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6.1A: Overview of Melting Point

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    93431
  • Measurement of a solid compound's melting point is a standard practice in the organic chemistry laboratory. The melting point is the temperature where the solid-liquid phase change occurs. In some reference books it is listed as a single value (e.g. \(98^\text{o} \text{C}\)), but in chemical catalogs it is more often listed as a range of values (e.g. \(96\)-\(98^\text{o} \text{C}\)). The melting "point" is therefore more of a melting "range," and in part, reflects how melting points are experimentally determined.

    A melting point is determined by loading a small amount of sample into a capillary tube (Figure 6.1), and then slowly heating the sample. Figure 6.2 shows a close-up view of a sample inside a melting point apparatus, where the sample is slowly heated through contact with hot vertical metal blocks on either side of the capillary tube. The sample is kept small in this technique to ensure adequate heat transfer between the metal and sample.

    Figure 6.1: White solid (indicated with an arrow) in a capillary tube.

    The first value recorded for the melting range is with the very first appearance of liquid. As this temperature is approached, the solid may begin to glisten (Figure 6.2b), and the temperature is recorded with the first hint of liquid movement (a droplet) inside the tube (Figure 6.2c). The second value recorded for the melting range is with the melting of the entire sample, which occurs when all areas of opaque solid have turned into a transparent liquid (Figure 6.2h).

    Figure 6.2: Time-lapse melting of benzoic acid: a) Well below the melting point, b) "Glistening" of the solid, c) First liquid droplet is seen (the temperature is recorded as the lower value of the melting range), d-g) Melting, h) Sample is completely melted (the temperature is recorded as the upper value of the melting range).

    Contributor

    • Lisa Nichols (Butte Community College). Organic Chemistry Laboratory Techniques is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. Complete text is available online.