MELTING POINT DETERMINATION UW （熔点测定华盛顿大学）.pdf

Melting Point Determination The normal melting point of a solid is defined as the temperature at which the solid and liquid are in equilibrium at a total pressure of 1 atmosphere. In contrast to the volume change that accompanies the vaporization of a liquid, the change in volume that takes place upon the melting of a solid is very small. This makes the melting point of a solid, unlike the boiling point of a liquid, practically independent of any ordinary pressure change. Since the melting point of a solid can be easily and accurately determined with small amounts of material, it is the physical property that has most often been used for the identification and characterization of solids. We will first describe how to take melting point readings in the lab. The rest of the material describes empirical observations of how structure affects melting point, while providing some theoretical explanations of these facts. Experimental Determination of the Melting Point There are several methods by which melting points can be determined, and the choice of method depends mainly upon how much material is available. Melting Points from Cooling Curves If large amounts of the solid are available (a gram or so), the most accurate method for determining the melting point is to heat the sample until it is melted and then allow it to cool slowly for crystallization. Keep track of the temperature of the sample as a function of time by means of an immersed thermometer or thermistor. At first, the temperature falls as the liquid loses heat to the surroundings. When crystallization begins, however, the heat evolved during this process (-∆Ηf, the heat of fusion) will maintain the temperature at a constant value until crystallization is complete. At this