The melting point of a compound is the temperature at which the solid is in equilibrium with its liquid. A solid compound changes to a liquid when the molecules acquire enough energy to overcome intermolecular forces holding them together in an crystalline lattice structure. The melting point range is defined as the span of temperature from the point at which the crystals first begin to liquefy to the point at which the entire sample is liquid. This data can be tabulated experimentally through multiple trials for an unknown and referenced against the chemistry literature for a known compound. In Macroscale and Microscale Organic Experiments, 5th ed, Williamson notes that most pure organic solids will melt repeatedly over a narrow temperature range of 1°C. As a measure of purity, impure compounds will have a different melting point expected for the pure compound and a broadening of the melting point range. Such an observation would indicates that a compound is impure. In experiment 3.2, 0.120g sample of benzophenone was obtained from stock and crushed into a fine powder. Two samples of the benzophenone were loaded into a capillary tube. The capillary tubes were then individually placed in a Mel-Temp apparatus where they were gradually heated at a steady rate of 1º/minute. The melting rate was manually controlled by a dial on the Mel-Temp on a relatively low setting of 2.5. The temperatures were recorded at the sign of the first drop of liquid and when the sample had turned completely to liquid. The same procedure was followed in experiment 3.4 to observe the melting point of the Unknown #51 sample. Two trials were done for each sample, taking care to let the temperature fall 20°C between trials. The melting point ranges was compared to the melting points of compounds found in Table 3.1 Melting Point Standards and Table 3.2 Melting Point Unknowns in the text. It should be noted that while the procedure suggests using compounds listed in...
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