Physical Constants of Solids The Melting Point

Many reference books list the physical properties of substances. You should consult Technique 4 for a complete discussion on how to find data for specific compounds. The works most useful for finding lists of values for the nonspectroscopic physical properties include 

Complete citations for these references can be found in Technique 29. Although the CRC Handbook has good tables, it adheres strictly to lUPAC nomenclature. For this reason, it may be easier to use one of fhe ofher references, particularly The Merck Index or the Aldrich Handbook of Fine Chemicals, in your first attempt to locate information (see Technique 4). 

Some commercial melting-point instruments have a built-in vibrating device that is designed to pack capillary tubes. With these instruments, the sample is pressed into the open end of the capillary tube, and the tube is placed in the vibrator slot. The action of the vibrator will transfer the sample to the bottom of the tube 

If the melting point of the sample is unknown, you can often save time by preparing two samples for melting-point determination. With one sample, you can rapidly determine a crude melting-point value. Then repeat the experiment more carefully using the second sample. For the second determination, you already have an approximate idea of what the melting-point temperature should be, and a proper rate of heating can be chosen. 

When temperatures above 150°C are measured, thermometer errors can become significant. For an accurate melting point with a high-melting solid, you may wish to apply a stem correction to the thermometer as described in Technique 13, Section 13.4. An even better solution is to calibrate the thermometer as described in Section 9.9. 

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Technique 9 Physical Constants of Solids The Melting Point... 

Determination of the physical constants and the establishment of the purity of the compound. For a solid, the melting point is of great importance if recrystalhsation does not alter it, the compound may be regarded as pure. For a hquid, the boiling point is first determined if most of it distils over a narrow range (say, 1-2°), it is reasonably pure. (Constant boUing point mixtures, compare Section 1,4, are, however known.) The refractive index and the density, from which the molecular refractivity may be calculated, are also valuable constants for liquids. 

Physical Properties. Almost all liquid diacyl peroxides (16) and concentrated solutions of the solid compounds are unstable to normal ambient temperature, storage many must be stored well below 0 C. Most of the solid compounds are stable at ca 20"C but many are shock-sensitive. Other physical constants and properties have been reviewed. The melting points and refractive indexes of some acyl peroxides are listed in Tables 10-12. 

In this example, we consider the classic Stefan-Neumann solution. The solid is initially at a constant temperature Tq. At time t — 0 the surface temperature is raised to T, which is above the melting point, Tm. The physical properties of each phase are different, but they are temperature independent, and the change in phase involves a latent heat of fusion A. After a certain time t, the thickness of the molten layer is Xi(t) in each phase there is a temperature distribution and the interface is at the melting temperature Tm (Fig. E5.4). 

Experimental data for the DC conductivity pressure dependences of the conductivity at constant temperature near the liquid-vapor critical point. Comparison with the equation-of-state data displayed in Fig. 2.3(a) clearly shows a qualitative relationship between rapid variation in the conductivity and density. Conductivity data obtained along the liquid-vapor coexistence curve, shown in Fig. 3.20, demonstrate, furthermore that the electronic structures of the liquid and vapor are fundamentally different. The liquid structure of cesium just above the melting point is characterized by a high degree of correlation in the atomic positions (see Section 3.4) and, as we have noted previously, cesium is a normal liquid metal with physical properties very similar to those of the corresponding solid. The electrical conductivity, in particular, is ical for a material with metallic electron concentration, that is, an electron density comparable with the atomic density. 

Figure 2-5 The physical constants of straight-chain aikanes. Their vaiues increase with increasing size because London forces increase. Note that even-numbered systems have somewhat higher melting points than expected these systems are more tightly packed in the solid state (notice their higher densities), thus allowing for stronger attractions between molecules.

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