Many of the questions that we tackle about fire on this blog are relatively straightforward and involve what burns well or how to extinguish a fire. However, today, we look at something rather more complicated and the nature of fire itself. I was asked whether a fire has any mass and if so, what does fire actually weigh?
If we consider fire to be the hot air that is part of a flame, then, yes, it definitely has mass and it weighs slightly less than air as the heating of the air will cause it to rise above the colder air around it. But, if we acknowledge that fire is really heat and light then things become much more complicated.
This is an interesting, thought-provoking question. This is what you need to know about the mass of fire.
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Also read: What Makes Something Flammable?
Table of Contents
What Is Fire?
Fire is what happens when you use heat to burn fuel in oxygen and the resulting reaction is self-sustaining.
These four elements: heat, fuel, oxygen, and self-sustaining chemical reaction are the “fire tetrahedron” (4 sides).
There is also, technically, a fifth element, gravity.
Gravity is required to keep oxygen in contact with the fuel.
It’s not included in a “fire pentagram” because in almost all circumstances in day-to-day life gravity is a given.
There’s no way to switch gravity off, so if the fire is on Earth, gravity is present.
Interestingly, one of the reasons that fire on spacecraft, in space, is not particularly concerning is the absence of gravity, the flaming section can be quickly vented to remove all oxygen, and thus, it won’t burn anymore.
This just leaves burn up on re-entry as a possibility and thankfully, that’s been something that hasn’t happened in a very long time.
But this doesn’t really answer the question as to what fire is made out of.
And this is probably because it depends on who you ask as to the answer you get.
If you’d have asked an ancient Greek, she’d have told you that it was one of the four basic elements of existence and sadly, she’d have been wrong, chemistry has advanced quite a lot over the years and fire is not an element at all.
In the present day, if you ask an engineer, they will insist that fire is the gases above the burning fuel.
But if you ask a physicist, they will note that the fire is, in fact, just the heat and light being emitted from the oxidation reaction.
Well, that decision is way above our pay grade but we can look at both possibilities and ask, what is the mass of fire and thus, what is the weight of fire?
Does Fire Have Mass?
Let us assume for a second that our engineering friend is right, and that fire is made up of the gasses in the flame above the burning fuel.
If they are correct, then fire must have a mass as gasses definitely have matter inside them.
Calculating the mass of fire, however, is not an easy process.
To work it out, we would need a sealed container in which we could weigh the fuel present at the beginning of the fire and the gas present at the beginning. We would also want to take measurements during the fire and weigh the material outside of the flame and then at the end of the fire, we’d need to weigh the remaining fuel/oxidized product.
We could then, using some fairly complicated mathematics, work out how much gas had been in the flame and the mass of that gas, and from there, we could relatively easily calculate its weight.
As activities go, this is pretty pointless and very hard work, so to date, nobody has actually tried to calculate the mass of gas involved in a fire in this manner.
The Engineer’s Model Approximated By A Mathematician
A mathematician has tried to approximate this idea though and they use the “ideal gas law” to come to grips with the idea that the density of a gas is inversely proportional to its temperature in Kelvin.
(This is fairly obvious when you think about it, when things are cold, they contract and become denser and when they are hot, they expand and become less dense).
They then take the density of air at 300K which is 1.3 kg/m3.
And then the average temperature of a flame (about 1300K).
Combining these two data points with the ideal gas law and you know that the density of the gas in the flame is going to be roughly 0.25 times the density of the surrounding air.
Given that air weighs around 1.3kg per cubic meter, this would give fire a mass of about 0.3kg per cubic meter.
If this approximation comes true, you’ve “only” got to measure the approximate volume of your fire and you have its approximate mass.
The Problems With This Approach
But it will be very approximate because the gasses in a fire are not air and, in fact, may vary dramatically from air based on the material which is burning.
Not only that, but this approximation also requires our engineer to be correct that fire is the gas that’s floating about in the flame, and we’d argue that it’s quite possible that the engineer is not at all right and that the physicist is correct in saying that heat and light are the fire and thus, it’s much harder to calculate the mass present.
Relationship Between Heat, Light, and Mass
Heat and light don’t technically have any mass at all, they are, in fact, expressions of energy in photons that are famous for not having any mass.
So, is the mass of fire zero if heat and light are fire?
There’s one equation in physics that almost everyone on Earth has heard of, even if they don’t know what it means and it was put forward by Albert Einstein as part of his approach to relativity.
It is, of course, E=mc2
That equation means that Energy equals mass x the speed of light squared.
Yes, energy and mass are equivalent.
So, if we measured the energy released in a fire and then converted it using this formula, we’d get a mass of fire.
This is better than the engineer’s model because it takes into account the fact that fire is based on a chemical reaction of a fuel-burning in oxygen.
We can burn aluminum in oxygen to form aluminum oxide, but there are no gasses created in that reaction.
So, how does the aluminum burn if there is no gas created and thus, no fire, from the engineer’s perspective?
In this instance, however, even if your fire is very large because the speed of light squared is a huge number – the effective mass of fire is zero.
How Much Does Fire Weigh?
It depends on whether you’re following our engineer’s path or our physicists.
“Weight” is distinct from mass because it’s the action of mass under gravity.
So, a fire that had a mass of say 10 Kg using the engineer’s model would weigh approximately 100 Newtons (because the acceleration under gravity here on Earth is approximately 10m/s2).
But using our physicists’ energetic model of fire, the weight of fire is zero because zero x 10m/s2 is still zero.
But as one online wit wryly observed, if you are ever caught in a fire, you will not have to worry about being crushed by the weight of the flame because burning will be a much bigger priority to avoid.
So, does fire have mass? It depends on whether you’re a physicist or an engineer and the weight of fire is simply found by multiplying acceleration under gravity by the mass of fire.