Identifying An Unknown Solid Using Melting Point

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Identifying an Unknown Solid Using Melting Point and Mixed Melting Point Data Shultz, Joshua T. Chemistry 2210L Results The experimenters inserted a capillary tube containing approximately 2 to 3 mm of benzoic acid into a manual Mel-Temp melting point apparatus. With the Mel-Temp dial set between 3 and 4, the temperature increased at a steady rate of approximately 2° C per minute from a plateau set at 90° C. The first appearance of liquid occurred at 119° C, and the benzoic acid completely melted by 122° C giving a melting point range of 119° C to 122° C for benzoic acid. Repeating the procedure for 2-naphthol returned a melting point range of 118° C to 123° C. Next, a 1:1 mixture of benzoic acid and 2-naphthol received the same…show more content…
Melting Point Ranges of Known and Unknown Substances and Mixtures Compound or Mixture Melting Point Range in °C Benzoic Acid 119 - 122 2-Naphthol 118 - 123 1:1 Benzoic Acid and 2-Naphthol Mixture 88 - 92 Unknown 126 - 128 1:1 Unknown and Benzoic Acid Mixture 92 - 97 1:1 Unknown and Succinimide Mixture 118 - 122 1:1 Unknown and Urea Mixture 93 - 107 Discussion A pure organic compound melts or freezes fairly sharply over a specific temperature range called its melting point. This physical constant can help identify a substance. Generally, any impurity added to a pure substance will lower its observed melting point and increase the range over which melting occurs. Therefore, when two pure organic substances possess a similar melting point, a mixture containing both substances will often melt at a lower temperature and over a broader range. Conversely, if a mixture of a known and unknown substance actually contains a single substance, the melting point of the mixture will not deviate from the melting point of the known. Davis (1913) demonstrated that pairs of substances with similar melting points melted at lower temperatures and over broader melting ranges when mixed. Since the separation and purification of a compound from food or excretions often leaves toxicologists with very little material to examine, Davis (1913) recommended the use of melting point and mixed melting point data for

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