The Liquid Nitrogen Cloud

One of my favorite experiments to conduct is the liquid nitrogen cloud. As long as you can get some liquid nitrogen, it is incredibly easy to carry out. All you have to do is pour some hot water into liquid nitrogen and a massive cloud results. Check it out (also, be sure to check out the video posted further below):

What a fantastic experiment! It’s hard to imagine that such absolutely remarkable results originate from an experiment that is so easy to set up and carry out. I’ve wondered for some time how the cloud forms in this experiment. I’ve seen claims that the cloud forms as hot water vaporizes the liquid nitrogen upon contact, and the resulting cold, rising nitrogen vapor cools atmospheric water vapor, condensing it into a cloud.¹ I’m not convinced of this explanation for two reasons. First, you will note that the cloud formed is massive and thick. It seems to me that there isn’t enough atmospheric water vapor in the immediate vicinity of the rising, cold nitrogen gas to produce a cloud of this size. Also, this experiment is reminiscent of the dry ice cloud, in which a cloud forms when dry ice is placed in water. During the dry ice experiment, it is easy to show that the cloud forms not from atmospheric water vapor, but rather from the water into which the dry ice is placed.^2,3 I decided to run some experiments to see if the water in the liquid nitrogen cloud originated from the water poured onto the nitrogen rather than atmospheric water vapor. I ran various trials of pouring water into liquid nitrogen – and also liquid nitrogen into water – and observed the results. The results of my explorations led to a working hypothesis that the cloud originates at the interface between liquid nitrogen, hot water, and expanding nitrogen gas (Figure 1). In this scenario, the contact of the two liquids causes rapid vaporization of nitrogen, resulting in large pockets of expanding nitrogen gas that rises and pushes through the liquid water (Figure 1, middle). Some of this liquid water evaporates into these expanding plumes of cold nitrogen gas (Figure 1, right, blue arrows), and then rapidly condenses into large pockets containing condensed water vapor – a cloud (Figure 1, right, red equations).

Formation of liquid nitrogen cloud

Figure 1: Diagram summarizing the hypothesis for the formation of the cloud during the liquid nitrogen experiment.

The image in Figure 1 is overly schematic; the explosive nature of this process certainly pulverizes the liquid water into innumerable liquid droplets. The basic idea, however, is that all surfaces of contact between liquid water and cold gaseous nitrogen results in tiny droplets of condensed water vapor. To summarize in equations:

H₂O _{(l, in bulk water and pulverized droplets)} à H₂O _{(g, in pockets of cool gaseous nitrogen)}

H₂O _{(g, in pockets of cool gaseous nitrogen)} à H₂O _{(l, droplets, in cool gaseous nitrogen)}

You can see the results of my experiments in the video below, and judge for yourself what you think of my working hypothesis.

If you have comments or criticisms about how I think the liquid nitrogen cloud forms, I’d love to hear from you – especially if you have suggestions for possible experiments to improve my understanding of what is happening in this fascinating experiment.

References:

1. See for example, https://www.noble.org/videos/liquid-nitrogen-demo/ (Accessed July, 2019).

2. https://www.chemedx.org/blog/dry-ice-water-cloud (Accessed July, 2019).

3. Kuntzleman, Thomas S., Ford, Nathan, No, Jin-Hwan, Ott, Mark E., A Molecular Explanation of How the Fog is Produced when Dry Ice is Placed in Water, J. Chem. Ed.201592