She wanted to know if cold water would put out a fire faster than hot water? How do kids come up with this stuff? It's great! I have been a volunteer fire fighter for some time, and I haven't thought about this--until now!

Well, anyway, I do know that one of the main ways water puts out a fire is by cooling the burning material to below the temperature at which it burns. So maybe cold water would work better at extinguishing fire.

Fearing that this was too obvious, I checked with James Swihart in our Physics Department. Now I can tell you that technically cold water will douse a fire faster than hot water. But of course it's not that simple--is it ever?!

Remember, water takes heat from the fire--it cools it. The main way it takes heat from the fire is by becoming steam. When the water hits the fire and turns to steam it absorbs a lot of heat, making the burning material too cool to burn.

So, although it is true that technically cold water will put out a fire faster than hot water, most of the cooling effect comes from changing the water into steam, not from raising the temperature of the water. In fact there's less than a one percent advantage in using cold water. Not really enough to be helpful to the fire department. I mean if my house were on fire I wouldn't want the fire fighters taking the time to cool down the water to get less than a one percent advantage in cooling. They could probably put out the fire faster than they could cool the water.  

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A Moment of Science®