Soup

In preparation for this fall and just for the love of a good Physics book, I've been reading the Feynman Lectures on Physics (which are a great fun read that I'd highly recommend). In one of the earlier lectures, Feynman asks a good question:

Does blowing on your soup cool it down? And perhaps more importantly, were our mothers right when they told us this? 

I just loved reading his explanation - it ties together so many different concepts in Physics - the idea of "breaking free" of forces reminded me of escape velocities for a rocket to "leave" earth; the idea of looking at averages to see macroscopic properties reminded me of the beautiful nature of Physics in taking the most complicated system, and finding ways to talk about it in simple terms. So I thought I'd share Feynman's explanation in my own words - enjoy! Note that there's also a lot more going on in the cooling down of soup than described here, but this part is my favourite, so we'll have to save the rest for another day.

To keep this explanation prettier, we’ll pretend your soup is made of water only, though we know there’s delicious chicken and noodles in there too. If the water is cooling down, the chicken and noodles will cool down too, so it's fine to just ignore these in our explanation. (But definitely don't omit the chicken and noodles from your recipe or you will have rather bland soup!)

First, let’s think about what will happen to your soup whether or not you blow on it.

Molecules everywhere are always moving. In fact even if you cool a substance down (by slowing down the molecules) to absolute zero, the molecules can’t stop moving entirely – if they did, they’d violate Heisenberg’s uncertainty principle, which is a real crime. So molecules in everything just keep jiggling around.

Here you have a bowl of soup – in it are lots of water molecules, which are all attracted to each other. What attracts them? Well, if you look at a water molecule, it's a little bent, and you may recall from chemistry that this leaves one positive and one negative end (aka. it's a polarized molecule). So the negative end of one molecule is attracted to a positive end of another, etc, and we see all the water molecules "staying together". A really lovely example of this "sticking togetherness" is a raindrop. 

Even though the water molecules are all staying together, they are definitely still jiggling (because, as we just learned, molecules are not allowed to take a break from moving). Now, what we call the temperature of the soup is really just a fancy way of describing the average speed of these jiggling molecules. Naturally, (and for the word "average" to mean anything significant) some molecules must be moving faster than average and others a little slower. And - now here is the dramatic part you've been waiting for - every once in a while, a water molecule is near the surface and is moving so fast that it has enough energy to break free of that attraction to the other molecules around it! (Quick analogy: it's a bit similar to when you throw a paper clip past a magnet - if the paper clip is moving slowly, it'll just stick to the magnet, but if it's going fast enough it will have enough energy to escape the force and continue on it's merry way.) So back to our story: away flies the energetic little water molecule into the air above the soup - and this is evaporation.

The story of evaporation is pictured below. I couldn't show speed in a still image of course, so I illustrated faster molecules as having bigger smiles, because, well... I just like to think molecules are happier when they're moving fast - it seems more fun.

Now things get quite interesting. Let’s think for a moment about the molecules who are left behind when evaporation happens. In the above paragraph, the soup just lost one of its most energetic/fastest moving molecules. So the average speed of those left behind must therefore go down. It’s not that the other molecules have physically slowed down – it’s just that one of the fastest molecules is now gone, so when we take the average speed of those left behind, we see that it is lower. And since the average speed of the molecules directly tells us the temperature of the soup, the temperature must go down a little bit with every speedy molecule that evaporates.

After a little bit of evaporation, however, something else interesting happens. Remember that so far, we have not blown on the soup, so we can consider the air above the soup to be pretty much still. More water evaporates. The air above the soup gets more and more humidified. Eventually, the air will form an equilibrium with the soup where some water molecules are evaporating, lowering the temperature of the soup, and others are condensing back into the soup, raising the temperature of the soup back again. So we run into a problem where our great cooling mechanism of evaporation is not as being effective as we'd hoped.

What happens then when you blow on your soup? You are blowing away that humid air, so that the condensation happens less. That equilibrium of condensation/evaporation can no longer be established, so all the soup can do is keep evaporating! More evaporation means the average speed of the water molecules in the soup goes down, meaning the temperature of the soup goes down. So our mothers are correct! But then again, why did we ever doubt?