The law of conservation of mass – Todd Ramsey
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The law of conservation of mass – Todd Ramsey


Where does all this stuff come from? This rock? That cow? Your heart? Not the things themselves, mind you,
but what they’re made of: the atoms that are
the fabric of all things. To answer that question, we look to
the law of conservation of mass. This law says take an isolated system defined by a boundary that matter
and energy cannot cross. Inside this system, mass,
a.k.a. matter and energy, can neither be created nor destroyed. The universe, to the best
of our knowledge, is an isolated system. But before we get to that, let’s look
at a much smaller and simpler one. Here we have six carbon atoms,
12 hydrogen atoms, and 18 oxygen atoms. With a little energy,
our molecules can really get moving. These atoms can bond together
to form familiar molecules. Here’s water, and here’s carbon dioxide. We can’t create or destroy mass. We’re stuck with what we’ve got,
so what can we do? Ah, they have a mind of their own. Let’s see. They’ve formed more
carbon dioxide and water, six of each. Add a little energy, and we can get them
to reshuffle themselves to a simple sugar, and some oxygen gas. Our atoms are all accounted for:
6 carbon, 12 hydrogen, and 18 oxygen. The energy we applied is now stored
in the bonds between atoms. We can rerelease that energy by breaking that sugar back
into water and carbon dioxide, and still, same atoms. Let’s put a few of our atoms aside
and try something a little more explosive. This here is methane, most commonly
associated with cow flatulence, but also used for rocket fuel. If we add some oxygen
and a little bit of energy, like you might get from a lit match, it combusts into carbon dioxide,
water and even more energy. Notice our methane started
with four hydrogen, and at the end we still have four hydrogen
captured in two water molecules. For a grand finale, here’s propane,
another combustible gas. We add oxygen, light it up,
and boom. More water and carbon dioxide. This time we get three CO2s because the propane molecule
started with three carbon atoms, and they have nowhere else to go. There are many other reactions
we can model with this small set of atoms, and the law of conservation of mass
always holds true. Whatever matter and energy
go into a chemical reaction are present and accounted
for when it’s complete. So if mass can’t be created or destroyed, where did these atoms
come from in the first place? Let’s turn back the clock and see. Further, further, further, too far. Okay, there it is. The Big Bang. Our hydrogen formed from
a high-energy soup of particles in the three minutes that followed
the birth of our universe. Eventually, clusters of atoms accumulated
and formed stars. Within these stars, nuclear reactions
fused light elements, such as hydrogen and helium, to form heavier elements,
such as carbon and oxygen. At first glance, these reactions
may look like they’re breaking the law because they release
an astounding amount of energy, seemingly out of nowhere. However, thanks to
Einstein’s famous equation, we know that energy is equivalent to mass. It turns out that the total mass
of the starting atoms is very slightly more
than the mass of the products, and that loss of mass perfectly
corresponds to the gain in energy, which radiates out from the star as light,
heat and energetic particles. Eventually, this star went supernova and scattered its elements across space. Long story short, they found each other
and atoms from other supernovas, formed the Earth, and 4.6 billion years later
got scooped up to play their parts in our little isolated system. But they’re not nearly as interesting as
the atoms that came together to form you, or that cow, or this rock. And that is why,
as Carl Sagan famously told us, we are all made of star stuff.

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