Science Made Simple: Why Water Can Make a Grease Fire Worse
More than 50 percent of cooking fires are caused by grease, cooking oils or fats, according to the National Fire Protection Association (NFPA). Whether you’re preparing a holiday meal or a quick snack, you should know what to do if a grease fire starts in your kitchen.
One rule: Never try to put it out with water, according to the NFPA. Why? The answer boils down to basic chemistry, says Kate Biberdorf, Ph.D., a chemistry lecturer and director of demonstrations and outreach for The University of Texas at Austin.
Why Oil and Water Don’t Mix
You’ve probably heard the expression “oil and water don’t mix.” This is the first step to understanding why water makes a grease fire worse, Dr. Biberdorf explains.
Reason #1: Density
Density is the ratio of mass to volume, or how tightly particles are packed together. Oil has a density between 0.91 and 0.93 grams per milliliter, Dr. Biberdorf explains, while water has a density of 1.
“That slight difference in density is enough to make them separate,” she says.
Reason #2: Polarity
A molecule is polar when it has a positive charge at one end and a negative charge at the other.
You’ve likely heard the expression “opposites attract.” That applies to polar molecules: The positive end of a water molecule, for instance, wants to bond with the negative end of another water molecule. “Their bond is freakishly strong,” Dr. Biberdorf adds.
Oil or grease molecules, on the other hand, are nonpolar, she explains. The components of a nonpolar molecule are evenly distributed, so it doesn’t have an overall positive or negative charge.
Nonpolar and polar molecules want nothing to do with each other. “When you put together something nonpolar, like oil, and something that’s very polar, like water, they end up completely separating and being pushed apart from each other,” Dr. Biberdorf says. “They literally are doing everything they possibly can to minimize touching.”
What Happens When You Add Water to a Grease Fire
The polar water molecules and nonpolar oil molecules want to keep as far away from each other as possible. And because water is denser than oil, it sinks right to the bottom of the pan — underneath the grease fire, Dr. Biberdorf explains.
One more thing to consider, she adds: Water boils at a lower temperature than oil does.
“Because the pan is already at a really high temperature if the oil is on fire, when you pour water into it, the water is going to hit the pan and instantaneously boil,” Dr. Biberdorf explains. “So it’s going to go from a liquid to a gas in less than a second.”
The water molecules turn to steam. “As the water goes from the liquid state to the gas state, the water vapor molecules leave the pan as fast as possible. And as they do that, they take the grease with them,” she says. “It physically flings the grease out of the pan and shoots fire everywhere.”
Picture the steam escaping from a boiling pot of water, she explains. “Now attach a small fire to every single water vapor molecule. It’s very dangerous.”
What to Do if You Have a Grease Fire
If your cooking oil does ignite, turn off the flame and cover the pan with a lid as quickly as possible, the NFPA recommends. That will cut off its oxygen supply and help smother the flames, Dr. Biberdorf says. “I always have a metal or glass lid ready if I’m cooking with grease,” she adds.
Once the fire is out, let the pan cool for a long time, the NFPA advises. Removing the lid too early could cause the fire to start again.
To help prevent the spread of a grease fire, use a damp cloth to wipe your counters before, during and after cooking with oil, Dr. Biberdorf says.
“Any time you see grease on your kitchen counter, you should try to wipe it down in order to avoid a ‘jumping fire,’” she warns. “Essentially, a grease fire will immediately ‘jump’ to any other sources of fuel, which in this case is oil on your counter.”
If a fire ever gets out of your control, evacuate your home, call 911 and let the professionals take care of it, the NFPA says.