Organic Chemistry Lab #4: Sublimation


Sublimation is the transition of a substance from the solid phase to the gas phase without passing through an intermediate liquid phase. The process of sublimation is an endothermic phase transition that occurs at temperatures and pressures below a substance's triple point in its phase diagram (Figure 1). Sublimation is used to purify substances in small scale quantities, typically less than 100 mg, because there is very little loss of the material in the process. Some common occurrences that involve the process of sublimation include dehydration of food, freezer burn and dry ice.

external image phasediagramco2.png

Figure 1: Phase diagram.

Below are the steps of the Sublimation lab experiment. Detailed lab instructions are described in the following citation: Williamson K.L., 2003. Macroscale and Microscale Organic Experiments 4th Edition. Boston (MA): Houghton Mifflin Company, p. 120-125.
  • Place 50 mg of an unknown into a flask.
  • Assemble small scale vacuum sublimation apparatus (Figure 2).
  • Close the flask with a rubber pipette bulb, and then place ice water in the centrifuge tube.
  • Cautiously warm the flask until sublimation starts, and then maintain that temperature throughout the sublimation.
  • Then fit the flask with a loose cone of aluminum foil to direct heat up the sides of the flask, causing the product to collect on the tube.
  • Once sublimation is complete, remove the ice water from the centrifuge tube and replace it with water at room temperature.
  • Then collect the product, determine its weight and the percent recovery.
  • Record the melting point (degrees Celcius) using a MelTemp apparatus, comparing with values in Williamson (2003) to determine the name of the unknown.

external image sublimationApparatus.jpg

Figure 2: Example of a small scale vacuum sublimation apparatus. Note NH4Cl is only an example and does not apply to this lab.


The identification number of the unknown substance is 241-7-8. The tables below summarized the weights and melting points of the unknown before and after sublimation.

Before Sublimation
Weight of Unknown (g)
Melting Point of Unknown (Deg.C)
After Sublimation
Weight of Unknown (g)
Melting Point of Unknown (Deg. C)
always report experimental melting points as temperature ranges.
The percent recovery of the unknown substance is equal to the weight of the substance recovered divided by the weight of the substance before sublimation multiplied by 100. The percent recovery of the unknown substance is 74.5%. sig figs here: you have two to work with, not 3.

Comparing the post sublimation melting point value of the unknown (123.9 degrees Celcius) with the values given in Table 7.3 in Williamson (2003) on page 124, the closest melting point value is Benzoic Acid (mp 122 degrees Celcius). Therefore, the unknown product is concluded to be Benzoic Acid (see Analysis). this is correct!

General Observations:
The unknown substance before sublimation had the appearance of small white crystals. The process of sublimation required heat, time and a cold point for the gaseous product to collect and turn directly into solid crystals. When heated in the hot sand bath from below, the solid turned to gas, and the cold tip of the test tube worked as a point for the gas to collect and solidify. Not long after placing the container in the hot sand bath the crystals started to vaporize, within ten minutes of the start of sublimation, vapors started to form crystals on the cold tip of the test tube. After heating for approximately 27 minutes, sublimation had completed, leaving a substance with the appearance of small, clear/white, somewhat prismatic symmetry, with a low density, appearing like a spiderweb.

The lab experiment followed the procedures as described in Williamson (2003). The unknown substance was kept warm and the collection site was kept cold. Aluminum foil was also used, which kept the crystals from forming on the inside of the filter flask. The percent recovery of 74.5% suggests that some product was lost in the sublimation process; however this percentage is large enough to adequately determine the unknown substance from melting point data. The experimental melting point data was determined used a MelTemp apparatus, which was 123.9 degrees Celcius. In Table 7.3 in Williamson (2003) on page 124, the sublimation unknowns given in which are within the range to the experimental results are both Acetanilide (mp 114 degrees Celcius) and Benzoic Acid (mp 122 degrees Celcius). The experimental unknown value was 123.9 degrees Celcius, which is 1.9 degrees Celcius warmer than the theoretical melting point of Benzoic Acid. Based on the procedures followed, the percent recovery and close proximity of the melting point values; the unknown product is concluded with fairly high confidence to be Benzoic Acid.
this is all good.
error? Please address in a very deliberate way.

Post Lab Questions:
What is lyophilization and why is it used?
Lyophilization is the process of subliming a solvent from heat and oxygen sensitive substances to obtain the solid that remains after sublimation is done. Water is a widely used solvent with this type of sublimation and low pressures to obtain natural substances such as proteins, vitamins, nucleic acids and other biochemicals. The common name of this process is freeze drying. The aqueous solution is frozen, vacuumed and then kept frozen throughout the process, which makes the water sublime and condense as ice on the surface. The primary application of lyophilization is used to dehydrate food that is commonly used for backpacking, such as instant coffee and prepared meals. good answer.