We all know the effect: Have a bite off a pizza straight off the oven and your mouth nerves will immediately complain about your stupidity. However, you can still eat the crust without too much of a problem. This post will explain why this happens and it all comes down to a simple physics property called "heat capacity".
Heat capacity measures how much heat (also known as thermal energy) can be stored in any material before their temperature is significantly raised. For example, the heat capacity of stainless still is about 10 times smaller than water's. Put your pan on a stove and it will get hot very quickly. This is because having a small heat capacity, it cannot store much energy without increasing its temperature. On the other hand, try to heat up an equal amount of water and it will feel like it takes ages for its temperature to rise. This is because water has a much higher heat capacity. As a matter of fact, you need more energy to increase the temperature of one kilogram of water than almost any other substance you can think of!
But what does this have to do with pizzas? Well, during cooking the ingredients of a pizza will go from fridge temperature (or room temperature if it's fresh) all the way up to about 200°C. Now here is the interesting thing: The bread has a low heat capacity so it does not store much energy as its temperature increases. The cheese on the other hand, being 30%-40% water, has a much higher heat capacity. That means that it has been absorbing and storing much more energy as its temperature increases.
Once you have a bite, all of that energy is released into your mouth burning you! But biting the bread, even though it is also at 200°C is not as painful, because the bread has less energy stored in it. So next time you enjoy a nice slice of hot pizza, feel free to tell your friends all about heat capacity and how it works!
Thanks for reading!
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p-a has a PhD in physics and has taught in a variety of universities and schools in two different continents and three different countries. He now works full-time as a science journalist.
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The Explain Everything Series is aimed at a broad audience, sharing interesting science facts relevant to everyday life. Comments, feedback or suggestions for future posts? Leave a comment!