechanism, be sure to check out the reference provided below.

I have developed a laboratory experiment that guides students through the process of thinking about the dry-ice-in-water experiment. In the exercise, students explore properties of liquids that aid in the production of clouds upon addition of dry ice. Students are also guided to use evidence collected to propose a mechanism for how the cloud forms in this experiment. When conducting this exercise, it is helpful if students are somewhat familiar with the ideas of vapor pressure and surface tension. The write up for this experiment is still a work in progress, but I’d like to share it here (see below) in case anyone would like to try it out. If you do try this laboratory exercise, please be sure to let me know ways the write up can be improved. How might the directions guide students more clearly? Do you have any suggestions for additional experiments that would guide students in the right direction? I would be especially interested to hear how things go if you use the write up with your students.

Reference:

1. Kuntzleman, Ford, No, and Ott J. Chem. Educ., 2015, 92, 643–648.