Why Blowing on a Hot Drink Cools it Off
Why does that mug of super hot, hot chocolate get cooler when you blow on it? Can you really blow away heat? Ask Science discusses the science behind this everyday phenomenon.
Lee Falin, PhD
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Why Blowing on a Hot Drink Cools it Off
A few days ago, my family and I decided to go to the beach. Unfortunately, since we happened to be in the north of England at the time, it was a cold and rainy day, which is pretty typical weather for that part of the world. Long time Ask Science fans know that when my family has a cold day out of doors, we love to warm up with a steaming mug of hot chocolate. I’ve mentioned before that hot chocolate can spark a great discussion on density. But hot chocolate can also provide a great way to discuss heat. .
This Chocolate Is Too Hot!
As we sat preparing to sip our hot chocolate, it quickly became apparent that our steamy mugs of hot chocolate were a little too steamy. There’s a fine line between hot chocolate that’s hot enough to warm your soul, and hot chocolate that unpleasantly melts your tongue as you drink it, and unfortunately these mugs of hot chocolate had crossed that line.
Of course the standard way to deal with any hot drink is to use the time-tested method of blowing gently across the surface of the cup in order to cool off the drink within. But have you ever stopped to wonder why that works? Is heat something that you can really blow away? In order to find out the truth, we’re going to have to get a lot closer to the drinks.
Save Your Breath to Cool Your Porridge
As I’ve mentioned in previous episodes, the molecules in a liquid are packed together less tightly than molecules of a solid, allowing them to bounce around quite a bit. This is what allows liquids to take the shape of their containers, or to spill all over the floor when one of your kids knocks over their mug of chocolate. (This is not one of my favorite properties of liquids).
Well, the more a liquid heats up, the more bouncy those molecules get. In the case of our hot chocolate, the water molecules were bouncing around quite energetically. Loosely speaking, you can think of that bounciness, or kinetic energy, as where the heat energy is being stored within the hot chocolate.
This Mug Ain’t Big Enough for the Two of Us
Sometimes, near the surface of the hot chocolate, all of these water molecules bouncing around causes one of the ones near the surface to get bounced right out of the mug. This results in two things: first, the molecule takes its kinetic energy with it, meaning that it removes some of the heat energy from the mug, causing it to cool down slightly; second, the amount of hot chocolate decreases slightly, as it is now one molecule less than it was before. We call this process evaporation.
In order to speed evaporation up a bit, you need to move that water vapor out of the way, and one easy way to do that is by simply blowing lightly across the space just above your mug.
Now of course a single water molecule flying out of your hot chocolate isn’t going to decrease the heat enough for your tongue to notice a difference. But over time, as lots of these molecules get bounced out of the mug, it starts to cool down enough for you to notice. The temperature slowly starts to cross over that line from tongue-scorching to soul-warming.
The Winds of Change
Unfortunately, this evaporation stuff can take quite a long time, and you want to drink that hot chocolate, not sit around and stare at molecules bouncing around. It turns out that one of the limiting factors in this process is the amount of water molecules already in the air, something we usually call humidity. Near the surface of your mug, the air can get pretty saturated with water molecules because so many of them are bouncing out of your mug. This slows down the rate at which they are able to leave the mug.
In order to speed things up a bit, you need to move that water vapor out of the way, and one easy way to do that is by simply blowing lightly across the space just above your mug. This causes the excess water molecules in the air to get pushed aside, making room for more molecules to bounce out of your hot chocolate, taking their heat energy with them. After a minute or so of this, (depending on how hot your hot chocolate was to begin with), you’re finally ready to enjoy a drink of soul-warming hot chocolate, without melting your tongue.
Conclusion
So now you know why your hot chocolate is hot, why it slowly cools down, and why blowing on it helps to speed up the process. As you might have guessed, this process isn’t limited to hot chocolate. Porridge, soup, oatmeal, gravy, peppermint tea, and any other hot liquid works the same way.
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Ned Flanders Hot Chocolate image, Christopher Carfi at Flickr. CC BY 2.0.
