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Lab Discussion about melting point...

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Gonzalez 1 Melting Point Lab Discussion and Conclusions Melisa S. Gonzalez The melting point of a substance is the temperature at which it changes from solid state to liquid state. At the melting point the solid and liquid phase exist in equilibrium. A solid is said to melt sharply if the melting point range is very narrow (no more than 3 C). A pure solid will generally melt sharply because the forces of attraction between its particles are the same. However, an impure solid have a broad and depressed melting point because the presence of a different particle in the crystal lattice of a pure substance interrupts its uniform structure, weakening its intermolecular attractions. For that reason, the melting point of the solid is very useful not only as a method of identification of substances but also as an indication of a substance’s level of impurity. A mix melting technique is used to identify if two compounds A and B with the same melting point are equal or not. This technique involves the mixing of equal amounts of both A and B (50:50 mixture) and then determining the melting point of the mixture. If A and B are identical, then the melting point of the mixture is going to be equal to the melting point of each individual compound. On the other hand, if A and B are not the same compound, then the mixture will have a broad and depressed melting point range compared with that for each individual compound since one will act as an impurity in the other. In this experiment, the melting point of a known and an unknown solid organic compound was measured through a fast and slow heating. After that, a mix melting technique was applied to identify another solid organic compound through a positive and negative test. The compound selected for the determination of its melting point was glutaric acid, a white solid with a literature melting point of 95 - 98 C. Before starting measuring its melting

Gonzalez 2 point, the compound was grinded into a fine powder form to make the heat transfer into the sample more efficient and reproducible. After that, fast heating was performed to make a rough estimate of the melting point of the sample, which resulted in a melting point range of 95.7 - 98.6 C. Then, a slow heating was applied to make a more accurate determination of the melting point of glutaric acid. It was observed that, during this slow heating, the sample changed color from white to yellow at a temperature of 80.4 C, which indicates that the sample decomposed before melting. However, its melting point was not affected since a range of 95.6 – 97.2 C was obtained. The melting point of unknown 104, a white solid, was determined using the same procedure described previously. A melting point range of 151.2 – 151.4 C was obtained through fast heating. During the slow heating an error was made because the Melt Station must have been cooled to a temperature of at least 20 degrees below the expected melting temperature (i.e. approximately 130 C); however, it was cooled to approximately 40 C. For that reason, it took almost an hour to reach the expected melting point, forcing the Melt Station to start cooling automatically and a faster heating rate was needed to be applied. As a result, an erroneously higher and broad melting point range was obtained, which was reported to be 150. 4 – 153.3 C. Taking the fast heating values into consideration, it was concluded that the compound was 2bromobenzoic acid, with a literature melting point of 148 – 150 C. Unknown 204 was obtained to determine its identity using a mix melting technique. First, its melting point was measured through a fast and slow heating, resulting in a melting point range of 68.8 – 73.9 C and 66.9 – 67.9 C respectively. From a list of possible unknowns, it was determined that unknown 204 was benzhydrol, which has a literature melting point of 67 – 68 C. To confirm the identity of unknown 204 a positive test (with benzhydrol) and a negative test

Gonzalez 3 (with myristic acid, which has a literature melting point of 53 – 54 C), was performed using a mix melting. The mixture of unknown 204 and myristic acid exhibited a broad and depressed melting point range of 51.1 – 58.7 C, which indicates that the compounds were not equal, being unknown 204 an impurity in myristic acid. On the other hand, the mixture of unknown 204 and benzhydrol exhibited a very narrow melting point range of 67.0 – 67.8 C, which leads to the conclusion that unknown 204 was benzhydrol....