A solid that melts in the temperature range 50-70 °C is melted. The temperature of the melt is recorded while the melt cools to room temperature.
Many compounds and elements can be melted from a solid crystalline state to a liquid state. The heating and cooling curves for these processes have the same general features. During heating, the temperature rises steadily until the melting temperature is reached. Once melting begins, the temperature remains constant until all of the material has melted. The temperature then rises again.
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 7 seconds.
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Select 2, "GRAPHIC DISPLAY".
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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 70°C) and time range (10 minutes or 600 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 tripod, gauze, beaker, water, and Bunsen burner, or use a hot plate to prepare a water bath at 60-70 °C. Place 10 g of lauric acid in a clean, dry 25-mm x 150-mm test tube. Heat in a hot water bath to a temperature of 50-60 °C until all of the solid crystals have melted.
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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 tap water into the inner beaker. Prepare a cardboard cover. Insert one thermistor (GC1) into this 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 thermistor (GC0 or Probe 1) to a glass stirring rod. Remove the melted lauric acid from the hot water bath. Insert the thermistor probe. Place through the cardboard cover into the calorimeter. Stir gently.
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++Collecting data with the Apple II:
When the setup is complete, select option 3, Accept Values-Start Collection.
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Stir continuously during data collection. A set of axes scaled according to your specifications will appear. Data points will be plotted every 7 seconds. The final graphs will look somewhat like the Figure in Sample Data. 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 LAURIC4 has been typed in. Press Return and the file will be saved under this name. Follow the screen instructions to continue working or shut down.
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++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.
++More Wet Procedures:
A heating curve also can be studied. Remove the tube from the calorimeter. Discard the water. Heat 300 mL of water to a temperature of 70 °C. Pour this hot water into the calorimeter. Replace the tube of solidified lauric acid, and start recording a new set of temperatures.
Save the data when requested.
Name ___________________________ Class ________
Teacher__________________________
DoChem 126 Cooling and Heating Curves
Watch the movies and answer the questions.
1. The heating (cooling) curve of a substance is one of its most characteristic "fingerprints." Explain the meaning of this statement. Does it matter how much of the material is heated (cooled)?
To study the energy changes associated with cooling and heating a substance through a phase change.
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(per apparatus available)
1 400-mL beaker
1 600-mL beaker
1 Bunsen burner, igniter
1 25 x 150 mm test tube
15 g lauric acid (or other suitable substance)
1 thermometer, -10 to 110 °C
1 20-cm length glass stirring rod
paper towels
++either
1 suitable computer
1 suitable adapter box
1 suitable SERAPHIM software floppy disk
2 calibrated thermistor probe
++or
Macintosh Computer with ULI or serial box interface from Vernier software
2 "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.
Alternate solids to use include benzophenone, stearic acid, and thymol.
Teacher preparation: 15 minutes
Class time: 40-50 minutes
Reheat the test tube to loosen the probe, if necessary. Rinse the probe in running water and wipe dry. Save the lauric acid in the tube for reuse during later periods. At the end of a class day, melt the lauric acid and pour it into a disposable glass or plastic vessel. Cool, and dispose of with ordinary solid waste. The remaining lauric acid may be disposed of at the sink. Use some sodium carbonate or detergent and warm water to clean lauric acid from glassware.
The notion of adding heat to anything without having a temperature increase confuses most students at first.
Many compounds and elements can be melted from a solid crystalline state to a liquid state. The heating and cooling curves for these processes have the same general features. During heating, the temperature rises steadily until the melting temperature is reached. Once melting begins, the temperature remains constant until all of the material has melted. The temperature then rises again.
See Figure V126V1
Closure Questions:
1. The heating (cooling) curve of a substance is one of its most characteristic "fingerprints." Explain the meaning of this statement. Does it matter how much of the material is heated (cooled)?
Answers to Closure Questions:
1. Most substances melt at different temperatures. Melting temperature is, therefore, a characteristic. Since there are lots of substances known, however, melting temperature alone nearly always cannot provide a unique identification of a material. Another rather unique characteristic of a substance is its ability to hold heat. (This is called the heat of fusion.) A cooling curve depends upon both the melting temperature and the heat of fusion, so it combines two characteristics and, therefore, serves as an excellent fingerprint. The amount of heat needed to bring about melting does depend upon the amount of substance. For this reason, characteristic heating curves should be based upon molar quantities. Rather than use time as the x-axis in a curve, we could use amount of heat added. This would allow a comparison of heating curves to be made between different sets of apparatus.
The original module was written for Project SERAPHIM by PatriciaÊBarker 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
thermistor
calorimeter
heat of fusion
melting point