Na2SO4(c) + 10H2O(l)A sample of Glauber's salt (Na2SO4•10H2O(c)) is heated above the phase transition temperature. The temperature of the melt is recorded as the melt cools and solidifies.
The reaction for Glauber's salt is:
Na2SO4•10H2O(c) Na2SO4(c) + 10H2O(l)
The water and Na2SO4(c) proceed to form a solution.
Burns are possible.
Handle heating equipment cautiously. Check glassware by holding finger nearby to sense heat before touching.
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++Setting up the Apple II:
Install a pair of thermistors by connecting their 9-pin D-connector to the game port at the rear of the computer.
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Insert the GENERAL LABORATORY INTERFACING software in Drive A and turn on the computer and monitor.
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A title screen will appear. Follow the instructions at the bottom of the screen.
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Select the GENERAL LABORATORY INTERFACING program.
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Select 1, the THERMISTOR option.
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Follow instructions to continue.
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Follow the screen instructions. Choose 3, "Sample Temperatures."
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Choose a sampling time interval of 5 seconds. Select graphic display.
Adjust the maximum and minimum temperatures.
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++Setting up the Macintosh with Vernier Software ULI Interface:
Use two "Budget Temperature Probes" (TPB-DIN). Load the calibration file for the specific temperature probes being used. Use the "Data Logger" software to record the temperature. Select the "Display" menu item "Timebase" to choose minutes or seconds. Select the "Display" menu item "Labels" to change the labels on the graph. Select the "Display" menu item "Axes..."
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Type in the temperature range (10 to 60°C) and time range (6 minutes or 360 seconds) you wish to use. Be sure your time units are consistent with the "Timebase" you set earlier.
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++Wet Procedures:
Use a hot plate or a Bunsen burner with a tripod and gauze to prepare a water bath at 60-70 °C.
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Place 35 g of Glauber's Salt (sodium sulfate decahydrate) in a clean, dry 25-mm x 150-mm test tube. Heat in the hot water bath to a temperature of 50-60 °C. Swirl the tube to keep the liquid in contact with the solid (which is undissolved sodium sulfate.)
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Prepare a calorimeter. Insert a 600-mL beaker into a 1-L beaker containing crumpled newspaper or paper towels for insulation. Place 400 mL of cold tap water (10-15 °C) into the inner beaker. Prepare a cardboard cover. Insert one thermistor (GC1) through the cover. The cover should have a hole large enough to accommodate the test tube. Place the cover over the beaker.
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Prepare a thermistor probe by using a small rubber band to attach the second thermistor (GC0) to a glass stirring rod. Remove the heated Glauber's salt from the hot water bath. Insert the thermistor probe. Place through the cardboard cover into the calorimeter. Stir gently and continuously.
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++Collecting data with the Apple II:
When the setup is complete, select option 3, Accept Values-Start Collection. Stir continuously during data collection. A set of axes scaled according to your specifications will appear. Data points will be plotted every 5 seconds.
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The final graphs will look somewhat like the sample data shown below. Gather data until the two probes indicate the same or nearly the same temperature.
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Enter 'S' to save the data. Enter 'C' to view a catalog of existing file names, or just type in a name for the file. The file name GLAUBER.DWB has been typed in. Press Return and the file is saved under this name.
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Follow the screen instructions to continue working or shut down.
++Collecting data with the Macintosh:
Click "Start" to begin data collection. Stir gently and continuously. Collect data until the two probes indicate the same or nearly the same temperature.
You may copy the graph to the clipboard with the "Edit" menu or the usual command-C. The graph may them be pasted into your laboratory report in any word processor. Save the experiment with the "File" menu.
Name ___________________________ Class ________
Teacher__________________________
DoChem 125 Change of Phase in Glauber's Salt
Watch the movies and answer the questions.
Closure Questions:
1. Explain why the temperature inside the tube containing Glauber's salt remains constant while the outside temperature of the water in the calorimeter drifts up continuously.
2. Hollandaise sauce is prepared by heating gently a mixture of egg yolk, lemon juice, and butter while stirring continuously. If heated too quickly, the lemon juice causes the egg to curdle, and the mixture separates into a water and an oil phase. Solid butter is always kept present until the very end. Suggest a role played by the solid butter in this recipe.
To study the energy change associated with the phase change in Glauber's Salt.
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(per apparatus available)
1 400-mL beaker
35 g Glauber's salt (Na2SO4•10H2O(c))
1 Bunsen burner, igniter or hot plate
1 25- x 150-mm test tube
1 thermometer, -10 to 110 °C
1 20-cm length glass stirring rod
++either
1 suitable computer
1 suitable adapter box
1 suitable SERAPHIM software floppy disk
1 calibrated thermistor probe
++or
Macintosh Computer with ULI or serial box interface from Vernier software
1 "Budget Temperature Probes" (TPB-DIN)
1 "Data Logger" software
Check that the thermistor probes have been calibrated with the computers to which they are attached. The student must have the proper thermistor calibration file on the floppy disk. See DCExperiment 122 to calibrate the thermistors. (Click here to see the experiment.)
If you have the ULI interface, use the Macintosh instructions which follow the Apple II instructions.
Teacher preparation: 15 minutes
Class time: 40-50 minutes
Save the Glauber's salt in the tube for reuse during later periods. At the end of a class day, the Glauber's salt may be disposed of at the sink.
The notion of adding heat to anything without having a temperature increase confuses most students at first. It seems contrary to their view of nature. Take sufficient time, using as many examples as you can think of, to illustrate this phenomenon.
The reaction for Glauber's salt is:
Na2SO4•10H2O(c) Na2SO4(c) + 10H2O(l)
The water and Na2SO4(c) proceed to form a solution.
See Figure V125V1
Closure Questions:
1. Explain why the temperature inside the tube containing Glauber's salt remains constant while the outside temperature of the water in the calorimeter drifts up continuously.
2. Hollandaise sauce is prepared by heating gently a mixture of egg yolk, lemon juice, and butter while stirring continuously. If heated too quickly, the lemon juice causes the egg to curdle, and the mixture separates into a water and an oil phase. Solid butter is always kept present until the very end. Suggest a role played by the solid butter in this recipe.
Answers to Closure Questions:
1. During a phase change (such as the melting of ice or the boiling of water) brought about by heating, all added heat goes into the newly formed phase (liquid, steam) and the temperature remains constant until the original phase is gone and replaced completely by the new phase. (That is, until all of the ice melts, or until all of the water boils.) During cooling, the energy flows in the other direction, but the consequence is the same. (You can't cool an ice/water mixture below 0 °C until all of the water has frozen solid.) Even though the Glauber's salt is releasing heat, because it is undergoing a phase change the temperature is remaining constant. On the other hand, the water in the outside bath is not undergoing a phase change. By adding the heat to that water, coming from the Glauber's salt phase change, the temperature rises. Ultimately, in any "system" insulated from and isolated from its surroundings like this, heat will have a net flow in one direction or another (that is, to the salt from the water, or to the water from the salt) until all of the subparts of the system in contact with one another reach the same temperature.
2. The butter tends to keep the temperature constant while it undergoes a phase change of melting in somewhat the same way as the Glauber's salt. The stirring ensures that the added heat gets into the butter for melting rather than into the other components for raising the temperature.
The Glauber's salt absorbs heat energy as the phase changes, and releases this energy during the reverse process. During daylight hours, a solar energy device containing Glauber's salt (or a similar substance with a phase change temperature in the region of 75-88 °C) is placed indoors in front of a sunlight source (say in front of a window). It absorbs heat, and prevents the indoor temperature of the room from rising until the phase change is complete. In dark hours, when the sun is gone and the outside temperature is dropping, the reverse phase change takes place. The salt continues to maintain a high temperature and release heat indoors until the phase change is complete. This is a clever way to use a simple chemical change as a means of storing solar energy during the day for use at night.
Citrus growers fight "frost" by spraying trees with a mist of water. The trees are not damaged until the temperature goes below 0 °C. By spraying with a water mist, the temperature is kept at 0 °C until the water freezes. Thus the outside air temperature can go below 0 °C, but the temperature inside the tree remains at 0 °C until the water freezes. This procedure works so long as the outside temperature does not remain below freezing for long, and soon rises above freezing.
The original module was written for Project SERAPHIM by Patricia Barker and Kenneth Hartman.
Project SERAPHIM is supported by the National Science Foundation.
For additional information write:
Project SERAPHIM
Department of Chemistry
University of Wisconsin-Madison
1101 University Avenue Madison, WI 53706
phase
phase change
Glauber's Salt
thermistor
calorimeter