IDIOT’S GUIDES: SCIENCE MYSTERIES EXPLAINED
Why is frozen carbon dioxide called “dry ice”?
Frozen carbon dioxide is used in ﬁre extinguishers and some fog machines. But they call
it “dry ice.” What’s so dry about ice?
Water ice turns into a liquid before it then evaporates into steam, but carbon dioxide skips the liquid part
when it melts. So CO
ice never gets wet.
Under normal surface conditions on our planet,
there are three so-called “states of matter”—gas,
liquid, and solid. And the substance we see change
states the most often is water.
Key to life on Earth is the way our planet is just
hot enough, with just the right air pressure, for
what’s called the “triple point” of water. That means
with just a little addition or subtraction of energy,
we can make water a gas, a liquid, or a solid.
All solids can be melted, and you can think of
any solid material as being “frozen.” Water ice
has some special chemical properties that make
it dierent from a solid block of, say, iron, but
the basic idea is the same.
If you heat iron to 2,800°F, it will melt into
a liquid. If you keep heating it all the way up to
5,182°F, it will boil into a gas.
Carbon dioxide is the same. Under normal
conditions here on Earth, CO
is a gas. If you
chill it down to -109°F, it will freeze into a white
ice that looks quite similar to water ice.
But when it comes to melting CO
discover there’s more to melting and boiling
a chemical than just its temperature. The air
pressure around the chemical is also very
People who live high up in the mountains already know
their tea boils a couple degrees lower than that of people
who live by the ocean. That’s because the air is thinner at
high altitude, and water boils at a lower temperature. In
a similar way, carbon dioxide, unless it’s kept frozen, will
boil into a gas if the air pressure around it is less than ﬁve
atmospheres—that is, ﬁve times the air pressure at sea
Since there’s nowhere on Earth with air pressure
that high except in special chambers and labs, any time
carbon dioxide ice melts it skips the liquid phase and boils
straight into a gas.
When you look at a block of dry ice melting, all the
fog you see is just water in the atmosphere condensing
against the very cold CO
gas. The CO
itself is invisible.
When the dry ice melts completely away, there’s no puddle
or residue left behind. Thus the name: dry ice!
Dry ice is very useful because it’s much colder than
water ice. It’s especially useful in insulated containers
because it can keep water frozen without needing an
external power source. We also use it in ﬁre extinguishers
because, as pure CO
, it can smother a ﬁre—which needs
oxygen to burn.
The way CO
turns straight into gas from solid is called
“sublimation.” And it highlights why it’s so important that
Earth’s temperature and air pressure be at the triple point
of water. If our air pressure was very low, water ice would
be like dry ice: it would boil into steam without forming
a liquid ﬁrst. Without liquid water, many of the chemical
reactions in our bodies and the bodies of all living things
Ice (Water, H O)
Dry Ice (Carbon Dioxide, CO )
IDIOT’S GUIDES: SCIENCE MYSTERIES EXPLAINED
What’s so special about carbon, anyway?
Carbon, carbon, carbon. It’s all you hear about these days. Carbon economy, carbon
emissions … what’s so great about this particular element?
Carbon is the foundation on which all life is built. Without carbon, there may not be any life at all. Yet
carbon could end up killing us all ….
Life is made of chemicals, and life depends on
chemicals. Without two important molecules—
oxygen and water—nothing on Earth that we know
about could survive. But there’s a chemical even
more fundamental to life than oxygen and water:
Even though oxygen is essential to make our en-
ergy, and water is essential to keep our cells work-
ing, none of this would happen without so-called
organic compounds to carry the energy and use the
water. And these compounds all have long chains of
carbon atoms in them.
If oxygen is the walls of the house of life, and
water is the roof, then carbon is the foundation.
And also the mortar between the bricks. And all
Carbon’s chemical superpower is its ability
to connect with up to four other atoms at a
time. Not only can it make four connections,
it requires relatively small amounts of energy
to make it give up these atoms and break the
chemical bonds that keep them attached.
Because of this, carbon can be part of
millions of dierent chemical compounds.
Think about it: there’s carbon in the molecules
that make up your eyelashes, but carbon also
forms diamond—one of the hardest natural
substances. Carbon ﬂoats around in the air as
carbon dioxide, and it also makes up the wood of
mighty trees. Wherever there’s biology, there’s a
lot of carbon.