MATERIALS NEEDED:

Safety goggles
Clock or watch that shows seconds
Ring stand
Two 250-mL beakers
Test tube
*12 to 13 g unknown test chemical
Iron ring
Wire gauze
Utility clamp
2 Thermometers (-10^oC to +10^oC)
Bunsen burner
Beaker tongs
Test tube tongs
Paper towels
Ice
Graph paper

*Four chemicals appropriate for this lab are given in table 3, along with their melting points. In addition, you can use paraffin(candle wax), with a melting point of 50-54^oC. However, it will not work in comparison with the other chemicals due to the wide range of its melting temperature.

PROCEDURE:

PART 1
The cooling behavior of a pure substance:

1. Obtain a test tube containing an unknown sample of solid to be tested. Fill a 250-mL beaker about three-fourths full of water. Place it on top of the wire gauze and iron ring combination above the burner and heat the water in the beaker. BE CAREFUL: THE BEAKER OF WATER WILL BE HEAVY AND COULD TIP EASILY. When the water boils, turn off the flame. Remove beaker with tongs, and set it on the desk top.

2. Place the test tube containing the sample in the hot water and place a thermometer in the test tube. While the solid melts, prepare step 3.

3. Prepare a cold water bath by placing a hand-full of ice in the second 250-mL beaker, and filling it about three-fourths full of water. Place the second thermometer in the beaker and stir. When the bath temperature is approximately 10^oC, remove any remaining ice(this is very important).

4. If you are working with a partner, clearly divide the labor. One partner will stir and make thermometer readings while the other partner will time and record temperature readings and other observations.

5. When all is ready, check the temperature of the LIQUID SUBSTANCE (remember not to take the thermometer out of the substance to read temperatures) and record it ti the nearest 0.5^oC as temperature at time zero. Quickly immerse the lower half of the test tube in the cold water bath, start timing, and clamp the test tube in place. The water level should be above the level of the substance in the test tube. Now move the thermometer gently and continuously up and down, WITHOUT HITTING THE BOTTOM OF THE TEST TUBE. Record the temperature indicated by the thermometer in the tube every 30 seconds, even though the thermometer eventually sticks in the "frozen" substance and cannot be removed. Note this temperature. Note also when solidification starts and when it is completed. Record your data in table 1. Record 8 - 10 minutes of readings.

TABLE 1: Cooling behavior data.
Time (min)	Temperature (oC)	Observations
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0

6. Remove the test tube containing the solid and the thermometer from the cold water bath and place it in a test tube rack. DO NO TRY TO REMOVE THE THERMOMETER

PART 2
The warming behavior of a pure substance:

7. Use the beaker that contained the hot water and prepare the warming apparatus (figure 2), with the exception of attaching the utility clamp to the test tube in the test tube rack, and do not put on the apparatus until told to do so.
Adjust the water temperature to about 80^oC. Have the bunsen burner ready to warm the water bath a few seconds when necessary to keep the temperature between 75 and 85^oC.

8. Record the temperature of the solid in the test tube to the nearest 0.5^oC, for temperature at time zero, on table 2. With the second thermometer, keep track of the bath temperature incase you need to heat it as described in step 7. On the signal of the recorder, immerse the lower half of the test tube until the water level is above the level of the solid (timing starts). Clamp the tube into place (figure 2). Record the temperature of the sample in the test tube every 30 seconds You should note when melting starts, and when it is completed.

TABLE 2: Warming behavior data.
Time (min)	Temperature (oC)	Observations
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0

CALCULATIONS:

9. Plot heating and cooling temperatures on the same graph. For each cooling temperature and its corresponding time, make a small cross (x). Use small circles (o) for the heating data.

10. Study the experimental results as shown on your graph. Write a one-paragraph summary describing the graph. Be sure to include the temperature where the two lines cross.

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11. What effect would increasing the amount of solid have on the shape of the heating or cooling curves?

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12. From your graph, what is the melting point of the solid you tested? This is simply where the two graph lines cross.

13. Compare your melting point to the list of chemicals and their melting points in table 3.

Table 3: Chemicals and melting points.
Chemical	Melting point (oC)
mesityl bromide. C9H11Br	40
thymol. (CH3)(C3H7)C6H3OH	50
para-dichlorobenzene. C6H4Cl2	53
dibromoaniline. C6H5Br2N	57

14. From the comparison, which chemical was yours?