Investigating Phase Changes

Procedure

Part 1 Heating Curve

1. Attach the ring clamp to the pendulum stand and place the wire mesh upon it. Place the Bunsen burner attached to a gas source underneath the ring clamp.
2. Fill a 250ml beaker three-fourths full of warm water and place it on the ring clamp. Light the Bunsen burner with a match and adjust the flame so that it is not too high.
3. Obtain a test tube of unknown substance and place it in the beaker of water.
4. Write down the initial temperature of the substance in a table in your lab book. One partner will call out the time every ½ minute (30 sec.) and will record temperature data in the data table. The other partner will read off the temperature of the sample at each half-minute interval.
5. Be sure to include observations such as when the solid pure substance first starts to liquefy and when there isn’t any solid pure substance left. Continue to record the temperature until the unknown substance is completely liquefied.

Part 2 Cooling Curve

1. As soon as the pure substance is completely a liquid, take it out of the beaker with the thermometer still in it and place it in a 500ml beaker filled with tap water. Write down the initial temperature of the substance in a second table in your lab book and proceed to record the temperature of the cooling substance every half minute (30 sec).
2. Be sure to include observations such as when the liquid pure substance first starts to solidify and when the substance has completely solidified.
3. Do not try to pry out the thermometer from the test tube else it be damaged. Put back the unknown substance with the thermometer in it to where you got it. Do not throw in the garbage or down the sink.
4. Clean up and put everything else back.

Data Table

Table 1: Heating Curve

Table 2: Cooling Curve

Observations

Heating Curve

        At 28.5 degrees Celsius, a small amount of the unknown pure substance starts to liquefy. At 37 degrees Celsius, a larger amount of the unknown pure substance has begun to liquefy.  At 47 degrees Celsius, the remaining solid clumps together and floats in the liquefied portion of the substance. At 59 degrees Celsius, the solid portion sinks to the bottom of the test tube. At 67 degrees Celsius the remaining solid has disappeared and the substance is completely a liquid.

Cooling Curve

At 48 degrees Celsius, a small amount of the liquid pure substance starts forming into little crystals. At 44.5 degrees Celsius, the substance starts solidifying more concretely. At 39.5 degrees Celsius, the substance has solidified into a paste-like substance.

 

Data Analysis

1. According to the graph, the unknown pure substance freezes and melts at around 42.5 degrees Celsius, but this figure may be subject to inaccuracies in the measurement reading procedure and the graphing by hand procedure.

2. Since the substance has a freezing-melting point of around 42.5 degrees Celsius, it is logical to guess that the unknown substance is Phenol Salicylate which has a freezing-melting point of ~43-46 degrees Celsius. Although it may be possible, based solely on the recorded freezing-melting point, that the unknown substance is Lauric Acid (mp ~44-46 degrees Celsius). However, the distinct coconut smell which comes from the heating of Lauric Acid was not observed. Therefore, it is most likely that the unknown substance is Phenol Salicylate.

3. Freezing is Endothermic. Melting is exothermic.

4. The diagonal parts of the cooling curve represent the decreasing kinetic energy and decreasing potential of the substance’s atoms as they are being cooled or removed of heat energy. The horizontal portions of the cooling curve represent the steady kinetic energy and increasing potential energy of the atoms as they gain energy to bond closer together as a solid.

Conclusion

        Based upon the gathered data and graph calculations, we have identified the unknown pure substance as Phenol Salicylate because of the unknown substance’s near matching melting-freezing point of 42.5 degrees Celsius to that of Phenol Salicylate which has a freezing-melting point of ~43-46 degrees Celsius. Also the unknown substance did not give off any smell, therefore we eliminate the possibility of it being Lauric Acid. Likewise, the substance’s melting-freezing point is much to far from that of the other possible acid, Stearic acid which has a freezing-melting point of ~67-69 degrees Celsius.