About a decade ago, I wrote briefly about the interesting experiment of throwing boiling hot water into air that is below -18°C (0°F) (see Ice Clouds).

WARNING: If trying this experiment on your own, be certain to NEVER throw boiling water into the wind. Make sure the water is thrown with the wind, away from you. Doing so not only keeps you safe but also creates a much more magnificent effect. Also, having the cloud backlit by the sun helps to accentuate the beauty of the experiment as well.

During a particularly cold snap here in Michigan, I repeated these experiments with some family members (Video 1).

Video 1: Boiling Water vs. Cold Air, Tommy Technetium YouTube Channel (accessed 1/16/24)

Why does throwing boiling hot water into cold air cause ice clouds? After doing the experiments this time around, I thought in a bit more detail about how this happens. The main thrust of the effect is the evaporation of water followed immediately by its rapid and immediate condensation:

H₂O(l) → H₂O(g)                   Eq. 1

H₂O(g) → H₂O(l)                   Eq. 2

Why do these steps (Equations 1-2) occur in rapid succession? I estimate that by the time we got the water off the stove and outside, it cooled to about 90°C, which means its vapor pressure was near 525 mmHg. This is over 20 times higher than the vapor pressure of water at room temperature (about 20 mmHg)! Naturally, a lot of water easily evaporates from the hot water into the cold, dry air (Equation 1). However, this gaseous water very quickly begins moving through the frigid air where it cools and recondenses back into tiny liquid microdroplets of water (Equation 2). In fact, some of these the microdroplets become so cold that they go on to freeze into tiny particles of ice:

H₂O(l) → H₂O(s)                    Eq. 3

So you get all three phases of water in a single, strikingly beautiful experiment (Equations 1–3)! What’s more, the tiny microdroplets of ice and water create stunning, steamy striations that streak from the hot liquid blobs.

Surface tension is another aspect of this experiment that I wondered about. The surface tension of water at 90°C is 61 J m^-2, which is about 15% lower than the surface tension of water at room temperature (73 J m^-2). I think about surface tension of a liquid as the energy required to spread the liquid out over an area (you can see this from the units of surface tension – joules per meter squared). The lower surface tension of water allows it to break apart and spread out into streams and tiny blobs much easier than water at room temperature. This separation spreads the hot liquid out, increasing its surface area in contact with the air. This higher degree of surface area contact between hot liquid and cold air translates into a greater volume of stunning icy striations streaking through the air.

I suppose it’s not at all surprising that chemistry is intimately involved in this experiment. After all, chemistry has a funny way of making everything a bit more beautiful.