How does dry ice work?

Dry ice is different from regular ice in several ways. Find out what makes dry ice so unique -- and why it's so good for transporting perishables over long distances -- in this podcast from HowStuffWorks.com.

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Episode Transcript

Speaker 1 (00:00):
Welcome to brain Stuff from houstuff works dot com where
smart happens. Join Josh and Chuck, the guys who bring
you stuff you should know, as they take a trip
around the world to help you get smarter in a
topsy turv economy. Check out the all new Superstuff Guide
to the Economy from how stuff Works dot Com, available
now exclusively on iTunes. Hi Am Marshall Brain with today's question,

(00:26):
how does dry ice work? Dry ice is frozen carbon dioxide.
A block of dry ice has a surface temperature of
minus a hundred nine degrees fahrenheit or minus seventy eight
degrees centigrade. Dry ice also has the very nice feature
of sublimation. As it breaks down, it turns directly into

(00:47):
carbon dioxide gas, rather than ever becoming a liquid. The
super cold temperature and the sublimation feature make dry ice
great for refrigeration. For example, if you want to send
something frozen across the country, you can pack it in
dry ice. It'll be frozen when it reaches its destination
and there won't be any messy liquid leftover like you

(01:09):
would have with normal ice. Many people are familiar with
liquid nitrogen, which boils at minus three hundred and twenty
degrees fahrenheit. Liquid nitrogen is fairly messy and difficult to handle.
So why is nitrogen a liquid while carbon dioxide is
a solid. This difference is caused by the solid liquid

(01:29):
gas features of nitrogen and carbon dioxide. We're all familiar
with the solid liquid gas behavior of water. We know
that at sea level, water freezes at thirty two degrees
fahrenheit and boils at two hundred twelve degrees fahrenheit. Water
behaves differently as you change the pressure. However, as you
lower the pressure, the boiling point falls. If you lower

(01:52):
the pressure enough, water will actually boil at room temperature.
If you plot out the solid liquid gas behavior of
a substance like water on a graph showing both temperature
and pressure, you create what's called a phase diagram for
the substance. The phase diagram shows the temperatures and pressures
at which a substance changes between a solid, a liquid,

(02:14):
and a gas. When you look at the phase diagram
for carbon dioxide, what you can see is that at
normal pressures, carbon dioxide moves straight between a gas and
a solid. It's only at much higher pressures that you
find liquid carbon dioxide. For example, a high pressure tank
of carbon dioxide or a carbon dioxide fire extinguisher contains

(02:38):
liquid carbon dioxide. To make dry ice, you start with
a high pressure container full of liquid carbon dioxide. When
you release the liquid carbon dioxide from the tank, the
expansion of the liquid and the high speed of operation
of carbon dioxide cools the remainder of the liquid down
to the freezing point, where it turns directly into a solid.

(03:00):
If you've ever seen a carbon dioxide fire extinguisher inaction,
you've seen this carbon dioxide snow form in the nozzle.
You compress the carbon dioxide snow to create a block
of dry ice. Do you have any ideas or suggestions
for this podcast? If so, please send me an email
at podcast at how stuff works dot com. For more

(03:22):
on this and thousands of other topics, go to how
stuff works dot com.