Demonstrating the Phase Changes of Carbon Dioxide

Introduction

As dry ice sublimes in a closed system, its three phases are clearly viewed and its phase diagram takes on concrete meaning to students.

Concepts

• Phase changes
• Phase diagrams
• Triple point
• Sublimation

Materials

Safety Precautions

Dry ice is extremely cold, handle only with heavy cloth gloves and never with wet hands: Frostbite is possible with only brief exposure. Never allow the Triple Point of CO₂ apparatus to exceed 100 psi, do not fill the tube with more than 8 inches of dry ice, and never leave a filled apparatus unattended. Always release the pressure and leave the ball valve open until all the dry ice has sublimed. Inspect the apparatus before every use. Over time, the Tygon tubing may develop weaknesses if misused. Replace the tubing if there are any signs of wear, cracking, or weak spots. Check the hose clamps to make sure they are tight. The demonstrator and all observers must wear safety eyewear.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Allow the dry ice to sublime in a well-ventilated room.

Prelab Preparation

Inspect the Triple Point of CO₂ apparatus. Replace the tubing if there are any signs of wear, cracking, or weak spots. Check the hose clamps to make sure they are tight; tighten with a wrench if necessary. Check to see that all other fittings are tight. 

Procedure

1. Using a hammer or heavy weight, break tennis ball size clumps of dry ice from a larger block of dry ice. Wrap the dry ice pieces inside a paper towel and pulverize the dry ice into fine crystals with the side of the hammer or large weight.
2. Open the ball valve on the triple point apparatus. Quickly scoop up some of the fine crystals with a metal scoopula and funnel them into the valve opening. It is important to work quickly at this point to utilize the energy (heat) available in the triple point apparatus tube and reduce the amount of time needed to reach the triple point.
3. When roughly six to eight inches of the tube has been filled with the powdered dry ice, close the valve and observe the pressure gauge. The pressure inside the tube will rise as the dry ice begins to sublime into carbon dioxide (CO₂) gas. The tube may need some additional warming or the frost may need to be removed from the outside of the tube. Use your hand but be careful not to hold the tube too tightly—frostbite may occur. Do not put any more than 8 inches in the tube.
4. The pressure will continue to rise to roughly 75 psi (5.1 atmospheres). At this point, liquid CO₂ will begin to form. To see the liquid at first, the tube may have to be tilted so the liquid–solid slurry moves. All three phases of CO₂—solid, liquid and gas—are present and in equilibrium. This is the triple point of CO₂. Caution: Pressure will quickly increase after this point; Do not allow pressure to rise above 100 psi.
5. Note that the pressure remains steady as long as both the liquid and solid carbon dioxide exist together. Boiling can also be observed near the ends of the slurry. Once the solid has completely melted or is covered by the liquid phase, the solid and gas are no longer in equilibrium and the pressure will rise again. Tilt the tube to cover the solid. The pressure rises. Move the tube to uncover the solid, if enough solid exists, the pressure will return to the triple point pressure.
6. Release the pressure in the tube by opening the ball valve. Do not point the end of the triple point apparatus at anybody when releasing the pressure. When the pressure is released, the liquid will return to the solid state. When the valve is opened quickly, solid and gaseous CO₂ will spray out much like a CO₂ fire extinguisher. When the valve is opened slowly, the liquid CO₂ will boil before turning to solid CO₂; and only gaseous CO₂ will be released.
7. Close the valve and observe. The pressure will return to the triple point pressure. Release the pressure and the carbon dioxide will again return to the more stable, solid state. This process can be repeated as long as there is adequate dry ice remaining in the tube.

Student Worksheet PDF

12609_Student1.pdf

Answers to Questions

1. Note what was happening in the apparatus at the following moments during the demonstration.

a. When the solid CO₂ was first enclosed in the apparatus

The pressure inside the tube rose and the carbon dioxide began to sublime, turning into a gas.

b. When the pressure stabilized (also give the pressure measurement here)

The pressure was about 75 psi. There is carbon dioxide present in all three states, solid, liquid and gaseous.

c. When the solid CO₂ was covered by the liquid carbon dioxide

The pressure began to rise again.

d. When the pressure in the apparatus was released

The liquid carbon dioxide froze back into its solid state.

2. Explain how the triple point was reached within the apparatus.

When the dry ice was sealed in the apparatus, the pressure within the apparatus began to rise. Solid carbon dioxide, which in normal room temperature and pressure sublimes, was able to remain in its solid state at a higher temperature. Both the temperature and the pressure continued to increase in a way that allowed both the solid and gaseous carbon dioxide to exist at once. Eventually the pressure and the temperature reached a point where the solid began to melt as well. At this point, all three states were in equilibrium with one another.

3. Following is a phase diagram for CO₂. Label each area on the diagram with the appropriate phase. Also mark and label the triple point.

Discussion

Dry ice is usually a fascinating and fun material for your students. From making “fog” to “boiling in water,” it is well-known for creating special effects. Carbon dioxide, however, also has fascinating and very useful chemical properties. At room temperature and pressure, solid carbon dioxide will warm to –78 °C and then begin to sublime to carbon dioxide gas. The carbon dioxide gas is, initially, also at –78 °C, which causes moisture in the air to condense and form the characteristic fog for which dry ice is famous.

One interesting feature of carbon dioxide is that at atmospheric pressure, it only exists as a solid or gas. In order to exist as a liquid, carbon dioxide must be subjected to a pressure of at least 5.11 atmospheres. Most chemicals will exist as a solid, liquid or gas depending on temperature and pressure. This relationship between phase, pressure and temperature can be presented graphically in the form of a phase diagram (see Figure 1).

A phase diagram has temperature as the independent (x) and pressure as the dependent (y) axis. Three distinct regions are represented as regions of pressure and temperature relative to the state of the substance as solid, liquid or gas. The boundaries between regions show the values of pressure and temperature when two phases are in equilibrium. For example, sublimation occurs at the boundary between solid and gas, evaporation/condensation occurs at the liquid–gas boundary and melting/freezing occurs at the solid–liquid boundary. The point at which all three phase boundaries meet is called the triple point and signifies the temperature and pressure at which all three phases exist and are in equilibrium.

Phase Diagram of Carbon Dioxide:

The phase diagram for carbon dioxide is shown on the previous page. If a sample of dry ice is placed on the desk at normal atmospheric pressure (1 atm. or 14.7 psi) and room temperature (25 °C), the solid will sublime spontaneously, maintaining its temperature of –78 °C (point ①) until the solid disappears completely. The gas formed will absorb heat from the room until it obtains a stable gaseous state at 25 °C. If a sample of dry ice is sealed in a closed system such as the triple point apparatus, the pressure begins to rise allowing the solid to exist at a higher temperature. Most of the energy absorbed from its surroundings will result in an increased temperature and corresponding increase in vapor pressure. The effect of this increased energy is a series of pressure–temperature equilibria along the solid–gas line between points ① and ② on the phase diagram.

Once the triple point is reached (point ②), the energy absorbed causes the solid to melt. The solid–gas and liquid–gas phases are also in equilibrium. While this phase change occurs, the temperature and pressure remain constant at 5.1 atm. and –56.6 °C as long as the solid, liquid and gaseous phases are in contact with each other. If the solid becomes covered with liquid, an equilibrium no longer exists between all three phases, and both the temperature and pressure will increase into the liquid portion of the phase diagram (point ③). For safety reasons, do not allow the triple point apparatus to exceed 100 psi.

When the valve is opened and the pressure is released, solid carbon dioxide is formed almost instantaneously due to a reduction in temperature and pressure. The temperature decrease is due to rapid vaporization (boiling) of liquid CO₂ to gaseous CO₂. The heat of vaporization of carbon dioxide is roughly 16 kJ/mol whereas the heat of fusion is –9 kJ/mol. The energy required to boil the liquid lowers the temperature of the system and causes the liquid carbon dioxide to freeze.


This demonstration also presents a good opportunity to discuss the various units that are used when measuring pressure.

References

Special thanks to Walter Rohr of Eastchester High School, Eastchester, NY, for providing us with this idea.

Atkins, P. W. Physical Chemistry; W. H. Freeman: New York, 1990; pp 132–135.

Becker, R. J. Chem. Ed. 1991, 68, 782–783. Orna, M. V., et al. Eds. Source Book; Chem Source: New Rochelle, NY, 1994, Volume 1, pp 28–29 (COND).

Tzimopoulos, N. D., et al. Modern Chemistry; Holt, Rinehart, and Winston: Chicago, 1993; p 385.