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