Pro-Human Extremist

Extremism in the defense of humanity is no vice

Why does water put out fires?

with 3 comments

We learn about our world so early—as little children. We find out what gives us pleasure and what causes pain. Even the most loving parents speak sharply to warn their children away from dangers. In this way, we all developed an emotionally charged idea of danger long before we had the mental tools to understand why dangerous things are dangerous.

Fire is one of those dangerous things we all learned about early, along with other hot things like stoves and ovens. Watch out—you’ll burn yourself! Keep away from that, it’s hot! From a respectable distance, fire is warm and cozy and so lovely to look at, but if you get too close it’s painful and destructive.

Most people’s understanding of fire pretty much ends there. We know what sort of things will or will not burn, and we have a sense of how fire behaves, and that’s all we really need to know about it. The chemistry and thermodynamics of fire have been thoroughly described by scientists, and most people once learned something about that in science classes, but relatively few people walk around with a very clear idea of it in their head. That’s fine for practical purposes, but if we don’t ourselves understand at least the basics of how fire actually works, then in effect fire is for us a kind of magic—producing familiar effects by mysterious means.

The science behind fire is discussed in the Wikipedia article on the subject. Basically, fire occurs whenever the temperature is high enough to oxidize some fuel. When wood burns, organic molecules that make up the wood react with oxygen in the air to produce carbon dioxide and water. Because the products of this reaction are held together by stronger chemical bonds than the reactants were, the reaction releases energy. That release of energy speeds up the motion of nearby molecules in the air, which further raises the temperature, because temperature is a measure of how rapidly molecules are moving.

Fire is called a chain reaction because the energy released through the oxidation of organic molecules helps keep the temperature high enough to permit the oxidation of other nearby organic molecules. If the release of energy stopped, the temperature would rapidly drop as heat flowed away. But if enough energy is released in each second, the temperature will stay high enough to keep the oxidation going, and so the wood will keep burning. That’s the chain reaction.

So how do you put out a fire? You can deprive it of fuel or of oxygen. Or you can get rid of the heat fast enough that the chain reaction stops, and that’s what water does.

Two things happen when water finds itself in a fire: the water’s temperature rises to boiling point, and then the water evaporates from liquid to gas. Both of these physical changes absorb energy—particularly the evaporation, which absorbs about six times as much energy as is needed to raise the temperature of water all the way from freezing point to boiling point. Any energy absorbed in evaporating water means less energy is left to help keep up the temperature of the fire. Toss in enough water, and the temperature falls below the threshold needed to sustain the chain reaction, and the fire goes out.

Water turns out to be an excellent liquid for lowering the temperature of a fire. It doesn’t combust like alcohol or gasoline, which is kind of important. But even in comparison to other non-combustible liquids, water has both a high specific heat and a high heat of vaporization. The specific heat is the amount of energy that must be absorbed to raise the temperature of the water, and the heat of vaporization is the amount of heat that must be absorbed to evaporate the water. Thus, water does a really good job of absorbing energy as its temperature is raised and as it evaporates, which makes it very efficient in putting out fires by lowering the temperature enough to interrupt the chain reaction.

© Joel Benington, 2012.

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Written by Joel Benington

July 17, 2012 at 3:14 pm

Posted in science

Tagged with , , ,

3 Responses

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  1. […] of slipping on a thick jacket on a cold day, or wrapping oneself in a warm blanket! Like our fear of burning ourselves in fires, our experience of snug warmth and our intuitive knowledge of how to get it date back long before […]

  2. What happens when water is used to put out kerosene?

    historypt2014

    August 26, 2014 at 2:52 am

  3. Water is a good fire quencher

    Gentle king

    June 15, 2016 at 10:15 am


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