Beeswax-measuring melting & freezing point
Introduction:
Matter can exist in one of three physical states- solid, liquid, or gas. For a pure substance, changes in state occur at a definite temp., which is characteristic of that substance. Water, for example, changes from a solid to a liquid at 0°C.
In a crystalline solid, the particles are arranged in an orderly, repeating, three-dimensional pattern. As a solid is heated, the kinetic energy of the particles increases. Eventually, at some temperature, which is called the melting point, the molecules overcome the forces of attraction holding the particles together, and the substance changed to a liquid.
When a liquid is cooled, the reverse process occurs. The temperature of the liquid decreases until the freezing point is reached. Only after the liquid is completely changed to a solid will the temperature begin to decrease again.
In this investigation, you will observe the melting and freezing behavior of bees wax (C15H31COOC30H61). You will measure the temperature at timed intervals as the bees wax is heated and cooled and determine experimentally its freezing point and melting point.
Pre-Lab: (read the procedure then answer the questions below)
1. Define melting point and freezing point.
2. Why is it necessary to have 2 water baths?
3. Describe what happens to the particles of a solid at its melting point.
Materials:
goggles hot plate thermometers 2 beakers, 400 mL tap water bees wax test tub
Procedure:
Part A
1. Put on goggles.
2. Get a 400 mL beaker and fill ¾ full of tap water. Heat it on the hot plate to make a warm water bath (start at 7).
3. Put in a thermometer in the beaker and try to keep the temperature constant at about 63°C. You may need to turn the hot plate down. CAUTION: Do not touch the hot plate or beaker.
4. Get a second 400 ml beaker to make a cold water bath. Add only 2 ice cubes and fill ¾ full with cold tap water. 5. Get a large test tube of bees wax from the hot water bath in back of room. You will return your sample to this same area when done.
6. Place a thermometer into the test tube record temperature immediately on chart at time 0.
7. Then place test tube with bees wax in cold water bath and record the temperature every 30 seconds until it is solidified.
8. Continue to record temperature for two additional minutes after solidification.
9. Record the approximate temperature at which freezing occurs below
Freezing Temperature: ______
Part B
1. Attach a test tube holder to test tube record temperature in time zero of the data table.
2. Transfer into warm water bath. DO NOT PLACE IN WATER BATH IF TEMPERATURE IS ABOVE 70.
3. Make sure the level of water in the beaker is above the level of the liquid in the test tube.
4. Begin taking temperature readings every 30 seconds until it is completely melted.
5. Continue to record temperature for two additional minutes after melting.
6. Return test tube to large hot water bath.
7. Turn off the hot plate and clean up supplies and work station. wash your hands before returning to classroom
Data Table:
Part A: Liquid to Solid / B: Solid to LiquidTime (min) / Temperature (°C) / Temperature (°C)
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
7
7.5
8
8.5
9
9.5
10
Critical Thinking:
1. Using graph paper and a ruler, make a line graph of your data from Part A. Plot time on the horizontal axis and temperature on the vertical axis. Connect the points in a smooth curve.
2. Using a different color pen or pencil, plot the data from Part B on the same graph.
3. Look for the place where the 2 lines cross or for areas that the lines flatten out. The temperature at this point (or area) will be the the freezing point and melting point of bees wax. Label it on your graph.
4. Would you expect bees wax to freeze and melt at the same temperature? Explain why or why not.
5. Calculate much energy is gained when a 20 g sample of Bees wax (C15H31COOC30H61). is melted?
ΔHfusion= 42.3KJ/mol for Beeswax
Post Lab Questions
1. Which graph (A or B) is the “melting curve”? Which graph (A or B) is the “freezing curve”?
2. How are the two curves similar?
3. Is there a flat region on each graph? Why or why not?
4. If possible use a ruler to draw lines on your graph (sloped where the temperature is changing, flat where it is constant, as shown in #10 below) through the data on both graphs A and B. (Note: not all data points fall on the lines).
5. Does the amount of the substance affect its melting point? Why or why not?
6. Does the amount of the substance affect its freezing point? Why or why not?
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7. raph below:
8. Are freezing and melting physical changes or chemical changes? How do you know?
9. How does the rate at which we cool or heat the material affect the measurement?
10. Calculate the percent error for your measured melting and freezing points. Why did your measurements differ from the actual values? (see board for actual values)
11. What are other sources of errors in your measurements?
12. How would you redesign the experiment for a more accurate measurement?