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Boiling Point And Melting Point Essay

1217 words - 5 pages

The Determination of Boiling Points and Melting Points of Organic Compounds
During this laboratory session, we will practice determining the boiling point of a liquid organic compound and the melting point (or we could call it the freezing point) of a solid organic compound. The boiling point of a liquid is the temperature at which the pressure of the vapor above a liquid equals the existing pressure. As we heat a liquid, the pressure of the vapor above the liquid slowly increases. When this pressure equals the pressure existing in the container, the liquid begins to boil– the liquid turns to vapor. If not contained, the vapor will “escape” into the atmosphere. In other words, the liquid ...view middle of the document...

The more polar the compound, the higher the melting point; the heavier the molecule, the higher the molecular weight. To determine the melting point of a compound, we simply put a very small amount into a very small “test tube”– actually a melting point capillary– and slowly heat the sample while watching it through a magnifying glass. We can see it begin to melt as liquid is observed and when the melting is completed, and only liquid is visible, we report a melting point range. A pure sample of a melting point generally has a very sharp melting range– no more than 3o– while an impure sample generally has a rather broad melting range, often 6-10° or more. By comparing the melting range of a sample to those in a list presented in a text or reference book, we can often make a case that we know what unknown we have. If possible, we then try a “mixture melting point” to prove our case. Let us assume that we have an unknown which melts at 120-122oC. Let us assume that we also know that it is an acid. From a text, we might guess that our sample is benzoic acid which has a reported melting point (or melting range) of 121-122oC. To prove the conclusion, we can take a small amount of our unknown and mix it thoroughly with some authentic benzoic acid that we have obtained from a different source. If we have mixed benzoic acid with benzoic acid, the m. pt. of the mixture will be the same as either sample; 120-122o. But if our sample really was not benzoic acid, then we have contaminated the authentic benzoic acid with our sample (or contaminated our sample with benzoic acid), and the melting point will now be lower and broader, perhaps 95-110o. Today’s Experiment You will set up a distillation apparatus at your workspace as demonstrated during the pre-lab lecture. Two important things to remember: water always goes into the condenser at the lower nozzle and exits from the higher nozzle (it wouldn’t fill up if you set it up backwards) and the bulb of the thermometer must be below the exit to the condenser. If it is higher, vapors will simply pass below the thermometer bulb without transferring energy to the bulb. (It wouldn’t warm up the thermometer bulb.) If the bulb is too low, then hot liquid splashes on the bulb and your reading will be in error. Have the instructor or the teaching assistant verify that you are correctly set up. Using the sample assignment at the end of...

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