I have often heard of people being burned at the stake, but if the body is 60% water shouldn't the fire just be put out?

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    $\begingroup$ An aluminium kettle on the fire might be 95% water, it's still dry and melts after an hour. $\endgroup$ – Karl May 7 '18 at 6:28
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    $\begingroup$ You know what happens to the water if you put out fire with it? $\endgroup$ – Mr Lister May 7 '18 at 10:49
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    $\begingroup$ You can put out small fires with your body. You know the "stop, drop and roll" advice for when your clothes are on fire? Doing that cuts off the air to the flames but it also dumps all the heat into your body. Since it's not a lot of heat when your clothes have just caught fire, that's okay. $\endgroup$ – zwol May 7 '18 at 13:30
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    $\begingroup$ And in the example @zwol mentioned the water we have in our bodies helps us with its high heat-capacity $\endgroup$ – JAD May 7 '18 at 14:06
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    $\begingroup$ I'm voting to close this question as off-topic because it's not really about chemistry and rather unclear. $\endgroup$ – Mithoron May 8 '18 at 19:17

We need to consider something. Are the people being truly burnt at the stake or are they being killed by the effects of fire? If the fire is very prolonged then their bodies will dry out and then finally be calcined to ashes.

It is an interesting (but a rather morbid) fact that cremation furnaces typically run at about 800 degrees Celsius for many hours, yet they tend to be unable to burn bodies to dust. Typically after cremation, some large fragments remain. These have to be crushed.

There are tales of spontaneous human combustion, these have been circulating for many years. I recall that the Victorians thought it was associated with extreme alcohol abuse. They thought a build-up of alcohol in the body would lead to the body being able to catch fire. A modern-day explanation is that humans wear clothing, the clothing can act as a wick.

A person dies as a result of something, then their clothing comes into contact with an ignition source. The clothing starts to burn, this heats up fat in their body. The fat melts and runs into the clothing. The clothing then acts like a candle wick and then the burning fat makes more heat which melts more fat thus turning the person into a candle.

I think that this type of human candle effect is very rare, it is likely to require a combination of several things at once. Many people who are killed in serious fires (who suffer burns as well as being exposed to hot and toxic smoke) leave behind bodies which have not been burnt to ashes.

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Water doesn't inherently beat fire. Throw a bit of water into a big fire and it will evaporate very quickly.

To find an answer, we have to take a look at what is required for a fire, and how water generally is used to fight fires. You might have heard of the fire triangle. Fire needs:

  • Fuel

  • Oxygen

  • Heat

Take away one of these, and the fire will die. Throwing water on a fire does two things:

  • It covers the fuel (at least during the initial splash, whether it will afterwards depends on the relative size of the fire), lowering availability of oxygen

  • It lowers the temperature, since the water is (I hope) colder than the fire itself. Whether this is sufficient to douse the flames depends again on the relative size of the fire.

Here we can see the things throwing water on a fire does, that putting a human on a stake does not. A human on a stake does not (make any attempt to) cover the fire. Since the human is on there since the start of the fire, warming up along with it, it doesn't accomplish a whole lot in the removing-heat-department either. If anything, as Nuclear chemist noted, it'll only add more fuel.

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  • $\begingroup$ Evaporation also uses a significant amount of energy. I believe you have to heat water over 500 K to use the same amount of energy that evaporation of the same mass of water requires. $\endgroup$ – Matt May 8 '18 at 1:56
  • $\begingroup$ @Matt hmm, this is where my knowledge of thermodynamics falls short. I can imagine that there is a difference between a sudden introduction of a colder body versus having the water slowly heat up along with the fire. I'm not sure though. $\endgroup$ – JAD May 8 '18 at 4:45

In a chemist's perspective, the fire is a high-temperature mixture of extremely reactive radical species, most importantly reactive oxygen species (ROS). Every method to reduce the movement of radical transfer to surrounding will stop the fire. Blocking the oxygen is also a way since throughout combustion reactions ROS are continuously depleted.

Simply put, the human body's water does not have any role in doing this. Our body has not only fatty parts of cells but also dry materials like hair. And whenever water of our body is in contact with fire around, it will either evaporate and escape or become another ROS source.

I can refer you to a book like Introduction to Fire Dynamics to really grasp what is fire, it is a very crucial topic in terms of our livelihood as well, yet in general, is covered in neither undergraduate nor graduate chemistry.

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On an average, a 70-kg man is made up of about 42 L of total water.

  • 28 litres is intracellular water
  • 14 L is found in the extracellular fluid of which 3 L is blood plasma
  • 1 L is the transcellular fluid (cerebrospinal fluid, ocular, pleural, peritoneal and synovial fluids)
  • 10 L is the interstitial fluid (including lymph), which is an aqueous medium surrounding cells.

So the majority (66%) of the body's water is inside the cells. When you're burned, you don't really have the ability to extract the water from the cells.

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    $\begingroup$ Cells will lyse (burst) quite quickly at the temperatures reached in a fire. Therefore the water from cells will be "extracted". $\endgroup$ – Jack Aidley May 7 '18 at 9:58
  • $\begingroup$ Could you provide a reference for these numbers? $\endgroup$ – Linear Christmas May 8 '18 at 19:14
  • $\begingroup$ @JackAidley Yes, but that water will be "extracted" to the already liquid interstitial fluid. $\endgroup$ – forest May 9 '18 at 1:32

Probably the simplest, most intuitive answer possible is to directly compare this to a very familiar instance of dousing fire with water. Did you ever see a team of firefighters spraying massive amounts of water with their fire hose? The simple answer to why a human being made mostly of water can't put out a good-sized fire with their body is that it takes way more than a single human being worth of water to douse a good-sized fire.

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Water puts out fire by coating the fuel and preventing it from getting oxygen. The fact that our bodies have water is irrelevant because we are not the source of fuel. We are the innocent victims of dissipating heat and noxious gasses. That's the simple explanation. In more detail...

How fire works - an overview

Fire is an energetic reaction where electrons transfer from a reducer (electron donor) to an oxidizer (an electron acceptor) in what is called a redox (reduction-oxidation) reaction. The acceptor is typically molecular oxygen, but oxygen is already moderately stable and needs a little "kick" in order to break its bonds and become willing to accept electrons. This kick is provided by heat*, and sufficient heat provides the activation energy necessary for the reaction to take place. As soon as the electrons are transferred, more heat is released, far more than what it took to initiate the reaction (making fire exothermic). Fire invests in heat, and it pays off as heat.

Fire starts as soon as the reactants so hot that their reaction rates become high enough to sustain the reaction without the input of any external heat. If the reaction rate is too low, more heat is lost than is produced, and all you get is smoldering. If the rate of heat production is significantly higher than the rate of heat loss, the excess heat manifests itself as a fire. The fire will die if you take away its heat, fuel, or oxygen. Exotic fires can be created with different reactants, giving them reaction rates that vary from barely fast enough to sustain itself, to so fast that the entirety of fuel is used up in a fraction of a second and a high-velocity explosion results. In this answer, I will be assuming a traditional fire. That is, a fire with a hydrocarbon fuel and molecular oxygen.

Humans are not very flammable

With very few exceptions, people just don't burn. We call heat-related injuries burns, but our bodies are not particularly good at sustaining a fire. If you were to hold your hand over a candle, your hand would not burst into flame, but you would be injured by the heat. Our bodies are already not very flammable. Fire is only harmful to us if we are near another source of flammable fuel, for example the wood used to burn someone at the stake, or our cotton clothes.

Despite this, there are three reasons fire is harmful to humans:

  1. Fire generates a lot of heat, and most of that heat is dissipated into the environment over quite a distance. Even if we are a small distance from the reaction itself, the heat is intense enough to damage us. The heat hurts, but it is not the most harmful thing fire has to offer.

  2. Fire is not a pure, clean reaction. A perfect traditional fire would take a hydrocarbon, oxygen, and heat, and convert it into pure carbon dioxide, water, and more heat, but fire is not perfect. It burns uncleanly, releasing toxic gasses like carbon monoxide and cyanide.

  3. Finally, fire kicks up particles of burnt material in smoke which, in an ironic twist, coats our lungs with burning, mucosa-irritating ash and prevents us from absorbing oxygen. We may be able to put fire out with water, but fire can put us out with smoke.

When someone is killed by fire, they are typically killed by the smoke inhalation, or by the toxic gases that are released when certain materials burn. Only rarely is it the heat alone which is fatal, in which cases it kills in a few days by damaging our protective epidermis and causing us to dehydrate rapidly and lose almost all our resistance to infection. Unlike plant material, we do not die because we ignite. We die because we need our skin intact and our lungs clean.

Water creates a barrier between fuel and oxygen

There is nothing magic about water that makes it able to put out fires, and its mere presence is of no concern to fire. In fact, traditional fires create water, but in a reaction so energetic that the water is in gas form. There are three reasons water is useful for extinguishing flame:

  1. Water is a liquid, so it will evenly coat the fuel, restricting access to oxygen.

  2. Water is not flammable, so it will not serve as additional fuel for traditional fires.

  3. Water has a high heat capacity, so it will be harder for the fire to evaporate it.

All of these require the water coat the source of fuel, acting as a barrier between the fuel and the oxygen. The water will absorb heat from the fire, all the while preventing it from gathering more oxygen, until the water can't absorb any more and evaporates. With enough water, the burning material will not have enough heat to get rid of the water and take another "breath". As the heat gets lower, it quickly becomes unable to sustain a reaction, and it rapidly cools.

Even if the fire is so big that the water evaporates before it hits the ground, it will still soak up heat that would otherwise go to increasing the ambient temperature and bringing nearby flammable materials closer to their ignition point. The water doesn't have to be cold, it just has to start out significantly cooler than its boiling point. This will decrease the overall heat, letting any more water put on the fire have a better chance of lasting long enough to coat the fuel.

Our water is nowhere near the fire

Humans do have water, but it is tucked deep inside us. We cannot put out fires near us (or right under us, or on our clothing) because the water doesn't leave our bodies in significant quantities. If we were somehow able to release a deluge of sweat and direct it anywhere we please, we might be able to put out small fires on our own (at the expense of severe dehydration). As it is however, the vast majority of our internal water is not in any position to blocking a fire's access to fuel.

Even if we are injured by the fire, our water is not kept in some big sack where rupture will result in it all being released at once. The water is inside and in between our cells, and if fire damages our cells, the water will simply be released into the already-aqueous interstitial medium. When our skin is sufficiently damaged (to a fatal extent), we will very slowly lose water through our skin. The rate will be high enough that we can die of dehydration in a matter of days, but far, far too low to put out a fire of any size. We will die of the effects of fire before we ever dry up.

* Life survives on the same reaction found in fire, using a hydrocarbon fuel, oxygen, and heat. Enzymes reduce the required energy of this "kick", making it possible to sustain a controlled exothermic reaction at body temperature.

† If the fire in question is an oil fire, water can make things much worse by causing the oil to float.

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Human bodies, most living creatures in fact, don’t burn like wood immediately or at all.

Human bodies technically boil.

Sometimes rapidly, or even explosively; other times human bodies “burn” or boil more slowly, progressing from 1-3rd degree burns, which spread with ongoing intensity and exposure.

When I was on active duty, I had to sit through many, many gross pictures and videos of war victims, via bombs, vehicle fires, fuel fires, electrical fires, etc... in order to learn how to provide combat/trauma medical aid to people who have basically been boiled. Some people I knew saw it in person, and it was basically the same as the pictures and videos. Boiled, but burnt crispy on some parts, usually extremities and membranes.

Like steak, or pork.

When grilling, if partially direct exposure to flames which cooks meat, but it’s mostly the fluids inside being heated to a point when the flesh is cooked.

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  • $\begingroup$ "Human bodies technically boil." Boil in what sense? In the culinary sense of "be cooked by being surrounded by boiling water?" In the physical/chemical sense of "be transformed from a liquid to a gas"? And I'm not sure what you mean by claiming that meat is cooked by the fluids inside being heated. The whole piece of meat, including both the liquids and the cellular structures that contain them, are heated simultaneously. It's not like, say, boiling penne pasta where the hot water moves freely through the solid pasta and heats it. The water in meat is mostly not mobile. $\endgroup$ – David Richerby May 8 '18 at 14:29

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