by Caleb Wehling and Ryan Gross
When most of us picture our ideal Thanksgiving break, we probably don’t imagine an explosive fireball that grows when you pour water on it. Surprisingly, grease fires are a real part of some people’s Thanksgiving. As it turns out, deep-fried Thanksgiving turkeys are the most common causes of the thousands of grease fire responses made by U.S. fire departments annually. Who knew?
What makes these odd catastrophes so dangerous is that grease fires can’t be put out by water. But why is that? Fire is really just energy given off as heat and light (mostly heat) by a burning material. The types of molecules that burn, like the oils in grease fires or the different types of sugars in wood fires, have a lot of energy that they can release. But these molecules don’t just release their energy automatically. They need a bit of starting energy to get them going. That’s why you have to light a fire with a match—the wood won’t start burning until it has some energy to get it started. Once it gets started, it creates its own heat to keep it going.
The important takeaway is that one of the essential components needed for a fire is energy (as heat). And that’s where water comes into the picture. Water is able to absorb a lot of heat, one of water’s many cool (ha, “cool.” Get it?) properties. So when you dump water on a wood fire, the water absorbs a lot of the heat from the combusting wood, preventing the rest of the wood from being able to release its energy. But with grease fires, since oil and water don’t mix, the water sinks and can’t come into contact with the surface of the burning oil very well to take away its heat.
Not only does water not put out grease fires, but it actually makes the fire worse. Why is that?
Since water is more dense than oil, water sinks below the oil to the hot surface below the oil (the metal fryer you in which you were trying to cook your Thanksgiving turkey, for example). The hot surface then heats the water enough to cause it to boil and evaporate. When liquids evaporate, however, they expand by about 1,700 times. That expansion of water as it evaporates shoots the burning oil out into the air, giving the oil more burning surface area, thus creating impressive and extremely dangerous fireballs. While this uses up the fuel more quickly, it certainly does not put the grease fire out.
The good news is that these fires (if they’re still small) can be put out in other ways, such as smothering them with a blanket or by dumping salt or baking soda on them.
Also, there are some easy ways to avoid deep-fried turkey grease fires in the first place: properly thaw the turkey before frying, don’t fill the fryer with too much oil and don’t get the oil too hot, keeping it at 350℉.
So there you have it. That’s why Thanksgiving can be a dangerous time of year. Maybe we should just stick with tradition and use an oven.
Have a question about the science behind everyday stuff? Email Caleb Wehling at firstname.lastname@example.org or Ryan Gross at email@example.com to have your question answered!