How Sublime Exploring and Measuring the Triple Point of...

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RChemistry

How SublimeExploring 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 sim-plifi ed 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

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

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 docu-ment, 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 Stan-dards for Mathematical Content as well as the Common Core Literacy Standards for Science and Technical Subjects.

Code Standard Level of Thinking

Depth of Knowledge

N-Q.2 Defi ne appropriate quantities for the purpose of descriptive modeling.

Apply II

N-Q.3 Choose a level of accuracy appropriate to limita-tions on measurement when reporting quantities.

Apply II

(LITERACY)RST.9-10.3

Follow precisely a multistep procedure when carrying out experiments, taking measurements, or performing technical tasks, attending to spe-cial cases or exceptions defi ned in the text.

Apply II

(MATH)A-CED.4

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.

Apply II

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RTeacher Overview – How Sublime

Code Standard Level of Thinking

Depth of Knowledge

(MATH)S-ID.4

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.

Apply II

(LITERACY)W.1

Write arguments to support claims in an analysis of substantive topics or texts, using valid rea-soning and relevant and suffi cient evidence.

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 ResourcesEach 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

Additional teacher materials:

hammer ice and ice chest 1 spool thread dry ice and cooler

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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

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.

Phase Diagrams

You may wish to use the two phase diagrams in the introduction to introduce the pressure-temperature 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 defi ned 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 fi ne powder that can fi t 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, stu-dents 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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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:

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 pres-sure 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: Ken-dall/Hunt Publishing Company, 1993.

Acknowledgements

Sharpie® is a registered trademark of Sanford L.P., A Newell Rubbermaid Company.

glue

} equal increments

V1 = 7/7 increments = 1 V

2 = 1.5/7 increments = 0.21

glue

trapped air

trapped air

Figure A. Pressure gauge

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Answer Key

Pre-Lab Exercises

1. Water:

Carbon dioxide:

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.

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.

Among others, www.dryiceinfo.com provides some useful information.

O

H H

C OO

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Answer Key (continued)

Data and Observations

Part I: Observing and Exploring Dry Ice and Water Ice

Table 1. Observations of Dry Ice and Water IceCondition Dry Ice Water Ice

Sitting on the table Buzzes around the table randomly

Stationary and quiet, begins to melt

Sitting on a coin High pitched squeal Stationary and quiet, begins to melt

pH of Universal Indicator solution before adding solid

pH* = 7–8 pH* = 7–8

pH of Universal Indicator solution after adding solid

Color changes to an orange-red color; pH = 5–6

No change in colorNo 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, forming some liquid and gas

The ice melts a bit, but no other observable change

After releasing pressure Solid state returns as gas is released

No change

* Answers will vary depending on local tap water.

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Part II: Measuring the Triple Point of Dry Ice

Table 2. Pressure Gauge Volumes and PressuresV1 Number of increments of trapped air in the pressure

gauge before sealing CO2 pipette7 out of 7

P1 The atmospheric pressure today (in atm) 1 atm

V2 Number of increments of trapped air in the pressure gauge at the triple point

1.5 out of 7

P2 The pressure inside the gauge (in atm) 4.7 atm

Analysis

1. Initial values will depend on the pH of your local tap water, but the fi nal pH of the dry ice in water should be about 5.

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.

5.2 4.7

% error 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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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

The product, carbonic acid, lowers the pH of the solution as indicated by the color change of the indicator.

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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Chemistry

How SublimeExploring 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.

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.

Temperature (˚C)100

1

Pres

sure

(atm

)

0

0.0098

critical point

0.0060

solid liquid

gas

Water

Figure 1. Phase diagram for water

Temperature (˚C)31

Pres

sure

(atm

)

-57

-78

critical point

?

solid liquid

gas

Carbon Dioxide

73

1

Figure 2. Phase diagram for carbon dioxide


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 mea-sure the pressure associated with the triple point of dry ice.

Materials

Each lab group will need the following:

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.

apronsgogglesicepaper towelsmarker, Sharpie®

ruler, clear metricscissorsspoon, plasticfood color, redhot 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



Procedure


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.

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 fi lled with dry ice.

Cut

Dry Ice

Figure 3. Filling the pipette



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.

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.


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.

Figure 4. Clamp and submerge the pipette

glue }

equal increments

thread

Figure 5. Micro-pressure gauge



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 fi ll the bulb about one quarter full. Slide the pressure gauge into the large pipette, fold the tip, and fi rmly grip both with the pliers. Be sure to place the setup into the water as shown in Figure 6 and observe.

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.

Figure 6. Measuring relative pressure



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 fi nd. Be sure to cite your sources.



Data and Observations


Table 1. Observations of Dry Ice and Water IceCondition 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


Table 2. Pressure Gauge Volumes and PressuresV1 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 ____ out of ____

P2 The pressure inside the gauge (in atm)



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.



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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