# Everyday Phenomenon: Soup Cooling Mechanism

Everybody knows it, everybody does it: If the soup is too hot, you just blow on it and it cooles down – albeit often slower than desired.
As it is in all aspects of life, I can truly recommend asking the simple and insight-bringing question: Why? – Why does the soup get cooler if I blow on it?

Certainly, whether you blow on it or not, the soup cools down either way, just because the heat slowly disperses into the environment. Why, however, does the soup’s heat go away faster when you drain your lungs over it? When not blowing, the soup is surrounded by air – as it is when you are actually blowing. (It’s just that the air is a little bit moving in that case.) Did the air gain some kind of special cooling skills by going through your lungs? (Don’t you think this would be a little bit strange since the air from your lungs is even slightly warmer than the surrounding air?)
So why is it that I can cool my soup by blowing on it?

The answer to this question can certainly turn out so long that every soup has got cold on its own in the meantime. That’s why I will try to keep my answer short and reasonable.

Well…why not?!
(Source: Aha! Jokes, http://www.AhaJokes.com/)

Before I start, let me briefly note what we actually mean when talking about temperature:
A measure of temperature is, to put it simply, the average speed of the particles in the soup (where I don’t mean the soup garnishes – which don’t really move, as long as they are not alive [Here I must also say that, of course, you should not throw living things into boiling water. That’s not cool!] -, but instead the molecular constituents of the meal). At it, a higher temperature means a higher average speed of the soup’s particles and a lower particle speed corresponds to a state of lower temperature.

Additionally, I should probably mention that the surface of the soup, on a molecular level, is not the plain and clear boundary between the soup and its surrounding air as one might think when sitting in front of the plate.
Now we are grudually cutting to the chase: How does the boundary layer look like? What does happen there?

Naturally, the velocities of the individual particles aren’t all the same. Some particles move faster, some move slower. Some shoot out of the “soup particle sea”, cannon into air particles, dash in the soup particle sea again. Others aren’t fast enough to overcome the attracting forces of the rest of the soup particles and therefore stay in the particle sea. Some particles, however, can even escape the soup particle sea.
Life on the very level of the soup surface is tumultuous indeed!
As we can imagine now, the boundary between soup and air is a bit wishy-washy and kind of blurred. There are always some particles, which have formerly been in the soup, in a small air layer above the surface. Concretely, we can even speak of a thin (water) vapor layer forming above the soup’s surface. Thus, in equilibrium, there’s always a layer (or a “phase”) which contains vapor as well as air.

Now comes the blowing trick!
If you blow on your soup, the just-described equilibrium gets disturbed: The vapor atmosphere above the soup is swept away. However, the equilibrium wants to be restored since now there’s again “room” for water vapor in the “fresh” air – the new air is not saturated with vapor yet.
Thus, particles from the soup dissolve in the air once more. But how does the soup get cooler thereby?

Two possibilities of how to imagine the cooling process come to my mind – one is fairly descriptive, the other certainly requires some basic knowledge of thermodynamics. (I will not go into the details of the latter, but rather mention it.)

For instance, we can ask ourselves which particles will go from the soup into the above vapor atmosphere after all. Statistically, mainly particles which have enough speed to overcome the attracting forces of the other soup particles will escape the soup particle sea. After these particles have left the soup, let’s average over the speeds of the remaining particles: Now the average of the soup particle speeds will have decreased since the faster particles haven’t entered into the equation. A lower average particle speed directly corresponds to a lower temperature, because that’s just the way we have previously introduced and defined “temperature”.
Tadaa! – The soup has cooled down!

Another way to understand the cooling process is by making use of a somehow inaccessible thermodynamical quantity – that is, the entropy. Let’s just note here that the entropy of the soup decreases when soup particles diffuse into the gaseous air phase. Now, the variation of the soup’s internal energy $U$ with the entropy $S$ (at constant volume $V$) is defined as the temperature $T$. The same sentence in a different (and more precise) language is: $(\partial U/\partial S)_V=T$. So if the entropy decreases in the process of particles diffusing into the air, so does the temperature of the soup.

Certainly, the soup also cools down on its own. At it, the mechanism is mostly the same, but it just takes longer due to the vapor layer above the soup persisting longer when no one is blowing. Therefore less particles can diffuse from the soup into the vapor layer in the same amount of time, because it can’t be made “room” for them in the vapor atmosphere as quickly as in the case of new air getting constantly blown on the soup surface.

By the way: With a layer of foam on it, the coffee stays warm longer. Now you can think about why this is.
(Picture by Christopher Michel, CC BY-NC-SA 2.0)

Well, now that I have written the word “soup” exactly 40 times all over, it seems that I’ve succeeded in getting myself hungry.
If there is still somebody who is able to take the following interesting example of an application of the above-explained mechanism without starving, I think she/he is very well prepared to understand the crazy process which can be used to cool liquid helium down to a few microkelvin – which is just some millionth degrees above absolute zero of -273 °C! (But when being in public, you should probably rather speak of “quantum cooling” instead of “soup cooling mechanism”. The former just sounds much more awesome!)

Let Andrea Morello, a refreshing professor from the University of New South Wales, explain the cool physics of “quantum cooling” to you in this video by 2Veritasium: