Chemistry
How Sublime
Exploring and Measuring the Triple Point of Dry Ice
About this Lesson
This activity has two parts, and can be used in a unit on states of matter or intermolecular forces.
In Part I, students are given the opportunity to explore the similarities and differences between
solid carbon dioxide and solid water. In Part II, students use a large pipette and dry ice to observe
the presence of solid, liquid, and gaseous states at the triple point for dry ice, and then use a simplified pressure gauge to measure the relative pressure at the triple point.
This lesson is included in the LTF One-Day Preview Module, LTF Chemistry Module 6, and as
an Open Lesson on the LTF website.
Objectives
Level
Chemistry
LTF Science lessons will be aligned with the next generation of multi-state science standards that
are currently in development. These standards are said to be developed around the anchor document, A Framework for K–12 Science Education, which was produced by the National Research
Council. Where applicable, the LTF Science lessons are also aligned to the Common Core Standards for Mathematical Content as well as the Common Core Literacy Standards for Science and
Technical Subjects.
Code
N-Q.2
N-Q.3
(LITERACY)
RST.9-10.3
(MATH)
A-CED.4
Standard
Define appropriate quantities for the purpose of
descriptive modeling.
Choose a level of accuracy appropriate to limitations on measurement when reporting quantities.
Follow precisely a multistep procedure when
carrying out experiments, taking measurements,
or performing technical tasks, attending to special cases or exceptions defined in the text.
Rearrange formulas to highlight a quantity of
interest, using the same reasoning as in solving
equations. For example, rearrange Ohm’s law
V = IR to highlight resistance R.
Level of
Thinking
Apply
Depth of
Knowledge
II
Apply
II
Apply
II
Apply
II
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T E A C H E R
Students will:
• Compare the behavior of solid carbon dioxide and solid water when placed in varying
scenarios
• Observe the presence of three simultaneous states at the triple point of carbon dioxide
• Calculate an experimental triple point pressure
Teacher Overview – How Sublime
Code
(MATH)
S-ID.4
(LITERACY)
W.1
Standard
Use data from a sample survey to estimate
a population mean or proportion; develop a
margin of error through the use of simulation
models for random sampling.
Write arguments to support claims in an analysis
of substantive topics or texts, using valid reasoning and relevant and sufficient evidence.
Level of
Thinking
Apply
Depth of
Knowledge
II
Apply
II
Connections to AP*
AP Chemistry: II. States of matter B. Liquids and solids 2. Phase diagrams of one-component
systems 3. Changes of state, including critical points and triple points
*Advanced Placement and AP are registered trademarks of the College Entrance Examination Board. The College
Board was not involved in the production of this product.
Materials and Resources
aprons
goggles
ice
paper towels
marker, Sharpie®
ruler, clear metric
scissors
spoon, plastic
food color, red
hot glue gun with glue stick
pipette, large bulb
pipette, thin stem
pliers, needle nose
thread, approx. 20 cm
Universal Indicator
3 cups, 9-oz clear plastic
dry ice
Additional teacher materials:
T E A C H E R
Each lab group will need the following:
hammer
ice and ice chest
1 spool thread
dry ice and cooler
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Teacher Overview – How Sublime
Assessments
The following types of formative assessments are embedded in this lesson:
• Assessment of prior knowledge
• Guided questions
The following additional assessments are located on the LTF website:
• Chemistry Assessment: States of Matter
• 2009 Chemistry Posttest, Free Response Question 2
Teaching Suggestions
Dry Ice
Phase Diagrams
You may wish to use the two phase diagrams in the introduction to introduce the pressuretemperature relationships students will investigate in this activity. Ask students to compare the
similarities and differences between the phase diagrams for water and carbon dioxide. These
simple three-phase diagrams are typical of what is found in most introductory chemistry texts.
Although a reasonable representation at this introductory level, you should be aware that most
phase diagrams are much more complex than these, containing several different defined areas
rather than just three. An example of a phase diagram of water showing eleven different solid
forms, multiple triple points, and multiple critical points can be found at
www.lsbu.ac.uk/water/phase.html.
Pressure Gauge
To prepare for this activity, you must pound the dry ice with a hammer into a fine powder that
can fit through the stem of a cut pipette. You may wish to distribute small quantities of dry ice
and water ice to students in insulated cups to avoid waste. To prepare the pressure gauge, students must close one end of the pipette stem with a drop of hot glue. A few hot glue guns placed
strategically around the room should be adequate as not all students will be using them at the
same time.
The pressure gauge works based on Boyle’s law. By creating a closed sample of air inside the
tube, we can monitor the relative change in volume of the trapped air and apply Boyle’s law to
determine the related pressure. For example, if the tube is marked into seven equal segments
between the top of the colored water plug and the glued end, we can say that the initial volume V1
of the trapped air sample is 7/7 units, as shown in Figure A. At this initial time, the pressure P1 is
the same as that of the atmosphere; assume 1 atm.
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T E A C H E R
You will need to locate a source for obtaining the dry ice. Many supermarkets now carry dry ice
for packing and shipping. The dry ice should be transported in a closed but not airtight cooler
and handled with insulated gloves. The freezing point of dry ice is about −78°C, enough to cause
frostbite with extended skin contact. A sizeable chunk of dry ice should last in a closed cooler for
at least one school day. Do not put the dry ice directly in the freezer, as the temperature reduction
caused by the dry ice may interfere with the freezer’s thermostat. For more information about
safe handling and transport of dry ice, please refer to www.dryiceinfo.com/safe.htm. Be sure to
educate your students about the safe handling of dry ice before letting them work with it.
Teacher Overview – How Sublime
glue
glue
trapped air
trapped air
} equal increments
V1 = 7/7 increments = 1
V2 = 1.5/7 increments = 0.21
Figure A. Pressure gauge
P1V1 = P2V2
(1 atm)(1) = (x atm)(0.21)
x = 4.7 atm
As you walk around the room, make sure that students are making observations and recording
them. Ask additional questions to stimulate their thinking. Some direction will be needed in
Part II with the construction and use of the pressure gauge. Some common errors include
clamping the pressure gauge and therefore not seeing a change, and allowing the tip of the pressure gauge to get too close to the dry ice and thereby freezing the drop of food color.
Be sure to post the current atmospheric pressure, the triple point pressure, and the triple point
temperature on the board so that students can calculate their percent error in Part II. For carbon
dioxide, the temperature at the triple point is −56.4°C and the pressure is 5.2 atm.
References
Ehrenkrantz, David and Mauch, John J. Chemistry in Microscale, Book I. Dubuque, Iowa: Kendall/Hunt Publishing Company, 1993.
Acknowledgements
Sharpie® is a registered trademark of Sanford L.P., A Newell Rubbermaid Company.
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T E A C H E R
If at the triple point the water plug is positioned such that only 1.5 of the 7 original volume units
contain the air sample (V2 = 1.5/7 = 0.21), we can use Boyle’s law to calculate the triple point
pressure, P2:
Teacher Overview – How Sublime
Answer Key
Pre-Lab Exercises
1. Water:
Carbon dioxide:
O
H
O
H
C
O
2. Both molecules are covalently bonded. Water is a polar molecule, and it has London
dispersion forces (LDF) as well as the ability to participate in hydrogen bonding with
neighboring water molecules. The intermolecular forces are much greater between water
molecules as a result of this hydrogen bonding.
Carbon dioxide is a nonpolar molecule and only exhibits LDF. As a result, the molecules
of carbon dioxide are not as strongly attracted to each other as the molecules of water
are attracted to each other. It takes less energy to overcome LDFs of carbon dioxide than
hydrogen bonds and LDFs of water, and therefore carbon dioxide exists as a gas at room
temperature whereas water exists as a liquid.
Among others, www.dryiceinfo.com provides some useful information.
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T E A C H E R
3. Answers will vary, but may include shipping temperature-sensitive materials (DNA, RNA,
and so on); keeping food cold over long distances; cleaning out well pipes to blow out the last
bits of oil; and making “fog” at parties.
Teacher Overview – How Sublime
Answer Key (continued)
Data and Observations
Part I: Observing and Exploring Dry Ice and Water Ice
Table 1. Observations of Dry Ice and Water Ice
Condition
Dry Ice
Water Ice
Sitting on the table
Buzzes around the table
Stationary and quiet,
randomly
begins to melt
Sitting on a coin
High pitched squeal
Stationary and quiet,
begins to melt
pH* = 7–8
pH of Universal Indicator
solution after adding solid
Color changes to an orangered color; pH = 5–6
No change in color
No change in pH
Placed in cup of tap water
Bubbles, turns cloudy, stays
on the bottom
Floats and begins to melt
Bulb under water
The solid melts and sublimes, The ice melts a bit, but no
forming some liquid and gas other observable change
After releasing pressure
Solid state returns as gas is
released
T E A C H E R
pH of Universal Indicator
pH* = 7–8
solution before adding solid
No change
* Answers will vary depending on local tap water.
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vi
Teacher Overview – How Sublime
Answer Key (continued)
Part II: Measuring the Triple Point of Dry Ice
V1
Table 2. Pressure Gauge Volumes and Pressures
Number of increments of trapped air in the pressure
gauge before sealing CO2 pipette
P1
The atmospheric pressure today (in atm)
V2
Number of increments of trapped air in the pressure
gauge at the triple point
P2
The pressure inside the gauge (in atm)
7 out of 7
1 atm
1.5 out of 7
4.7 atm
Analysis
2. P1V1 = P2V2
(1)(1 atm) = (0.21)(x)
x = 4.7 atm
3. At the triple point of carbon dioxide, the temperature is –56.4°C and the pressure is 5.2 atm.
% error 
5.2  4.7
 100  9.6%
5.2
Possible sources of error: If the triple point is too high, then the student may not have seen the
initial melting of the dry ice. If the triple point is too low or nonexistent, then the pressure did
not build up in the gauge, possibly because of a leak in the system.
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vii
T E A C H E R
1. Initial values will depend on the pH of your local tap water, but the final pH of the dry ice in
water should be about 5.
Teacher Overview – How Sublime
Answer Key (continued)
Conclusion Questions
1. Dry ice speeding around on the tabletop is caused by the gas forming between the dry ice and
the tabletop. The solid is sliding across the gas.
The coin was much warmer than the dry ice, and the metal is a good conductor of heat; it
quickly transfers heat to the dry ice, causing it to sublime. The gas that is released expands
and exerts pressure against the coin, pushing it away from the dry ice and then escaping. The
pressure is relieved and again the coin is again in contact with the dry ice, transfers heat,
sublimes more dry ice and again pushes the coin away. This sequence occurs rapidly, causing
the coin to vibrate and produce the resulting sound.
2. The tap water changes color, indicating the pH of the water from the faucet.
The color changed toward the acidic end of the color scale. This is the result of the carbon
dioxide dissolving and subsequently reacting with water molecules to produce carbonic acid,
H2CO3.
3. CO2 + H2O → H2CO3
4. Water keeps condensation droplets from forming on the outside of the pipette and obscuring
the view. Because the pressure buildup in the pipette can cause it to burst, plastic is used so as
not to potentially break a glass beaker.
5. Pliers keep a tight seal so that the pressure could rise. If a tight seal was not maintained, the
CO2(g) would escape and you would not observe CO2(ℓ).
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T E A C H E R
The product, carbonic acid, lowers the pH of the solution as indicated by the color change of
the indicator.
Chemistry
How Sublime
Exploring and Measuring the Triple Point of Dry Ice
How can ice be “dry”? Dry ice is the common name for compressed carbon dioxide, CO2. The
term “dry” refers to the fact that no liquid is left behind when a sample of dry ice is left out at
room temperature. The solid carbon dioxide undergoes a phase change directly from a solid to a
gas; it sublimes.
Water
critical point
liquid
Pressure (atm)
solid
1
0.0060
gas
0
100
0.0098
Temperature (˚C)
Figure 1. Phase diagram for water
Refer to the phase diagrams of water (Figure 1) and carbon dioxide (Figure 2). It is easy to see
that the most stable state for carbon dioxide is the gaseous phase under normal room conditions
(20°C and 1 atm). However, water (H2O) is most stable as a liquid under the same conditions.
Does liquid carbon dioxide exist? The answer to this question is not under “normal” conditions
but it is possible.
Carbon Dioxide
73
critical point
liquid
Pressure (atm)
solid
?
gas
1
-78
-57
31
Temperature (˚C)
Figure 2. Phase diagram for carbon dioxide
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1
Student Activity – How Sublime
Purpose
In Part I of this experiment, you will observe carbon dioxide in the melting and refreezing of the
dry ice and observe some of its properties. In Part II of this experiment, you will attempt to measure the pressure associated with the triple point of dry ice.
Materials
Each lab group will need the following:
aprons
goggles
ice
paper towels
marker, Sharpie®
ruler, clear metric
scissors
spoon, plastic
food color, red
hot glue gun with glue stick
pipette, large bulb
pipette, thin stem
pliers, needle nose
thread, approx. 20 cm
Universal Indicator
3 cups, 9-oz clear plastic
dry ice
SAFETY ALERT!
» Handle the dry ice with care. The surface temperature of the dry
ice is around −78°C.
» Never touch the dry ice with bare hands. Use the scoopula or
forceps when obtaining your sample.
» Wear safety goggles throughout the activity.
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Student Activity – How Sublime
Procedure
Part I: Observing and Exploring Dry Ice and Water Ice
1. Obtain a sample of dry ice and a sample of water ice from your teacher.
2. Using forceps or a scoopula, place a few small pieces of each sample on the lab bench.
Record your observations in Table 1 on your student answer page.
3. Place a coin on the lab bench. Carefully place a small sample of each solid on top of the coin.
Observe and record the results.
4. Fill two plastic cups about one third full with tap water. Label one cup “CO2” and one cup
“H2O.” Place a few drops of Universal Indicator into each of the cups. Note the color and
record the results.
5. Add the solids to their labeled cups containing the water and Universal Indicator. Observe
and record the results.
6. Fill a third plastic cup about half full with tap water.
7. Obtain a large pipette. Using a pair of scissors, cut the stem of the pipette as shown in
Figure 3.
Dry Ice
Cut
Figure 3. Filling the pipette
8. Place 8 to 10 pieces of dry ice through the stem of the pipette and into the bulb. The bulb
should be about one quarter filled with dry ice.
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Student Activity – How Sublime
Procedure (continued)
9. Fold the tip of the pipette stem over and clamp securely with the pliers, as shown in Figure 4.
Clamp securely so that no gas escapes. Immediately lower the pipette into the water until the
bulb is submerged. Observe and record the results.
Figure 4. Clamp and submerge the pipette
10. Carefully release the grip on the pliers, still holding the bulb under the water. Observe and
record the results.
11. Repeat this step several times until there is no dry ice left in the bulb.
12. Repeat Step 7 through Step 11 with a sample of water ice. Observe and record the results.
Part II: Measuring the Triple Point of Dry Ice
1. Construct a micro-pressure gauge by cutting the stem from a thin-stemmed pipette.
2. Use the hot glue gun to place a small amount of glue to seal the end. Tie a piece of thread
around this end, as shown in Figure 5.
thread
equal increments
}
glue
Figure 5. Micro-pressure gauge
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Student Activity – How Sublime
Procedure (continued)
3. Beginning with the sealed end, mark the stem of the pipette in 1.0 cm or otherwise equal
increments.
4. Prepare another large pipette as you did in Part I by cutting off the tip.
5. Check the length of the micro-pressure gauge by placing it inside the large pipette. Cut the
pressure gauge so that its length just comes to the top of the bulb.
6. Place a drop of colored water into the pressure gauge by squeezing the center of the stem with
pliers and placing the open end into a cup of colored water. Gently release the grip to allow
the water to be drawn into the pressure gauge. Based on the position of your colored water
droplet, count the number of increments occupied by the trapped air sample. Record this
value in Table 2 on your student answer page.
7. Obtain a small sample of dry ice and fill the bulb about one quarter full. Slide the pressure
gauge into the large pipette, fold the tip, and firmly grip both with the pliers. Be sure to place
the setup into the water as shown in Figure 6 and observe.
Figure 6. Measuring relative pressure
8. As the pressure climbs inside the pipette, you will notice the colored water plug rising in the
micro-pressure gauge. Note the position of the colored water droplet when the dry ice begins
to liquefy.
9. Gently release the grip and repeat until there is no more dry ice in the bulb.
10. Clean up your station as instructed by your teacher.
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Student Activity – How Sublime
Pre-Lab Exercises
1. Draw a Lewis structure for water and for carbon dioxide.
2. The water molecule has a molar mass of 18.0 g/mol and is a liquid at room temperature,
whereas carbon dioxide has a molar mass of 44.0 g/mol and is a gas at room temperature.
Explain both these states using bonding theories and/or intermolecular forces.
3. Research uses of solid carbon dioxide using online or other sources. Describe three of the
most interesting uses that you find. Be sure to cite your sources.
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Student Activity – How Sublime
Data and Observations
Part I: Observing and Exploring Dry Ice and Water Ice
Table 1. Observations of Dry Ice and Water Ice
Condition
Dry Ice
Water Ice
Sitting on the table
Sitting on a coin
pH of Universal Indicator
solution before adding
solid
pH of Universal Indicator
solution after adding solid
Placed in cup of tap water
Bulb under water
After releasing pressure
Part II: Measuring the Triple Point of Dry Ice
V1
Table 2. Pressure Gauge Volumes and Pressures
Number of increments of trapped air in the pressure
gauge before sealing CO2 pipette
____ out of ____
P1
The atmospheric pressure today (in atm)
V2
Number of increments of trapped air in the pressure
gauge at the triple point
P2
The pressure inside the gauge (in atm)
____ out of ____
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7
Student Activity – How Sublime
Analysis
1. Calculate the change in pH when dry ice was added. (Use the color chart provided with the
Universal Indicator.)
2. Calculate the pressure in the pressure gauge using Boyle’s law, P1V1 = P2V2, and record your
results in Table 2.
3. Ask your teacher for the theoretical value for the triple point of dry ice and calculate the
percent error. Cite possible sources for error in this activity.
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8
Student Activity – How Sublime
Conclusion Questions
1. Explain the observations of the dry ice on the tabletop and on the coin. What caused the
movement and the sound?
2. Describe what happened to the color of the water with Universal Indicator alone and then
when dry ice was added.
3. Write a balanced equation to show the reaction of dry ice with water. What is the product
formed? How does this relate to the color change of the Universal Indicator?
4. What purpose was served by submerging the pipette in a cup of water? Why was a plastic
cup used instead of a glass beaker?
5. Why were pliers used to secure the opening of the stem of the pipette? How might the
experimental results have differed if the pliers were not tightly gripped?
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