6 ChemMatters | FEBRUARY/MARCH 2018 www.acs.org/chemmatters
Liquid and Supercritical Carbon Dioxide
WE’VE ALREADY SAID THAT SOLID CO2 SUBLIMATES DIRECTLY INTO CO2 GAS.
So how is it possible to have liquid CO2 Take a look at the
phase diagram for CO2 on the right. At one standard ato-mospheric pressure unit (atm), CO2 will only change from
a solid to a gas when warmed (blue arrow). It can only be
liquefied if put under enough pressure—namely 5. 1 atm or
greater. However, the catch is that the temperature must
be between –56.7 and 31 °C to observe liquid CO2 (green
At 31 °C, CO2’s critical temperature, no amount of pressure will liquefy it. Gaseous CO2 can be liquified at room
temperature by putting it under pressure. Think about fire
extinguishers (red arrow).
If you heat CO2 above its critical point you get a
strange substance known as a supercritical fluid,
which has properties of both liquids and gases. Its
density is closer to that of a liquid, but its viscosity is more
like that of a gas. It can effuse through a solid like a gas, but flows more like a liquid. Supercritical fluids can dissolve
other substances. Since CO2 is nonpolar, it will dissolve other nonpolar substances. Some coffee beans are treated with
supercritical CO2 to extract caffeine and make decaffeinated coffee.
When the CO2 first begins to liquefy gas, liquid and solid CO2 can all be observed at the same time. The temperature
and pressure in which all three phases exist simultaneously is known as the triple point.
When dry ice is placed in water, its sublimation rate will increase dramatically. It is now
surrounded by a substance that is not only a
better conductor than air but also denser than
air. The water releases its heat to the dry ice at
a rapid pace, greatly accelerating the sublimation rate. The visible fog that comes from the
water is not carbon dioxide, however. The fog
contains carbon dioxide, but CO2 itself is invisible in the gas form. It is not even water vapor;
it is fine particles of condensed liquid water
brought upward by the sublimating dry ice.
Dry ice was first discovered by French
chemist Charles Thilorier in 1835. Like many
great discoveries, this one happened by
accident. He took the lid off a metal container
containing liquid CO2 that had been prepared
by subjecting CO2 gas to high pressures. As
expected, the liquid evaporated rapidly upon
exposure to atmospheric pressure and room
temperature. The pressure dropped, allowing
the molecules to spread out and gaseous CO2
to leave the container. But what he did not
expect to see was the layer of solid dry ice in
the bottom of the cylinder.
Evaporation, which is what Thilorier observed,
is an endothermic process—energy is
absorbed by the system from its surroundings. When the CO2 evaporated, energy was
absorbed by the liquid molecules to overcome
the weak intermolecular forces of attraction,
and enter the gaseous phase. But where did
that energy come from? The liquid CO2 in the
bottom of the cylinder provided that energy,
so rather than becoming a gas, it solidified.
Freezing is exothermic—it’s a process that
releases energy from the system to its surroundings.
Making dry ice
The first step in making dry ice is to start
with gaseous CO2. The gas is put under
high pressure and cooled, which allows the
molecules to get close enough to form weak
attractions (London Dispersion forces) and
liquefy. The liquid CO2 is released from the
tank through an expansion valve into an empty
chamber. The shift to lower pressure changes
the liquid to gas. This endothermic change
makes the temperature drop rapidly and about
half of the gas will immediately freeze into dry
On a small scale, dry ice can be made using a
liquid CO2 pressurized container and a special
attachment that captures and presses the CO2
flakes into a puck.
Temperature (not to scale)
–78.5 °C 31 °C
5. 1 atm
Carbon Dioxide Phase Diagram