/16 Points Lab: Phase Change Introduction Every substance has a characteristic freezing point and melting point. As you might expect, the substance changes phase at each of these temperatures. A pure substance will change from the solid state to the liquid state when the temperature reaches its melting point. At a temperature equal to the substance's freezing point, the substance will change phase from a liquid into a solid. Determining the melting point or freezing point for an unknown substance will often let you figure out the identity of the substance. Consider what happens when a solid substance, for example an ice cube, melts. Does the entire ice cube turn immediately into liquid water at some specific moment? Or is the process more gradual? What happens to the temperature of the ice cube while it is melting? Once the ice cube has melted, how could you reverse the process and turn the water back into an ice cube? What temperature conditions would you need for this to happen? This lab activity will help you answer these questions, as well as help you determine the identity of the mystery substance you will be melting. Predict How do you think the freezing temperature of a pure substance compares to its melting temperature? Are they identical, nearly the same, or significantly different (2 points)? Why? Computer setup- 1. Log into computer using your user ID. 2. Power up the GLX and plug two temperature sensors into the GLX as shown by Mr. Hart. 3. Open Datastudio. 4. Close the Welcome dialog box. 5. Go to File then Open Activity. 6. Browse for P: drive then Science then Hart then 01 Warming&Cooling CF.ds (This is the file you want to open). 7. You should now be ready for data collection. When using configured files, sampling rates and appropriate displays are automatically launched. Equipment setup- 1. Obtain a solid unknown sample from Mr. Hart.
2. Using a hot plate, prepare a 125ml hot water bath in the beaker. Insert one of the thermometers (B) into the water bath and try to keep the temperature just under 60C (50C-55C is ideal). This will require you to find the right setting on the hot plate. 3. Secure the test tube with the sample in a buret clamp on the ring stand. Position the stand close enough to the water bath so you can lower the test tube into the bath. (Reference diagram at bottom of back page) 4. Click the Start ( ) button to begin recording the temperature. 5. Continue heating the water to a temperature of 50 C - 55 C. Adjust the temperature setting on the hot plate so that it remains below 60 C. Lower your sample into the water when you reach a temperature of about 40 o C. 6. Do not move the Temperature Sensors during the initial portion of the procedure. 7. Have a lab partner note the temperature at which the solid begins to melt. You must understand which temperature is which. Use your brain! 8. When at least half of the unknown has melted, use the sensor labeled A to gently stir the sample. 9. When the entire solid has melted, carefully raise the test tube out of the water bath. Secure it with the clamp above the level of the beaker and off to the side, so that any rising water vapors do not warm the test tube. Turn off the hot plate for the duration of the experiment. 10. Continue recording temperature data as the unknown sample slowly cools and eventually solidifies. Do not stir the sample during this portion of the experiment. 11. Click the Stop ( ) button to end data collection. *Do not attempt to remove the probe from the solid!!!!! 12. If time permits, repeat the experiment. Do not reheat the water if it remains above the identified melting point of your sample. Analyze- 1. Enlarge your graph display by clicking the Scale-to-Fit button to best view the data. 2. Select the Smart Tool button. Drag the Smart Tool ( ) to pinpoint the melting temperature and the freezing temperature for your unknown sample. 3. Record the melting temperature and freezing temperature in the Data Table on the Student Response Sheet. 4. Use the Note Tool on the graph to label each interval in which the sample was melting or solidifying. 5. Print your data. 6. Clean your work area completely. 7. Leave the lab as instructed.
Data Make a sketch of your graph of Temperature versus Time data. Label the x-axis, the y-axis and the most significant features of the graph (2 points). Record the data from the graph in the data table below. Use this information to answer the questions that follow (2 points). Table 1 Initial heating and cooling data Melting Freezing Temperature at which phase change occurred: Total time needed for phase change to take place: Table 2 Additional data if time allowed Melting Freezing Temperature at which phase change occurred: Total time needed for phase change to take place:
Analyze 1. From your graph, what happened to the temperature during freezing? What happened to the temperature during melting (2 points)? 2. According to your data and graph, what is the freezing temperature of your pure solid? What is the melting temperature? How do they compare (2 points)? 3. Describe how the shape of the graph indicates whether the substance is in the solid phase, the liquid phase, or undergoing a change in its phase (2 points). Synthesize 1. Considering how energy is added to the system when heating your unknown substance, explain the shape of the graph you observed for melting your sample. (Hint: think about how the molecules in a solid are arranged compared to the molecules in a liquid, 2 points) 2. What effect would increasing the amount of solid have on the shape of the heating or cooling curves (1 point)?
Table 3 Chemicals and melting points Chemical Melting Point ( C) Lauric acid, C12H24O2 44 Thymol, (CH 3 )(C 3 H 7 )C 6 H 3 OH p-dichlorobenzene, C6H4Cl2 50 53 3. Compare your substance s melting point to the list of chemicals and their melting points in Table 3. Identify your unknown solid substance. The unknown chemical was (1 point):