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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? Everyday Einstein discusses the science behind this everyday phenomenon. 

By
Lee Falin, PhD,
October 5, 2013
Episode #071

Page 1 of 2

hotchocolateA 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 Everyday Einstein 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. 

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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.

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