(LAB) HEATING&COOLING CURVES OF A PURE SUBSTANCE!!

Lab: Heating&Cooling curves of a Pure Substance

Objectives:
1. to investigate the heating process for solid dodecanoic acid
2. to investigate the cooling process for liquid dodecanoic acid
3. to determine and compare the melting and freezing points of dodecanoic acid

Supplies:
Equipment                                                            
ring stand                                                                   beaker (400mL)
buret clamp                                                                thermometer for water bath                   
hot plate                                                                     lab apron
test tube (18mm×150mm)                                          safety goggles      
      assembly (half full with solid
                      dodecanoic acid with 
                      thermometer embedded) 

Chemical Reagents
dodecanoic acid, C11H23COOH
     (in test tube apparatus)

Procedure:
Part I: the Heating Process
1. put on lab apron and goggles
2. deciding the roles with your partner as one is recorder and one is obsever
3. obtain a test tube assembly embedded a solid dodecanoic acid
4. put 300mL water in a 400mL beaker and put it on the hot place, initially turn to the high temperature to boil the water to 55°C to 60°C
5. turn to the low heat as the temperature of water reaches the 55°C, lower the test tube on the clamp ring into the water and start recording the temperature of solid acid every 30s
6. continue recording until the acid reaches a temperature of 50°C. stir to mix the solid and liquid as the acid begins to melt and record the time it melts and it ends
7. turn off the hot plate and quickly move to Part II

Part II: the Cooling Process
1. may exchange the roles with your partner
2. raise the test tube out of the hot water and replace the hot water with the normal one with room temperature
3. immediately start monitoring the cooling process after put the test tube into the normal water, record the temperature
4. record the temperature every 30s until it reaches 25°C, record the solidification begins and ends
5. may stop recording when the temperature of acid is 25°C
6. put every single tool back to the right place properly
7. wash hands thoroughly before leave

Reagent Disposal:
The test tube assembly containing the solidified dodecanoic acid with embedded thermometer is to be stored for future use.

Post Lab Considerations:
The results you have collected can now be analyzed with graphs that will represent the heating and cooling curves of dodecanoic acid.

Experimental Results:
Table 1 Heating and Cooling of Dodecanoic Acid

Analysis of Results:
Part I: the Heating Process
1. using the results you obtained during the heating process, construct a graph of temperature versus time.  use small circles for these data points and sketch a smooth curve through these points.
2. indicate on the graph where melting began and ended.
3. from your heating curve, determine the melting point of dodecanoic acid.

Part II: the Cooling Process
1. on the same graph as the heating curve, plot temperature versus time for the cooling process.  use small squares to distinguish these data points and sketch a smooth curve through these points.
2. indicate on the graph where solidification began and ended.
3. from your heating curve, determine the freezing point of dodecanoic acid.
4. compare your melting and freezing points with each other and with those of two other lab groups.  explain any similarities and differences.
5. what can you conclude about the melting and freezing points of a pure substance?

Follow-up Questions:
1. how would you explain the plateaus in your heating and cooling curves?
2. suppose that more dodecanoic acid had been used in Part I.  what would be the change in appearance of the new cooling curve?  sketch it.

Conclusion:
State the results of Objective 3.