HEATING & COOLING BEHAVIOR
The purpose of this experiment is to observe the temperature of a substance as it changes from liquid to solid, and then back from solid to liquid. You will record the temperature of the substance at regular time intervals during each process, then display your results on a graph.
Procedure
Part 1. Warming behavior
1. Make a hot water bath by filling a tall beaker about ¾ full of hot water and placing it on a hot plate set at 2. The bath should be about 55 °C. A test tube of lauric acid with a thermometer and stirrer embedded in the solid is at your station.
2. When you are ready to begin recording data, lower the tube of substance with the embedded temperature probe into the water bath, making sure all of the solid in the test tube is surrounded by warm water. Record the temperature of the substance every 20 seconds until the solid is completely melted and the temperature is above 50 °C. As soon as you can, use the wire to stir the substance gently and continuously. Record on your data table when the solid begins to melt and when the wire becomes free to move.
Part 2. Cooling behavior
3. When you are ready to begin recording data, raise the sample out of the water bath and swing it aside, so that it just hangs in the air. Record the temperature of the substance every 20 seconds until it is completely solid and the temperature is below 35 °C. Use the wire to stir the substance gently and continuously until the wire sticks. Record on your data table when the liquid begins to freeze and when the wire becomes stuck.
Analysis
1. Prepare a single full-page graph on which to plot both sets of data. Put time on the x axis, starting at time = 0 and using 3 squares per minute until you have room for all your data. Put temperature on the y axis, starting at any convenient temperature below your lowest temperature reading and using 1 square per degree. For example, if your lowest temperature reading is 32.4 °C and your highest temperature reading is 58.6 °C, you would start your graph at 30 °C and go up to 60 °C (30 squares).
2. Plot both sets of data on your prepared graph, using different colors to distinguish between the data for part 1 and for part 2. Draw a smooth curve through each set of points. On each curve, mark the temperature where freezing or melting began and where the wire stirrer became stuck or free.
Discussion
1. Write a short description of the temperature behavior of the substance as it melts and freezes.
2. Identify the melting point and freezing point of the substance (these are temperatures, not times) and correlate these with the shape the curves.
3. How would the graph be affected if you used a larger amount of the substance for your experiment?
4. Suggest at least two sources of error in the experiment.