Melting point of a solid

Effect of pressure on the melting point of a solid, (a) When ihe solid is die more dense phase, an increase in pressure converts liquid to solid the melting point increases, (b) If the liquid is the more dense phase, an increase in pressure converts solid to liquid and the melting point decreases. 

The influence of surface energy on the melting-point of a solid has been calculated by P. Pawlow, who found experimentally that with salol granules of surfaces 228—1296 p2, the m.pt. decreased 2 8° C. per 100 times increase of surface (p = 0 001 mm.). 

Repulsive forces determine, for example, the melting point of a solid. Whenever the packing is efficient, the melting point tends to be high. The attractive forces, on the other hand, govern the heat of vaporization and therefore the boiling point. Trouton s rule, which relates the normal boiling point of a liquid to its heat of vaporization, is a manifestation of this relation. 

That such ought to be the case was first realised by James Thomson 1 who, in 1849, showed that from theoretical considerations a connection must exist between the melting-point of a solid and the pressure. The following year this was experimentally demonstrated, by his brother W. Thomson (Lord Kelvin),2 who found that under a pressure of 8-1 atmospheres the melting-point of ice was lowered by 0-059° C., equivalent to a fall of 0-0078° per atmosphere. In the table on p. 251 are given the more accurate determinations of Tammann,3 the third colunm giving the results calculated in terms of atmospheres.4... 

The Clapeyron-Clausius equation (1.52) for solid = liquid equilibrium cannot be integrated easily since Vs cannot be ignored in comparison with Vt. Also the laws of liquid state are not so simple as those for gaseous state. However, this equation can be used for calculating the effect of pressure on the melting point of a solid. Eq. 1.52 can also be used for calculating heats of fusion from vapour pressure data obtained at different temperatures. 

Some chemical examples of data likely to be distributed in a normal fashion are pH of natural waters melting point of a solid compound. To check whether data come from a normal distribution, you can ... 

In other words, the melting point of a solid, T p, is equal to molar enthalpy of fusion, AHf s, divided by molar entropy of fusion, ASfus-Boiling takes place when the drive toward disorder overcomes the tendency to lose energy. Condensation, shown in Figure 18, takes place when the tendency to lose energy overcomes the drive to increase disorder. In other words, when AH p > TASyap, the liquid state is favored. The gas state is preferred when AHy p < TASyap. 

If we consider the melting point of a solid as a function of applied pressure, then we are more interested in the reciprocal of the coefficient in (14.4) i.e. in STjSp. This must be equal to TAfVjAfh where AfV and Afh are the volume change and latent heat of fusion respectively. If therefore we measure the volume change which accompanies melting and also the slope of the curve of m.p. against pressure, we can evaluate the latent heat of fusion Afh. 

Melting is the conversion of a solid to the liquid state. The normal melting point of a solid is the temperature at which solid and liquid are in equilibrium under a pressure of 1 atm. The normal melting point of ice is 0.00°C, thus liquid water and ice coexist indefinitely (are in equilibrium) at this temperature at a pressure of 1 atm. If the temperature is reduced by even a small amount, then all the water eventually freezes if the temperature is raised infinitesimally, all the ice eventually melts. The qualifying term normal is often omitted in talking about melting points because they depend only weakly on pressure. 

The melting point of a solid is the same as freezing point of its liquid. It is the temperature at which the rate of melting of a solid is the same as the rate of freezing of its liquid under a given applied pressure. 

The equipment for measuring the melting point of a solid varies in complexity from a simple oil bath heated with a microbumer to a microscope with a heated stage as shown in Fig. 12.1. The essential components of a melting point apparatus are ... 

The transformation of liquid to solid is called freezing, and the reverse process is called melting, or fusion. The melting point of a solid or the freezing point of a liquid is the temperature at which solid and liquid phases coexist in equilibrium. The normal melting (or freezing) point of a substance is the temperature at which a substance melts (or... 

Over the years several semi-empirical melting or freezing rules have emerged in an attempt to correlate SFE with other features of the solid or fluid phases [153]. A distinguishing characteristic of these rules is that they are all formulated in terms of properties of just one of the phases. The best known of these is the Lindemann rule [154], which states that the melting point of a solid correlates with the mean-squared atomic displacement. When this quantity exceeds a particular value, the solid is observed to melt. This rule is actually a loose statement about the mechanical stability of the solid, rather than a statement of its thermodynamic stability relative to the fluid. 

The melting point of a substance is the temperature at which the liquid and solid phases are in equilibrium. A good way of experimental determination of the melting point of a solid is to place a small amount of the perfectly dry substance in a thin-walled capillary melting point tube attached to the bulb of a thermometer suspended in some suitable liquid and to raise the temperature of the liquid bath until the solid melts. 

When a crystalline solid is heated to the temperature at which it melts and passes into the liquid state, the solid/liquid system is univariant. Consequently, for a given pressure value, there will be a definite temperature (independent of the quantities of the two phases present) at which the equilibrium can exist. As with any univariant system, a curve representing the equilibrium temperature and pressure data can be plotted, and this is termed the melting point curve or fusion curve. Since both phases in a solid/liquid equilibrium are condensed (and difficult to compress), the effect of pressure on the melting point of a solid is relatively minor unless the applied pressures are quite large. 

The melting point of a solid organic substance is frequently adopted as a criterion of purity, but before any reliance can be placed on the test, it is necessary for the experimental procedure to be standardized. Several types of melting point apparatus are available commercially, but the most widely used method consists of heating a powdered sample of the material in a glass capillary tube located close to the bulb of a thermometer in an agitated bath of liquid. 

The melting point of a solid and its response to shock waves depends on the form and depth of the interatomic potential. Its compressibility is simply related to the Born repulsion parameter. The mechanical hardness is a function of the bulk modulus (inverse compressibility), which in turn correlates strongly with the volume density of the chemical bonding energy, i.e., the bonding energy per unit volume the smaller the atoms and the stronger the atomic bond, the harder the solid. In the box on p. 34, a simple relation is derived between the bulk modulus and the Bom repulsion parameter. 

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