Water puts out fire. Everybody knows that, right? That is why firefighters hose down burning things with water, no? Actually, the truth is more complicated. There are certain fires that can actually be made worse by pouring water on them. Such is the case of oil or grease fires. You can see what happens when you add water to an oil fire in the video below by Greg Foot from BBC Earth Lab.
The way water puts out a regular fire such as a wood fire is by covering it and depriving the fire of oxygen. But with oil the water displaces the oil rather than covering it. There are three potential explanations for this:
1) Water is denser than oil and will sink to the bottom pushing the oil upwards. This is why the water doesn’t cover the oil.
2) The mechanical force with which the mass of poured water hits the oil makes it splash.
3) Part of the water may be quickly converted to steam as a result of the heat, and will boil through the oil making it splash.
An oil or grease fire will burn at the surface because only that area of the oil has access to oxygen. When water dropped on an oil fire causes the oil to splash, the mass of the oil underneath the fire is displaced upwards and more of the oil gains access to oxygen and the combustion reaction resulting in more oil igniting.
Which of these explanations is the most accurate?
To figure that out we can check what happens when trying to put out water with liquid nitrogen. Nitrogen is a gas that displaces oxygen and can put out a fire. When nitrogen gas is cooled down to - 320 °F, it becomes a really cold liquid with a density lower than that of oil, so liquid nitrogen should float on top of oil not sink under it like water. Therefore explanation #1 does not apply here. Also the mechanical force of a mass of poured liquid nitrogen would be similar to that of a mass of water poured into the oil fire (they are both liquids). Therefore any difference between liquid nitrogen and water would not be due to explanation #2.
What happens when you pour liquid nitrogen into an oil fire was investigated by Cody from Cody’s lab in the video below.
As you can see, it was actually worse than water! The liquid nitrogen turns to nitrogen gas so rapidly upon heating that it violently displaces the oil making it splash and creating an explosion with a loud bang. It seems to me that of the three explanations, the third one is most likely responsible for most of the effect we observe with both water and liquid nitrogen. And in case you are wondering who in their right mind would try to use liquid nitrogen to put out an oil fire in the real world, there is at least one known case of a scientist who tried this. It didn’t go well.
If you ever face a pan of oil or grease that has caught fire, the easiest and safest way to put out the fire is to smother it with a cover such as the lid of a frying pan. This will deprive it of oxygen and extinguish the flames.
So never try to put out an oil fire with water, or liquid nitrogen for that matter.