Super Powered Water: Specific Heat and Heat of Vaporization
Why does cheese on pizza, fresh from the oven, burn the roof of your mouth? Blame water. In Part 2 of this series, Ask Science reveals two more unique superpowers of water.
Lee Falin, PhD
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Super Powered Water: Specific Heat and Heat of Vaporization
Have you ever picked up a just-cooked slice of pizza fresh from the oven, taken a bite, and screamed in pain because the cheese just melted the roof of your mouth? If you have, don’t blame yourself, blame water.Â
As I mentioned in a Part 1 of this series, water has a wide variety of superpowers, thanks to its ability to form hydrogen bonds. To review, water molecules are formed when one oxygen atom forms covalent bonds with two hydrogen atoms. The oxygen atom shares one electron with each hydrogen atom, which in turn share one electron each with oxygen, forming a water molecule. .
However since these shared electrons tend to spend more time on the oxygen side of the molecule, this leaves oxygen with a partial negative charge and the hydrogen atoms with partial positive charges. These partial charges allow water molecules to form hydrogen bonds or polar bonds with one another. These bonds are the source of water’s amazing powers.
Is it Hot in Here?
One of the amazing abilities granted to water by its hydrogen bonds is its high specific heat. The specific heat of something is a measurement of how much heat must be gained or lost by a substance in order to change its temperature. (Usually this is measured as the amount of heat required to change 1 gram of a substance’s temperature by 10 Celsius).
Recall that heat is a form of energy, and as we add heat to a substance, we are adding energy. As heat energy is added to a substance, the particles of that substance increase in kinetic energy, causing them to vibrate faster. The faster this vibration, the higher the substance’s temperature.Â
However, water molecules form hydrogen bonds with one another. These extra bonds allow the water molecules to resist vibrating faster. Hydrogen bonds also absorb heat when they form. This means that to increase its temperature, water has to absorb a lot of heat. In fact, only ammonia (which also forms hydrogen bonds) has a higher specific heat than water.Â
Water’s high specific heat doesn’t just mean that it requires a large increase of energy to heat up, it also means that it requires a large decrease of energy to cool down, almost twice as much as most carbon compounds.
Mmmm Pizza…
So let’s assume that when you take a piece of pizza out of the oven, that it’s 2000 Celsius. Now, since the temperature of the air in your kitchen is (hopefully) considerably less than 2000, heat energy leaves the pizza and enters the surrounding air.Â
While this happens at approximately the same rate for both the cheese and the crust, since mozzarella cheese is over 50% water it has a higher specific heat than the crust . This means that the cheese has to lose a lot more energy than the crust to cool down the same amount. So even if the crust feels cool enough to pick up, there’s a good chance the cheese will still melt the top of your mouth.
Time to Cool Off
If all of this work of making pizza is making you sweat, you’re in luck. You probably know that sweating is the body’s way of cooling off. What you may not have known is that the reason sweating cools you off is once again thanks to the hydrogen bonds of water.Â
Just like its high specific heat, water also has high heat of vaporization. This means that it takes a lot of heat energy to convert water from a liquid to a gas. The reason behind this is because extra energy is needed to break the hydrogen bonds between water molecules before they can evaporate. Because of this, as water evaporates, it takes quite a bit of heat energy away from our skin. This is why sweating helps our bodies to cool off.
Try This at Home
If you’ve got two thermometers handy, the next time you cook some pizza, try sticking one thermometer into the cheese and another into the crust and see how their temperature changes differ over time.Â
Another fun experiment you can try is to hang two thermometers in front of a fan. Wrap the bulb of one thermometer with a wet napkin, then turn on the fan and see which one cools off more quickly.Â
If you liked today’s episode, you can become a fan of Ask Science on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.comcreate new email..
Pizza 002 image, British Mum at Flickr. CC BY 2.0. Thermometer image courtesy of Shutterstock.
