One of the most fundamentally human characteristics of humanity is our ability to completely by-pass
natural process’ and pathways to facilitate our own needs and desires.
Arguably the most important of these innovations was our discovery of how to create and harness
the energy from fire.
Essentially all later inventions throughout history can be attributed to or benefit from
our ability to convert stored chemical energy into another energy type.
Because of this and its essential role in both todays society and the development of
it, understanding not only what fire is but more importantly why it occurs will greatly
benefit our understanding of the world around us.
*intro*
At some point in everyones life we’ve all stared intently at a fire and tried to classify
what exactly it was we were looking at.
Depending on your knowledge of the natural world, you may have tried to think of it as
one of the states of matter.
Solid, liquid, or gas.
It certainly isn’t a solid, but it kinda seems like a gas, yet I can see it plain as
day unlike any other gas I’ve ever seen.
So it almost seems unclassifiable.
For a short and sweet explanation I like to describe it as ‘painted air’.
The moving heated air current is the canvas and tiny solid glowing particles are the paint.
So if you had to classify fire based on its state of matter it’s actually a mix of gas
and solid.
But that’s certainly an incomplete explanation and completely ignores the process’ that
lead to fires creation.
So why does fire exist?
The first thing to understand when discussing fire and surprisingly just about anything
in the universe, is that when you wonder why something occurs, the answer can almost always
be boiled down to either:
A) the final result of an action is more stable than the starting conditions, or
B)
the pathway to reach the end result is easier.
What does this mean?
The universe has an intrinsic property that causes it to prefer its energy to be in the
lowest possible energy state.Now, even with this preference, it doesn’t mean all energy
is already in its lowest energy state, but it does mean that if you give something or
someone somewhere the opportunity to lower its energy state, it will seize that opportunity.
We can visualize this as a ball on a hill.
As it sits still it is stable and will remain in its place.
However, if we give it a little nudge, or an “opportunity” to lower its internal
energy the ball will roll down the hill.
Keep this example in mind as you can use this analogy when describing the majority of chemistry.
Something gives chemicals a “push” and they roll down a symbolic hill.
So fire occurs because the resulting products or end state is more stable than the initial
reactants or energy state.
But what causes this difference in energy states?
To fully understand this, one needs a basic understanding of the subatomic particles and
their interactions.
There are a lot of other resources that can go in-depth on the basics but the main thing
you must know is this: The electron has a negative charge and is attracted to the positive
charge of the protons in the nucleus.
The more protons in a nucleus, the more attraction an electron feels towards said nucleus.
The strength at which electrons are attracted to the positive charge of protons in the nucleus
is called “electronegativity”.
A stronger attraction between protons and electrons physically shortens the distance
between the two, so like a ball rolling down a hill and being closer to the center of the
earth’s gravity, an electron being brought closer to a proton exhibits the same reduction
in energy and increase in stability.
When trying to visualize elements and their electronegativities, I like to describe various
concerts of increasing entertainment value on the periodic table of elements.
Each column is a single concert or performance, and each element is a row of 8 seats for that
concert.
The further right a concert or performance is, the bigger and more exciting that concert
is and the further down an element is, the further the seats are from the stage.
On the left here we have the alkaline metals, this is like a drunk transient singing in
the subway, and this is like a drunk transient singing in the subway with a guitar.
All the way over here we have the noble gases, this is like Queen at Live Aid, the concert
of the decade, it’s packed and all the seats are taken.
Therefore obviously the best place for the best concert experience is front row in the
last remaining seat at the second best concert.
Right here with Fluorine.
If we look at a table of electronegativity we can see that Fluorine is indeed the most
electronegative element.
However, because Fluorine is SO electronegative this seat is almost never ever available and
those who get the last seat are really difficult to convince to leave, so Fluorine almost never
exists as just fluorine and it’s resulting compounds aren’t as reactive as we’d imagine.
So what’s the second best seat?
Well some may say they’d have no problem with the second row of the second best concert
of the decade.
But unfortunately electrons are really tall and really hateful, and any electron sitting
behind another electron is gonna spend a lot of their time staring at the back of that
electrons head being upset.
This in turn tarnishes their experience and enjoyment.
For that reason, the second best seat is actually front row here at the third best concert,
Oxygen.
If we consult our table again, we see that Oxygen is indeed the second most electronegative
element and thus the second most reactive element.
And unlike Fluorine, oxygen has two available seats at their concert so it’s slightly
easier to get a seat here than at fluorine.
Not only is oxygen the second most reactive element, its also found in abundance in our
atmosphere and as a result, is the primary driver to all our fires here on earth.
This is not to be confused with explosions, as an explosion can be the result of many
different types of chemical reactions even in the absence of oxygen.
But sustained and prolonged exothermic reactions like fire are usually the result of reactions
with oxygen.
Since oxygen is so electronegative and electrons and compounds bonded with oxygen are generally
more stable than when they weren’t bonded with oxygen, whats stopping everything from
simply reacting with all of our oxygen?
Well just like the ball in the earlier example, chemical compounds exist because in their
current environment they actually are stable.
In order to change, something needs to come along and disturb their stability.
The ball doesn’t simply need just a push, it
needs a bit of extra energy before it can roll down.
In the chemistry world the most common way to supply energy is heat.
When we heat up an atom or compound, that energy can be transferred to the electrons
themselves which can cause them to excite into new and different orbitals, sometimes
they can become so excited they become completely detached from their respective atom or compound.
When detached, the electrons are highly reactive and will look for any place or seat at a concert
they can go to.
If multiple seats are open they will choose the best concert seat available.
So lets go through a simple example to complete our understanding.
Lets combust a single atom of methane.
Normally oxygen and methane coexist peacefully as the external environment isn’t extreme
enough to destabilize their internal bonds.
If we start to add heat to the system both the methane and oxygen molecules start vibrating
faster and faster, this vibrational energy at some point will be enough to transfer into
the electrons binding the atoms of the molecules together.
In the case of oxygen, this double bond is made of two different types of bonds, so this
less stable, higher energy bond (the Pi bond) splits as both of these electrons move into
higher energy orbitals.
Now despite oxygen molecules being made up of two very electronegative atoms, this bond
isn’t that strong.
All of these electrons are stuck between two absolute bangers of a concert and can’t
decide which to focus on, so they aren’t as happy as they could be.
As a result, Oxygen begins to split apart at energy levels only a little bit higher
than the energy needed to split methane bonds.
So after the Pi bond of oxygen splits, these valence electrons usually react with the hydrogens
of an already dissociated methane forming hydrogen peroxide.
At these temperatures this bond between the oxygens lasts but an instant and splits forming
two hydroxyl radicals.
These then go on to react with two more hydrogens forming two water molecules.
Another oxygen molecule reacts with Carbon in very similar fashion to produce carbon
dioxide.
During this process, the electrons originally bonded in methane molecules free themselves
into higher orbitals.
In these higher orbitals they will eventually return to a lower energy state, And as mentioned
previously, if you give the universe an opportunity to lower its energy state it will take it.
So the electrons associated with methane grab onto nearby oxygen atoms forming water and
CO2, two molecules whose overall stability is greater than before.
And if we look at the electrons, before there were only 8 electrons enjoying an oxygen concert.
Now there are 16 electrons who get to enjoy one.
As a result of greater stability there is now less energy in these resulting molecules.
So the energy that they had before is released in the form of both light and heat.
The heat energy produced from this reaction is roughly double needed to dissociate methane
and oxygen which results in further reactions of nearby molecules.
And thus the reactions sustain themselves until the concentration of methane drops low
enough that the dissociated oxygen and methane have trouble finding one another and the heat
being given off by the reaction isn’t enough to trigger another subsequent reaction.
Now that we understand why fire occurs the last question we need to answer is why does
fire look the way it does?
The light from fire is caused by black body radiation.
This phenomenon is discussed in my previous video on light, but essentially what happens
is that the heat from the fire excites the electrons within the water and carbon dioxide
molecules, but this isn't enough to dissociate them or break the bonds.
So these excited electrons return to their ground states and as they do it they release
the excess energy in the form of light.
The chemical world however isn’t perfect, methane is such a simple molecule that it
almost always forms CO2 and Water, but most molecules dont always rearrange into their
lowest energy states.
Reactions can be incomplete or imperfect.
These reaction imperfections can be amplified depending on what is burning.
Candles and wood have large molecules like cellulose and paraffin which can have upwards
of 40 carbon atoms.
The odds of all of these atoms reacting with oxygen are pretty low which results in a lot
of leftover solids.
These tiny solid particles are then carried up with the heated air where some of it can
still continue to combust.
As a result when we burn solids with larger molecules, the flames tend to be taller as
the combustion is spread out further in time and space and yellower since we have less
of a concentration of heat.
This is different compared to gas with a constant source of fuel which burns blue and short
as the gas particles react quickly.
If the gas concentration isn’t significant though, gas can burn yellow as well.
As the particles or gas molecules rise in the air they cool off very quickly as they
move further away from the active reaction beneath them, as a result we can see the change
in color or wavelengths of the black body radiation they give off.
The tip of a flame is simply when the particles become too cool to release visible light.
If we look at a flame with an infrared camera it looks significantly bigger.
The last thing to explain about fire is the behaviour and why the flame has it’s shape.
When matter and molecules heat up, they move around and as a result kind of push away other
molecules to make more room for their new movements.
As a result, warmer matter is less dense than cooler matter.
So when air is heated up around fire, it also rapidly loses density and thus quickly rises
as cooler denser air pushes in underneath it.
You can think of this movement as wind, and this wind is what carries the glowing soot
and particles to give a flame its shape and movement.
These “convection currents” also bring in new and fresh oxygen to continue the combustion
process.
In environments without gravity like on the space station, the less dense hot air doesn’t
have gravity trying to shove denser air underneath it.
As a result the flame moves out slowly in a sphere away from the combusting material.
Without convection currents a flame burns much slower as it’s harder for oxygen to
flow into the system to continue the combustion.
This is also why embers behave the way they do.
The heat generated from combustion is still hot enough to stimulate further combustion,
but because the combusting material deeper in the matter isn’t directly exposed to
oxygen it burns at a much slower rate.
This is why blowing on coals or embers makes them glow and heat up more, you’re helping
them circulate air and bring in fresh oxygen.
So just like an ember, when it feels like your light is dimming or things are looking
darker, just take a deep breath, let the oxygen flow, and trust in yourself to light they
way.