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1. 2 Effect of Temperature on Solubility of a Salt GLX Version Note: This is a Vernier lab that has been modified by ASIM for use with the GLX. Introduction Fluctuations in temperature cause changes to ocean salinity. Warmer surface waters dissolve more salts creating a dense solution that begins to sink. Colder, less dense water rushes up to replace the sinking denser water, creating convection currents in our oceans. Ocean currents bring warmth from the equator toward the poles of the Earth. As global temperatures continue to rise, the increase in ocean salinity or dissolved salts will disrupt ocean currents. Without the movement of warmer waters from the equator, the end result could be the next ice age. In this experiment, you will study the effect of changing temperature on the amount of solute that will dissolve in a given amount of water. Water solubility is an important physical property in chemistry, and is often expressed as the mass of solute that dissolves in 100 g of water at a certain temperature. In this experiment, you will completely dissolve different quantities of potassium nitrate, KNO3, in the same volume of water at a high temperature. As each solution cools, you will monitor temperature using a Temperature Probe and observe the precise instant that solid crystals start to form. At this moment, the solution is saturated and contains the maximum amount of solute at that temperature. Thus each data pair consists of a solubility value (g of solute per 100 g H2O) and a corresponding temperature. A graph of the temperature-solubility data, known as a solubility curve, will be plotted using the GLX. Materials GLX Temperature Probe Hot plate Support rod for hot plate or Ring stand four 20 X 150 mm test tubes Test tube rack 400 mL beaker 10 mL graduated cylinder 250 mL beaker ring stand 2 utility clamps stirring rod potassium nitrate, KNO3 Procedure 1. Obtain and wear goggles. NASA and Auburn University Global Climate Change Education Chemistry Module Page 12 2. Label four test tubes 1–4. Into each of these test tubes, measure out the amounts of solid shown in the second column below (amount per 5 mL). Note: The third column (amount per 100 g of H2O) is proportional to your measured quantity, and is the amount you will enter for your graph in Step 8. Test tube number Amount of KNO3 used per 5 mL H2O (weigh in Step 2) Amount of KNO3 used per 100 g H2O (use in Step 8) 1 2 3 4 2.0 4.0 6.0 8.0 40 80 120 160 3. Add precisely 5.0 mL of distilled water to each test tube (assume 1.0 g/mL for water). 4. Plug in and turn on the GLX. Connect the Temperature Probe to port 1 located on the left side of the GLX. A temperature vs. time graph should be displayed. 5. Fill a 400 mL beaker three-fourths full of tap water. Place it on a hot plate situated on (or next to) the base of a ring stand. Heat the water bath to about 90°C and adjust the heat to maintain the water at this temperature. Place the Temperature Probe in the water bath to monitor the temperature and to warm the probe. Press to start and stop the temperature probe reading. CAUTION: To keep from damaging the Temperature Probe wire, hang it over another utility clamp pointing away from the hot plate, as shown in Figure 1. Figure 1 6. Use a utility clamp to fasten one of the test tubes to the ring stand. Lower the test tube into the water as shown in Figure 1. Note: In order to dissolve all of the KNO3, test tubes 3 and 4 need to be heated to a higher temperature than test tubes 1 and 2. Use your clean stirring rod to stir the NASA and Auburn University Global Climate Change Education Chemistry Module Page 13 mixture until the KNO3 is completely dissolved. Do not leave the test tube in the water bath any longer than is necessary to dissolve the solid. 7. You are now ready to collect temperature-solubility data. a. When the KNO3 is completely dissolved, press to start collecting data. b. Remove the Temperature Probe from the water bath, wipe it dry, and place it into the solution in the test tube. c. Unfasten the utility clamp and test tube from the ring stand. Use the clamp to hold the test tube up to the light to look for the first sign of crystal formation. At the same time, stir the solution with a slight up and down motion of the Temperature Probe. d. At the moment crystallization starts to occur, press to stop data collection. Record this temperature in the data table beside the appropriate mass of KNO3. e. After you have saved the temperature-mass data pair, return the test tube to the hot water bath for just long enough to redissolve the crystals. f. Place the Temperature Probe in the water bath for the next trial and return the test tube to the test tube rack. g. To clear the memory for the next data collection, select the Home key check mark Delete and then use the to select Data Files. The current, and probably only, file is highlighted. Select and then OK . Press the Home key again and then Graph . 8. Repeat Steps 7 and 8a – 8g for each of the other three test tubes. Here are some suggestions to save time. One lab partner can be stirring the next KNO3-water mixture until it dissolves while the other partner watches for crystallization and records data pairs. Test tubes 1 and 2 may be cooled to lower temperatures using cool tap water in the 250 mL beaker. This drops the temperature much faster than air. If the crystals form too quickly, briefly warm the test tube in the hot-water bath and redissolve the solid. Then repeat the cooling and collect the data pair. 9. When you have collected the fourth and last data pair and have cleared the memory, disconnect the temperature probe. Discard the four solutions as directed by your instructor. 10. Graph the four data points. NASA and Auburn University Global Climate Change Education Chemistry Module Page 14 a. From the Home screen Column, press , select Table . Select Edit and then New Data to select. Repeat for a second data column. b. To label a data column, press the desired title, and press Properties…, and press to highlight the titles, sse the arrow keys to select it. Use the arrow keys to scroll to to scroll to Data to select it. Measurement Name is highlighted. Press it, and type in solubility. Press to make the change. Use the arrow keys Measurement Unit, and press to select it. Type in g solid/100g water. Press the change. Select OK to select to scroll to to make when done. Repeat the process to name the second column temperature with degree c as the unit. c. Use the arrow keys to scroll to the first cell in the first data column. Select Edit Cell and type in the first solubility value. Press are entered in the first column. Press to enter it. Continue until all four values and use the arrow keys to scroll to the first cell in the second data column. Repeat the process to enter the temperature data. d. From the Home screen , select Graph . Make sure that solubility is on the x-axis and temperature is on the y-axis. To change a title press arrow keys to scroll through the titles. Press to scroll to the desired title, and press e. Press Auto Scale to highlight the titles and use the to select the title, use the arrow keys to make the change. . 11. Print a copy of the graph of solubility vs. temperature. Draw a best-fit curve for the data points on your printed graph if needed. Label both axes and show correct units. Give your graph an appropriate title. Label tick marks with the numerical values they represent. Note: If you did not print a graph, manually plot a graph of solubility of KNO3 vs. temperature using your data. NASA and Auburn University Global Climate Change Education Chemistry Module Page 15 Name _____________________________ Name _____________________________ Period ____________ Date ____________ Data Table Trial Solubility (g / 100 g H2O) 1 40.0 2 80.0 3 120.0 4 160.0 Temp (°C) 1. According to your data, how is solubility of KNO3 affected by an increase in temperature of the solvent? 2. Using your printed graph, tell if each of these solutions would be saturated or unsaturated: a. 110 g of KNO3 in 100 g of water at 40°C _____________________________ b. 60 g of KNO3 in 100 g of water at 70°C _____________________________ c. 140 g of KNO3 in 200 g of water at 60°C _____________________________ 3. According to your graph, will 50 g of KNO3 completely dissolve in 100 g of water at 50°C? Explain. 4. According to your graph, will 80 g of KNO3 completely dissolve in 100 g of water at 30°C? Explain. 5. At 30°C, approximately how many grams of KNO3 would dissolve in 100 g of water? NASA and Auburn University Global Climate Change Education Chemistry Module Page 16 EXTEND 1. Visit NASA’s website http://nasascience.nasa.gov/earth-science/oceanography/physicalocean/salinity. Answer the following questions: Where are the highest salinity and the lowest salinity of oceans found? Why higher seawater salinity is found in the middle of oceans near the equator? Why lower seawater salinity is found near the Arctic and Antarctic areas? Describe two techniques used for measuring ocean water salinity. Which method can provide a complete global data with relatively short periods (a few days) of time? EVALUATION 1. 2. What do you think about the effect of temperature on the solubility of a salt on our oceans? What do you think will happen if the earth’s average temperature continues to rise? NASA and Auburn University Global Climate Change Education Chemistry Module Page 17