Observing and Measuring the Triple Point of CO2
(adapted from Chemistry in Microscale by Ehrenkrantz and Mauch, Kendall/Hunt, 1993)

Introduction:  A look at the partial phase diagram of carbon dioxide shows, as one would expect, that CO2 is most stable in the gaseous state at normal room conditions (approximately 1 atm and 20oC).  The phase diagram also shows how solid CO2 earned its nickname “dry ice:” if a sample of it is left out in the room, it does not melt the way ordinary ice does; it sublimes.  That is it undergoes a phase change directly from a solid to a gas.  Liquid CO2 does exist, just not at 1 atm pressure.  To be able to observe liquid CO2, the sample’s pressure and temperature must be at or exceed the triple point temperature and pressure.  In Part 1 of this lab you will observe the melting and re-freezing of CO2.  In Part 2 we seek to measure the pressure that is associated with the triple point. 

PART 1:  Observing the triple point of dry ice

(In your lab notebook record observations to each step as appropriate.  I recommend that you keep you lab notebook on a side bench top since this activity involves lots of water.)

  1. Observe a few small pieces of dry ice sitting on the lab bench.  Record observations.

  2. Add tap water to a 400 mL or 600 mL beaker to about 2/3 full.

  3. Cut off the small tapered tip of a graduated plastic pipet.

  4. Slide 8 to 10 tiny pieces of dry ice into the pipet as quickly as you can.  I suggest using your hands and gently picking up the dry ice since it is cold.

  5. Fold the open end of the pipet over and clamp the fold firmly with a pair of pliers.  Be sure no gas can escape.  Immediately lower the pipet into the beaker so that the bulb is submerged.  View the dry ice from the side of the beaker through the glass.

  6. As soon as the dry ice begins to melt, release your grip on the pliers and release the pressure.  Observe and record melting and what happens after the pressure is released.

  7. If any dry ice remains re-clamp the pliers and try again.

Questions to Part 1:  (Answer these in your lab notebook.)

a.       What differences did you observe between when the dry ice sublimed versus when it melted?

b.      How is the melting (not sublimating) of dry ice different than the melting of ordinary ice?  What ideas in terms of the intermolecular forces of the different molecules can you offer to explain the differences?

c.       What purpose(s) does the water in the beaker serve?  (I can think of three.)

d.      What might have happened if the grip on the pliers had not been released once the dry ice melted?

PART 2: Measuring the triple point pressure of dry ice

(In your lab notebook record observations to each step as appropriate.)

  1. Obtain a micro pressure gauge from the end of the lab bench.

  2. Mark the pressure gauge with a “Sharpie” at 1 cm intervals starting from the closed, glued end.

  3. Place a small drop of colored water inside the open end of the tube by inserting the “super-micro” pipet tip into the micro pressure gauge and slowly pulling it out as you leave a drop of colored water.

  4. Note and record the length of the air column in the micro pressure gauge.  (This is the length of the tube from the glued end to the top of the colored water drop.)

  5. As in Part 1, prepare a graduated pipet by cutting off the tapered tip.  Fill it with 8-10 tiny pieces of dry ice.  Insert the micro pressure gauge with the open end pointing downward toward the dry ice.  Using the thread suspend the micro pressure gauge above the dry ice but low enough so that you can fold over the top of the pipet and clamp the folded end with the pliers.

  6. Clamp the folded end of the pipet and lower it into your beaker of water.  CAREFULLY observe the position of the colored water noting the changing length of the air column.  Specifically, record the position or length of the air column based on the top of the colored drop at the instant the dry ice begins to melt.  Once the position or length is noted release the pliers carefully.  Try to observe the colored drop as the pressure is released.

Questions to Part 2:  (Answer these in your lab notebook.)

e.       What happened to the pressure inside the bulb before, during and after the dry ice melted?

f.        What was the initial length of the air column inside the micro pressure gauge?

g.       What was the length of the air column when the dry ice started to melt?

h.       What was the initial pressure inside the tube?  (What is this pressure equal to?)

i.         Using your answers to f., g. and h. determine the pressure inside the pipet when the liquid CO2 first appeared?  (Hint: Consider how the pressure and volume of a gas sample are related.)

j.        The actual triple point pressure of CO2 is 5.1 atm.  Calculate your % error.