Sublimation

At normal atmospheric pressure, most substances undergo phase changes from solid to liquid to gas as the temperature elevates; however, sublimation is the phase transitioning of an element or compound from a solid directly to a gas, omitting the liquid phase. This can only occur at a temperature and pressure below a substance's "triple point," or the temperature and partial pressure at which the substance's three phases (solid, liquid, and gas) are in perfect equilibrium as shown by a phase diagram. It is an endothermic process, and the energy required to sublime a substance is known as the "enthalpy of sublimation." Only select organic compounds are used in sublimation in the pursuit of purification of a compound. One of the most common examples of the sublimation process is when dry ice, solid carbon dioxide, is exposed to air and begins to transform into its gaseous phase. This happens at standard atmospheric pressure and temperature. The preservation of foods through the process of freeze-drying is another example that includes sublimation of water in its solid state within frozen food. This process is referred t o as lyophilization. Sublimation is also an important topic for scientists who study the water cycle and weather. Under the right conditions, snow vaporizes upon impact with warm, dry winds when they come into contact with snowpack on mountains. This phenomenon explains the reduction of snow volume on mountain ranges without any melting occurring and in the Western United States is known as the "Chinook Winds". It is also noted that sublimation occurs frequently on Mt. Everest because of the powerful sunlight, low air pressure, cold, and winds. Sublimation also causes the visual "tail" on comets, as the frozen parts of the comets vaporize upon nearing the heat of the sun.
 * Introduction:**

The focus of this lab was to purify an unknown substance by sublimation which would later be identified based on its observed melting point. Sublimation was achieved in this process whereby an unknown solid sublimed from a warm surface and condensed on a cold surface, again as a solid. In order to prevent loss of any material, the apparatus for small-scale vacuum sublimation was used. Any impurities of the solid can be observed as they will not sublime and will be left behind on the floor of the flask. Sublimation is one of the most effective ways of separating solid impurities from the solid substance used in the activity, given that the substance was able to sublime at normal pressure. By scraping the crystals off of the cold, condensed flask, analysis of the melting temperature of the purified substance could be determined using an MelTemp apparatus. nice intro: the second paragraph could be placed at the end, as conclusion! Had you thought of this?


 * Procedure: **

__Setting up the apparatus__:
 * The unknown should be placed in the bottom of the 25mL flask. Number 241-7-13 was used . If you are reporting what happened, then that information should be in the data section rather than the Procedure.
 * 15mL Centrifuge tube, ‘cold finger’, should be put through a Pluro stopper into the 25mL filter flask allowing the tube to rest 3 to 8 mm above the flask bottom.
 * A pipette bulb should be placed on the side arm of the filter flask
 * Fill the centrifuge tube with ice and water.
 * Loosely wrap the flask with an aluminum jacket to insulate it and allow for an endothermic reaction to occur.
 * The filter flask should then be placed onto a sand bath to be heated.
 * Sublimation is observed slowly as solid product starts to appear on tip of centrifuge tube.
 * When the sublimation is complete (i.e less than 5% of the solid remains on the floor of the flask), remove the ice water from the cold finger with a syringe or pipette. Replace the ice water with room temperature water.
 * Gently remove the centrifuge and collect the sample while taking caution not to scrape the bottom of the flask as that will effect the purity.
 * Determine melting point of the purified substance using a MelTemp apparatus in order to identify unknown. Note: list of possible unknowns on table 7.14 of the Williamson text.
 * Dispose of waste appropriately in the containers in the hood.

The sublimation apparatus The filter flask on the sandbath The filter flask with crystals formed
 * Observations:**

Start time: 8:23am Unknown substance # 241-7-13 was heating in the flask. Appearance: white, fine powder 8:39am: Substance appeared like it was condensing on the centrifuge tube. However, it was difficult to tell whether sublimation had begun yet and if crystals were forming on the centrifuge. 9:05am: As the unknown substance was sublimed, small crystal like solids began to appear on the tip of the centrifuge tube. 9:05-9:25: Many crystals appeared and the matrix was described as a "Spiderweb of crystals." End time: 9:25am Increased "web" of crystals observed. A small amount of unknown solid remained in the bottom of flask but was greatly reduced in mass and had taken on a pale yellow hue.

Crystals were removed from the flask, they appeared to be fine, clear, spiny. There were a few medium and large sized chunks of crystal bunches. Crystal product weight: 0.399g Observed Melting point range:156.4-156.7 degrees C. This narrow range is addressed below in the conclusion. Percent recovery calculation: (0.399g inital unknown substance) / (0.502g of recovered crystalline product) = 79.5% data collection includes everything I want to see, and you calculated yoru % recovery correctly. good.

Rather than crystals only forming on the centrifuge tube, a crystal-lattice structure was formed that filled the entire flask, somewhat resembling spiny cotton candy as the solid on the bottom began to shrink in mass. The solid unknown substance also changed from its original white color to a pale yellow. Notice the gap between the bottom of the flask and the bottom of the lattice structure, it did not extend all the way to the bottom of the flask, although crystals did form on the remaining solid on the bottom. It was observed that a larger volume of crystals were formed than other groups. The crystals were clear-white in color and not very dense.

The data collected through observation indicates that the unknown substance changed from a solid within the flask into an entirely new solid in a crystal-lattice form without any noticeable liquid phase indicating that the solid-to-vapor-to-solid process of sublimation occurred. This also indicates that the temperature and partial pressure during the activity remained below the substance's triple point, a necessity for sublimation. The sublimed product's melting point was determined and found to be 156.4-156.7 degrees C. The substance melted quickly resulting in this narrow melting point range. According to the Williamson text, the unknown substance was determined to be salicylic acid whose melting point is 159 degrees C. Yep! The percentage of recovery was 79.5% as the amount of solid started with was 0.502 g unknown (number 241-7-13) and the amount of solid recovered was 0.399g of was identified as salicylic acid. Not all of the solid transformed, however. It is possible that if more time had been utilized, more solid would have sublimed leaving a smaller, more concentrated impure substance. It was noticed that some of the original substance remained and had a few crystals connected to it. The initial substance had immensely decreased in size and appeared to have a yellow tint although it had remained white. The temperature range was an inexact challenge. The instructions warned of overheating, and yet it remained a subjective decision as to whether the heater was hot enough for sublimation to occur. Unfortunately, the exact temperature at which the solid began to sublime is unknown. If the exact temperature at which sublimation occurred was known, the exact identity of the unknown substance would be easier to pinpoint if the melting point was not exact. It was discovered that 0.500g rather than 0.050g unknown solid was used, which explained the additional volume of crystals formed as compared to the results of other groups. The total weight of the crystalline product was 0.399g. However, this likely contributed to the vast, lattice structure, that perhaps would not have been so elaborate had a smaller amount been used. Other possible sources of error include an improperly sealed flask/cold finger apparatus, which would allow vaporized product to escape rather than solidify on the cold finger; imprecise heating, which could "cook" the solid; and ending the process before complete sublimation had occurred, which would reduce the potential amount of purified product collected and reduce the recovery percentage.
 * Analysis:**

The process of sublimation was readily observed in the lab. Sublimation occurs when the temperature and/or pressure acting on a compound moves below its triple point allowing the compound to pass between its solid phase to its gas phase. This can also be observed going in the opposite direction, which is called deposition. Because the liquid phase is skipped altogether, this offers several advantages in purification of organic substances including that sublimation usually yields a higher percent recovery by omitting the liquid phase, and the process happens faster than recrystallization. It can therefore be concluded that more purified substance was recovered using sublimation in this activity than would have been recovered using other purification methods.
 * Conclusion:**


 * Resources:**
 * http://ga.water.usgs.gov/edu/watercyclesublimation.html**
 * http://www.windows2universe.org/comets/sublimation.html**

Lyophilization, also referred to as freeze drying, is the process of subliming a solvent, usually water, to recover the solid that remains once the solvent is removed. The process is made possible at low temperatures and low pressures. The recovered product is then usually frozen and vacuumed and weighs dramatically less than pre-sublimation. It is important that during lyophilization that the material temperature does not exceed the material maximum allowable temperature but is maximized to accelerate the sublimation drying process. This process is employed to make "instant" coffee, soup, rice, tea and other dehydrated foods. Lyophilization allows foods to remain stable and allows them to be stored for long periods by dehydrating them. "Astronaut'" food is created by lyophilization. It is also used commonly in biological or biochemical labs, to preserve sensitive molecules like proteins from degradation caused by microorganisms.
 * What is lyophilization?**

This lab earned the following scores for: format (2/2) style (2/2) data (3/3) quality of the result (1/1) quality of reported data (1/1) conclusion (2/2) error (1/1) post-lab question (2/2) for total of 14/14. Nice job!