Polar Molecules, Vaporization and Grease Fires

By Tracy V. Wilson, HowStuffWorks.com

TIME WARP VIDEO

Video

HOWSTUFFWORKS.COM

Fighting a fire is no easy task. Find out more in How Smoke Detectors Work, How Fire Engines Work and How Firefighter Training Works at HowStuffWorks.

Even if you're not much of a cook, you've probably heard someone say, "Never put water on a grease fire." If you do, you won't just make the fire a little harder to put out -- you'll produce a pillar of flame that can easily reach and ignite the kitchen ceiling. There are two concepts to blame: polar molecules and vaporization.

The old adage that oil and water don't mix is true largely because of water's polar molecules. One region of a water molecule has a partial negative charge while the other has a partial positive charge, almost like a microscopic bar magnet. Because of this trait, water molecules attract each other and can dissolve substances that have compatible electrical charges easily.

Oil, on the other hand, is nonpolar. Its molecules have no complementary charges to help it mix with or dissolve in water. Instead, oil molecules stick to each other, and since water is denser than oil, they form a layer on the water's surface.

If you make the dangerous decision to pour water on a grease fire, the water will sink to the bottom of the pan, displacing the oil on top. But that's only part of the problem. Water's boiling point is 212 degrees Fahrenheit (100 degrees Celsius), and burning oil is much hotter. So the water heats up rapidly, causing it to expand and vaporize. As the water droplets rise, they carry some of the oil with them. The droplets of oil and water move through the air, giving them lots of access to oxygen. This mixture is extremely flammable, and a dramatic fireball is a result.

So if you're trying to put out a grease fire, don't turn to water. Turn the stove off and cover the pan with a lid. If you don't have a lid handy, pour baking soda on the burning oil to smother the fire. Or use a fire extinguisher -- one that contains dry powder, not water.