|Why is it that dry ice is colder than regular
ice? What different "elements" make it colder?
|Question Date: 2004-03-13|
Regular ice is made from freezing water. Water
freezes at zero degrees Celsius, or +32 degrees
Fahrenheit. (Most of Europe and all scientists use
Celsius because we're lazy and it's easier, but
the US is still stuck on the Fahrenheit
temperature scale.) "Dry ice" is made from frozen
carbon dioxide gas. Carbon dioxide is an odd
substance because at regular pressure, it is only
found in gas and solid forms: the stuff in the air
you breathe and dry ice. So when dry ice "melts"
it goes directly into the gas form!At really high
pressures, carbon dioxide does form a liquid
phase. Anyway, dry ice is much, much colder
because carbon dioxide freezes at a lower
temperature:-79 degrees Celsius or -109 degrees
Fahrenheit! It's very dangerous to handle dry ice
because it will freeze your skin and cause frost
bite in less than a minute, causing a painful "dry
ice burn". Did you know that in Antarctica,the air
can sometimes get so cold that the carbon dioxide
in the air will freeze, turning into dry ice? This
makes it very hard to breathe the air, because
without carbon dioxide, our brain doesn't know
when to make our body take a breath. (The brain
uses the amount of carbon dioxide in our blood as
a signal to breathe rather than the amount of
oxygen. Go figure!)
Dry ice is made of solid carbon dioxide
(CO2) while ice is made, of course, of
solid water. Almost all substances have a freezing
point which depends on their unique physical and
chemical characteristics. I'm not sure,though,
how well we understand why a particular substance
has the exact melting point it does.
of the stuff we deal with on a daily basis is
solid,meaning that it has a freezing point
(transition from liquid to solid) that is higher
than "room temperature," or about 20 degrees
Celsius A lot of room temp. liquids, such as
water, alcohol, and gasoline have freezing points
below 20 C but, perhaps, not too far below 0
Substances that are normally gases at 20
C and one atmosphere pressure, such as
CO2, oxygen, and nitrogen,have much
lower freezing points. The temperature of dry ice
is around -80 C or 195 K (195 degrees above
absolute zero). Oxygen, argon, and nitrogen
freeze at around 65 K.
Neon and hydrogen freeze
at 10 to 20 K. Normal helium, as it turns out,
doesn't ever freeze exactly.
interesting thing about dry ice is that carbon
dioxide doesn't exist as a liquid at room temp.
and one atmosphere pressure. So dry ice doesn't
melt (hence it is "dry") but sublimates - meaning
that the CO2 goes directly from solid
to vapor as it absorbs heat. The CO2
cylinders that are used to put bubbles in soda are
filled with liquid CO2 but have to be
pressurized at around 17 atmospheres.
Lots of things that are gases at room temperature
can be frozen into liquids or solids at cold
temperatures. Ice is just water frozen into a
solid. Even though dry ice is called "ice", it's
not. Dry ice is actually a gas at room
temperature - carbon dioxide - frozen into a solid
at about -110 degrees. At ice temperatures,
carbon dioxide is a gas - and as you probably know
is already in the atmosphere and air you breathe.
So if you are playing around with dry ice, you
want to make sure to be in a large,
well-ventilated room so the carbon dioxide level
in the room doesn't get to high as the dry ice
melts. By the way, -110 degrees is cold enough to
hurt you if you touch it without gloves and
definitely shouldn't be tasted (which you probably
knew). Something to notice is that dry ice,
unlike water, melts directly into a gas without
being a liquid. This is called sublimation.There
are colder things than dry ice. 70% of all the
gas in air is nitrogen. Nitrogen can freeze into
a liquid at -320 degrees. Colder than that is
liquid helium which has a temperature very close
to absolute zero.Thanks for the questions.
Dry ice is carbon dioxide ice, not water ice.
Carbon dioxide freezes at somewhere around -70 C
(I think)whereas water freezes at 0 C. Water ice
can get that cold, of course, but if you have dry
ice, it takes energy (and a lot of it) to take
something from a solid to a gaseous phase, so it
stays at that temperature until it evaporates
(carbon dioxide does not form a liquid at
atmospheric pressure, no matter how hard you try).
As a result, the solid stays at its evaporation
temperature. Water, by contrast, heats up to its
melting point before plateauing for the same reason.
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