Phase Changes of Carbon Dioxide
Introduction: Dry ice (carbon dioxide in the solid phase) is interesting to most people due to its unique triple point
which prohibits it from being a liquid under normal atmospheric conditions of pressure and temperature. By adjusting the
temperature and pressure, it is possible to manipulate the carbon dioxide so that it undergoes various phase changes and
all three phases of matter can be observed.
[NOTE: Very few people have actually seen liquid dry ice. After this experience, you will be one of those few .]
SAFETY:
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The probability of the pipettes exploding in this activity is very high. Goggles are to be worn at all times.
Dry ice is very cold (-78 °C at the surface) and may cause frostbite if in direct contact with skin for more than a
few seconds. Do not directly touch the substance with bare hands.
Purpose: Observe a sample of dry ice as it undergoes various phase changes due to changes in temperature and pressure.
Materials:
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Dry ice (small pieces)
Water
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Wide stem plastic pipettes
Pliers, tongs or forceps
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Clear plastic cup
Scissors
Procedure:
1. Half fill a clear plastic cup with tap water. THE PLASTIC CUP OF WATER IS A REQUIRED SAFETY FEATURE!
2. Obtain a plastic pipette and cut off the narrow end of the stem.
3. Scoop small pieces of dry ice into the pipette without actually touching the dry ice. Tap the dry ice down into the bulb
until the bulb is 1/3 to 1/2 full of dry ice.
4. Fold the open end of the pipette stem over and clamp it shut with a pair of pliers. (No gas should be able to escape.)
5. Immediately lower the pipette bulb into the water in the cup. Watch for bubbles indicating a leak from your stem or
pipet. If bubbles come from the stem, try to re-clamp the stem by adjusting the position or tightness of your grip.
6. Observe from the side of the cup the behavior of the dry ice. As soon as the dry ice has melted, slowly loosen the grip
on the pliers, still holding the bulb in the water, and observe the CO2 in the bulb. Tighten the grip on the pliers again and
observe. Repeat several times. If your pipette explodes, still observe the CO2 in the bulb of the pipette before and after the
explosion.
7. Place a small piece of dry ice on the lab counter top and observe.
8. Drop a small piece of dry ice in your cup of water and observe.
9. Throw away damaged pipettes and return all materials to their original location on the lab counter.
Observations:
Analysis Questions:
1. How is the melting of dry ice different from that of water ice? Why is this so?
2. What was the function of the water in the cup?
3. What variable was being controlled by adjusting the grip on the pliers?
G13
4. Consider two identical dry ice chips: Which would sublime faster...one submerged in water or one placed on the
counter top? Support your answer with evidence from your observations.
5. Look at the phase diagram of carbon dioxide. What are the specific temperature and pressure necessary to observe the
triple point of dry ice?
7. Why does solid carbon dioxide not melt under regular classroom conditions?
8. What is misleading about the terms “melting point” and “boiling point?”
9. The statement below is not completely true. Change it to make it completely true. "Water always boils at 100 °C."
10. The statement below is not completely true. Change it to make it completely true. "Liquid CO2 is not stable, so we
can never see it."
11. Is it possible to have boiling ice water? If so, explain how. If not, why not?
12. Two identical pots of water, one in St. Louis and one in Denver, are placed on identical stoves and identical eggs are
dropped in at the same time.
a. Which water will start to boil first?
b. Which egg will finish cooking first?
13. In the lab, you witnessed what few people ever witness: liquid CO2. Explain how you were able to make liquid CO2.
PHASE DIAGRAM OF CARBON DIOXIDE
PHASE DIAGRAM OF WATER