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 point, the temperature will again fall as the solid loses heat to the surroundings. If the material is pure, the temperature of the sample remains constant during the entire process of solidification; this temperature is the melting point. This procedure is the one that should be used for calibrating a thermometer or checking the calibration of a thermometer by using solids of known melting point. In this case, any disagreement between the reading of the thermometer and the true melting point is attributed to an error in the calibration of the thermometer.
Capillary Melting Points
Capillary melting points, either in an oil bath or a melting-point apparatus, are most often used for the determination of the melting point of a solid. A few crystals of the compound are placed in a thinwalled capillary tube 10-15 cm long, about 1 mm in inside diameter, and closed at one end. The capillary, which contains the sample, and a thermometer are then suspended so they can be heated slowly and evenly. The temperature range over which the sample is observed to melt is taken as the melting point. The thermometer and sample must be at the same temperature while the sample melts, so the rate of heating must be slow as the melting point is approached (about 1 degree per minute). Otherwise, the temperature of the thermometer bulb and the temperature of the crystals in the capillary may not be the same. The transfer of heat energy by conduction takes place rather slowly. If the approximate temperature at which the sample will melt is not known, determine a preliminary melting point determination by allowing the temperature of the sample to rise quickly. Then carry out a more accurate determination, with a low rate of heating near the melting point.
Filling a Capillary Tube
Usually, the melting point capillary can be filled by pressing the open end into a small heap of the crystals of the substance, turning the capillary open end up, and vibrating it by drawing a file across the side to rattle the crystals down into the bottom. If filing does not work, drop the tube, open end up, down a length of glass tubing about 1 cm in diameter (or a long condenser) onto a hard surface such as a porcelain sink, stone desk top, or the iron base of a ring stand. The solid should be tightly packed to a depth of 2-3 mm. A variety of oil baths can be used in a melting point determination, as well as in a boiling point determination. The simplest use a burner flame and depend upon convection for mixing; the more elaborate and accurate use an electric immersion heater and are stirred. It is easy to heat at a low and steady rate with an electric heater, but almost impossible with a flame. When an oil bath is used, the capillary can be fastened to the thermometer by means of a small slice of rubber tubing used as a rubber band (see Figure below). Arrangement of sample and thermometer for melting point determination There is also a type of melting point apparatus in which the sample and thermometer are both supported in an electrically heated metal block and the sample in the capillary can be observed through a magnifying glass. Usually you can heat the block rapidly when the temperature is well below the melting point, and slowly as the melting point is approached. If a compound begins to decompose near the melting point, the capillary with the sample should be placed in the bath after the temperature has been raised to within 6 or 10 degrees of the expected melting point, so as to minimize the length of time that the sample is heated. Capillary melting points are properly compared with one another, but occasionally they are considerably different from melting points determined from cooling curves.
