Questions About Phase Change Lab With Solutions

Pre-lab: (As always, write in complete sentences) 1. What should happen to the temperature of a substance while the substance is melting? Why is that? The temperature should remain the same because all of the energy is going to change the substance from a solid to a liquid rather than changing the temperature 2. If a substance melts at 25 °C, at what temperature should it freeze and why? The substance should freeze at 25 °C because the change from a liquid to a solid (freezing) is just the opposite of changing from a solid to a liquid (melting) so they should occur at the same temperature. 3. For the following cases, explain whether it is gaining or losing energy? Use kinetic theory of matter in your explanation. a) While melting While an object is melting it is gaining energy because it must take in energy (endothermic) to change from a solid to a liquid because the molecules must go from just vibrating to flowing around more. b) While freezing While an object is freezing it is losing energy because freezing is exothermic, so liquids give off energy to become solids because the molecules go from moving around more to just vibrating in place. Temperature 4. Draw what the temperature vs. time graph (no numbers needed) should look like for a solid that is heated to a liquid that is a few degrees above the melting point, and then the material is removed from the heat an allowed to solidify again and is cooled to a few degrees below the melting point. Label where the substance will be: 1) solid, or liquid, 2) where the substance is melting or freezing, and 3) where the object is taking in or giving off energy. Note some of these can occur simultaneously (so it could be cooling and a liquid for example) Melting & taking in energy Liquid & giving off energy Freezing & giving off energy Liquid & taking Solid & giving in energy off energy Solid & taking in energy Time 5. Explain why you drew the graph you did from number 4. The temperature rises as the solid is heated, but stays the same during a phase change to liquid. The temperature rises again when the substance is heated. Once the heat is removed, the substance starts releasing energy, lowering temperature, leveling off during the freezing, and then dropping again as the solid cools. Data and Results: Conduct the lab as described in the instructions. Your Data will be the table below and your Capstone graph. Once you’ve collected data as described in the instructions do the following: Using the Annotation tool (the A), do the following: Your names, period and date Where the substance is a solid and heating Where the substance is melting, listing the approximate melting point Where the substance is a liquid and heating Where the substance is a liquid and cooling Where the substance is freezing, listing the approximate freezing point Where the substance is a solid and is cooling. Using the coordinates tool record the following: Item Temp °C Starting temperature of the solid <43°C ~43°C >43°C <60°C ~43°C <43°C Melting point of your substance Maximum temperature your substance reached Maximum temperature you let your water reach Freezing point Temperature of substance when fully solidified a couple degrees below the freezing point Print off one copy of the graph for each person in the group. Conclusions: (complete sentences) 6. How did you know where to look for the freezing and melting points? Explain. The flat parts are the phase changes because the temperature doesn’t change during a phase change. 7. What happened to the temperature during melting, is it the same or different for freezing? How did that compare to your predictions? The temperature stayed about the same in both phase changes, which matches our predictions 8. How do the freezing and melting temperatures compare? Is that what you expected? The freezing and melting temperatures are about the same, which is as expected. 9. Why is it often more difficult to read the melting temperature than the freezing temperature? While melting the energy was still coming in from the hot plate, some of which is going into the probe (probe is touching glass), making the temp. reading rise slightly 10. Describe how the shape of the graph indicates what phase the substance is in. The flat parts indicate the phase changes, so since it started as a solid and never became a gas, below the flat spots is a solid, and above the flat spots is a liquid. 11. Considering how energy is added to the system as you heat it, and energy is removed as you cool it, explain the shape of your graph in terms of energy. At first the added energy increases the temp., then it goes into changing the substance from a solid to a liquid, and finally the added heat goes to increasing the temperature of the liquid. When cooling, the process is just reversed. 12. Using the following data, what is your solid? Dichlorobenzene, C6H4Cl2 Thymol, (CH3)(C3H7)C6H3OH Lauric Acid, C12H24O2 Rubidium Rb TEMPO (CH2)3(CMe2)2NO Galium Ga m.p. 53.5°C m.p. 51°C m.p. 43.2°C m.p. 39.31 m.p. 37°C m.p. 29.76°C 13. Look up the boiling point of the substance you found our sample was (using what you found above). The boiling point of the unknown substance is 298.9°C 14. If the substance was heated from room temperature (20°C) until it was up to 400°C, in a closed system so none of the material escaped, and then cooled back down to room temperature (20°C). Draw the temperature vs. time graph for the substance and label where the substance is a solid, liquid, gas, melting, boiling, condensing, and freezing, as well as where it is gaining energy and where it is losing energy (again it can be doing more than one at a time, like a solid losing energy). Temp vs. Time for the substance heating to 400 and then cooling back down 450 Gas, +E Gas, -E Condensing, 400 Temperature (degrees C) 350 +E 300 Boiling, +E 250 200 Melting, Liquid, +E 100 +E 50 150 0 Solid, +E Time Liquid, -E Freezing, +E Solid, -E 15. What could you do to improve your results? In other words, how could you have made your graph look better? Any ideas to improve the results, like have the temperature of the water lower so the melting point is more evident, more carefully measure temperatures, have the experiment all done in one day and allow the substance plenty of time to freeze. Phase Change Lab instructions Computer setup1. Keep the computers on the support platforms for this lab! 2. Log into computer using your user ID and password. 3. Copying file you need: a. You need to copy the desired file to your desktop. DO NOT OPEN IT FROM THE P DRIVE. i. From the Start menu, click “Computer” ii. Scroll down to the P: network drive (should be labeled as BHS) iii. Go to the directory “Science”  “Skare”  “Physical Science”  “Computer Files” iv. Select the file “Phase_change_lab.cap”. v. Right click the file and drag it over to the desktop (make sure to say “copy here” when asked.) 4. Power up the GLX and plug two temperature sensors into the GLX as shown by Mr. Skare. 5. Make sure your GLX is connected to the USB port on the back of the computer (both sides of the USB cable have a number that should match the number of your GLX) a. When connected and recognized by your computer, your GLX should display “DataSTUDIO®” on the screen. 6. Open Capstone from the desktop or Start menu 7. Go to “File” then “Open Experiment”. 8. Browse for the file you saved to your desktop 9. You should now be ready for data collection. When using configured files, sampling rates and appropriate displays are automatically launched. Equipment setup1. Obtain a solid unknown sample from Mr. Skare. 2. Using a hot plate, prepare a water bath in the beaker with about 125ml – 250ml. Insert one of the thermometers (T2) into the water bath and try to keep the temperature just under 55°C (50°C55°C is ideal). a. This will require you to find the right setting on the hot plate. (For most of the hot plates a good rule of thumb is to put the hot plate on the highest setting until the water reaches around 40°C, then turn it down to about the 2-3 setting until it gets close to 50°C and then turn the hotplate to low). 3. While heating the samples, make sure your temperature probe cord does not touch the hotplate. 4. 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 above) 5. Click the Record ( ) button to begin recording the temperature. DO NOT HIT STOP UNTIL THE INSTRUCTIONS TELL YOU TO DO SO 6. 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 55°C. 7. Lower your sample into the water when you reach a temperature of about 40oC. 8. Do not move the Temperature Sensors during the initial portion of the procedure. 9. Have a lab partner note the temperature at which the solid begins to melt. You must understand which temperature is which. Use your brain! 10. When the entire solid has melted around the temperature probe (so the probe is no longer encased in a solid), heat to a few degrees above where it 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. 11. Continue recording temperature data as the unknown sample slowly cools and eventually solidifies. Cool to a couple degrees below the freezing point. Do not stir the sample during this portion of the experiment. 12. Click the Stop ( ) button to end data collection. *Do not attempt to remove the probe from the solid!!!!! Analyze1. Enlarge your graph display by clicking the Scale to Fit tool ( ), says “Scale axes to show all data” if you place the mouse over it. 2. Select the Coordinate Tool ( ) says “Show coordinates and access delta tool” if you place mouse over it. Drag the Coordinate Tool to find the following temperatures: 1) Starting temperature of the solid 2) Melting point 3) maximum substance temperature 4) maximum water temperature 5) freezing point 6) final temperature of the substance after data collection was stopped Record these temperatures on your lab sheet 3. Use the Annotation Tool ( )on the graph to label each interval in which the sample was melting or solidifying. 4. Also Using the Annotation Tool, put your name, class period and date on the graph 5. Print your graph (one for each person). 6. Clean your work area completely. 7. Leave the lab as instructed.