Water is good for putting out fires because it has a very high latent heat of vaporization.

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.

There are three states of matter: solid, liquid and gas. A solid will keep its shape with or without a container and has a fixed volume. A liquid has a fixed volume but will assume the shape of its container. A gas will expand and disperse over time; thus it does not have a fixed volume or shape without a container.

Heat of Fusion and Heat of Vaporization

As solid or liquid fuels are exposed to heat, they absorb energy. The molecules increase in temperature depending on the specific heat capacity of the material, and can absorb enough energy from a fire to turn into vapour before they mix with air and burn.

When a solid heats and reaches its melting point, it absorbs additional energy, known as the "latent heat of fusion," until the solid material turns into a liquid. With additional heating, the temperature of the liquid increases in temperature until it reaches its boiling temperature, the highest temperature at which it can exist in liquid form at a particular atmospheric pressure. How much heat is required to increase the temperature of a unit mass of liquid (or any other material) by one degree in temperature is determined by the specific heat capacity of that material and designated as cP.

At the boiling point, the liquid undergoes a phase change from liquid to gas in a process called evaporation. During the phase change, the liquid will not increase in temperature, because all of the heat that is added is used (absorbed) to overcome the latent heat of vaporization (energy added into the liquid to change the phase of the material from a liquid to a gas).

The higher the specific heat capacity and/or latent heat of vaporization of a material, the more energy must be absorbed by a given mass of material for it to increase in temperature and become a gas. This is also a key part of why water is such an effective cooling agent when applied correctly to a heated fire environment. Water has a very high heat capacity as well as a relatively high latent heat of vaporization value. This means that water at ambient temperature first has to absorb heat to get to its boiling temperature (100°C). It then absorbs more heat from the environment as it evaporates and turns into steam.

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Hot water extinguishes fire faster, as the heat that must be absorbed to convert hot water to steam is much greater than the heat absorbed in bringing cold water up to 100°c.

“A fire seems to have started outside!! Quick, get me some water!” Charlie shouted.

“Hot or cold, sir?” questioned the servant.

“Doesn’t matter. Just get some water!” Charlie replied.

In the “heat” of the moment, it’s easy to forget, but the temperature of the water does actually matter, as it affects the water’s efficiency in extinguishing a fire.

It’s a common misconception that spraying cold water on a fire will help put it out faster, but in reality, hot water is more effective in putting out a fire than cold water or water that is at room temperature. This is credited to a variety of factors, including the high heat capacity and the high latent heat of vaporization of water. Before we get into those details, let’s cover the basics: how does water help put out a fire in the first place?

Hot water or cold water to extinguish a fire?

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How does water put out fire?

Fire is a three-headed devil. Cut any one of them off and you will effectively kill it. These three heads are heat, fuel, and an oxidizing agent, although, a fourth head (chain reaction) is sometimes important to consider. The application of heat to any fuel in the presence of an oxidizing agent is what starts a fire. Once a fire has commenced, it automatically burns through all the available fuel; this automated and perpetual propagation of reactions is called a chain reaction.

Heat, fuel and an oxidizing agent collectively form the powerfyl fire triangle.

Removing any of the aforementioned elements of a fire is fairly difficult. Starting with the first element, removing heat requires the employment of any material that absorbs heat, which will reduce the heat available to sustain the chain reaction. Next, fuel is anything that catches on fire.

However, removing fuel doesn’t instantly extinguish a fire, but rather restricts its further dispersal. Lastly, fire needs oxygen to breathe and continue its onslaught. Without a steady supply of oxygen, a fire starts to simmer down and ultimately dies.

Water extinguishes a fire by simultaneously severing two of its heads—heat and the oxidizing agent. Upon being sprayed, water first has a cooling effect on the fire. This cooling occurs due to the heat being absorbed by liquid water to transform itself into vapor (more on this later). Also, once liquid water is converted into vapor, it establishes a barrier between the burning fuel and atmospheric oxygen.

Thus, cutting off the supply of an oxidizing agent. This is exactly how a fire extinguisher works. When one releases pressurized CO2 from an extinguisher, the resulting CO2 cloud cuts off the supply of oxygen to the fire, eventually causing it to die.

Why is hot water better than cold water?

Water is more effective than most other liquids at putting out fires due to its chemical properties, specifically its heat capacity and latent heat of vaporization. Heat capacity is the amount of heat required to raise the temperature by one Kelvin. Water has one of the highest specific heats among other naturally occurring substances.

You would require about 4.182 KJ/kg to raise the temperature of water by 1 Kelvin. Thus, a large amount of heat is absorbed by the water when it is sprayed on fire in order to raise its own temperature.

However, the heat absorbed to raise the temperature only provides part of the cooling effect.

The heat absorbed when liquid water changes into steam is much greater than 4.182 KJ/kg and further boosts the cooling effect. Once the boiling point of water (100°c) is achieved, the heat being absorbed is no longer utilized to raise the temperature, but rather to break the bonds between the water molecules.

The amount of heat required to break all the bonds and thus convert liquid water to water vapor is called the latent heat of vaporization. For water, the latent heat of vaporization is quite high, standing at about 2,260 kJ/kg.

A large amount of heat is absorbed in converting liquid water to water vapor. (Photo Credit : Fouad A. Saad/Shutterstock)

When cold water is used, time is first spent in bringing the cold water up to the boiling point. Since the temperature of hot water is already near its boiling point, less time is required to reach 100°c. Thus, the high latent heat of vaporization of water comes into play sooner for hot water than it does for cold water, so heat is absorbed at a faster rate by hot water. Also, a quicker conversion to steam means a faster establishment of a barrier between the burning fuel and oxygen.

This combination of higher heat absorption when converting to steam and a faster barrier establishment helps hot water extinguish a fire more rapidly than cold water.

Final Words

Hot water may scientifically seem more efficient in putting out a fire, but you’re probably wondering how much more efficient is hot water than cold or regular water. Honestly, the difference might be too small to even consider, particularly in the practical moment of needing to put out a fire. However, the next time you see an accidental fire and have hot water on hand, you’ll be able to confidently toss the near-boiling water on the flames.

However, you must be careful when using water to put out electrical and chemical fires. Water is a good conductor of electricity and can help spread such fires even further, rather than extinguishing them. Chemical fires, on the other hand, may contain chemicals that are less dense than steam and will thus float over the barrier and continue to extract oxygen from the environment.

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References

1. Auburn University
2. UC Santa Barbara
3. Pennsylvania State University

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