Normal melting points

The normal melting point of a substance is the temperature at which solid and hquid are in equilibrium at atmospheric pressure. At the triple point, the pressure is the equilibrium vapour pressure of the system (solid liquid - vapour) and the temperature differs from the melting point. The difference is, however, quite small—usually only a fraction of a degree—since the line TV departs only slightly from the vertical within reasonable ranges of pressure. 

For a pure substance, the melting point is identical to the freezing point It represents the temperature at which solid and liquid phases are in equilibrium. Melting points are usually measured in an open container, that is, at atmospheric pressure. For most substances, the melting point at 1 atm (the normal melting point) is virtually identical with the triple-point temperature. For water, the difference is only 0.01°C. 

The solid is the more dense phase (Figure 9.7a). The solid-liquid equilibrium line is inclined to the right, shifting away from the y-axis as it rises. At higher pressures, the solid becomes stable at temperatures above the normal melting point In other words, the melting point is raised by an increase in pressure. This behavior is shown by most substances. 

We can expect the entropy to increase when a solid melts and its molecules become more disordered. Similarly, we can expect an even greater increase in entropy when a liquid vaporizes, because its molecules then occupy a much greater volume and their motion is highly chaotic. In this section, we develop expressions for the change in entropy at the transition temperature for the prevailing pressure. For instance, if the pressure is 1 atm, then these expressions are applicable only at the normal melting point, Tf (the f stands for fusion), the temperature at which a solid melts when the pressure is 1 atm, or the normal boiling point, Th, the temperature at which a liquid boils when the pressure is 1 atm. 

A new substance developed in a laboratory has the following properties normal melting point, 83.7°C normal boiling point, 177°C triple point, 200. Torr and 38.6°C. 

Use the phase diagram for compound X below to answer these questions (a) Is X a solid, liquid, or gas at normal room temperatures (b) What is the normal melting point ol X ... 

Energy must also be provided to melt a solid substance. This energy is used to overcome the intermolecular forces that hold molecules or ions in fixed positions in the solid phase. Thus, the melting of a solid also has characteristic energy and enthalpy changes. The heat needed to melt one mole of a substance at its normal melting point is called the molar heat of fusion, Ai/fas... 

Phase diagrams are constructed by measuring the temperatures and pressures at which phase changes occur. Approximate phase diagrams such as those shown in Figures 11-39 and 11-40 can be constmcted from the triple point, normal melting point, and normal boiling point of a substance. Example illustrates this procedure. 

C14-0050. Table lists molar enthalpies of fusion of several substances. Calculate the molar entropy of fusion at its normal melting point for each of the following (a) argon (b) methane (c) ethanol and (d) mercury. 

The normal melting points and boiling points generally increase as the intermolecular forces between the molecules in the compounds increase. 

The mixture is going to be identified by its ability to not mix with water (total immiscibility), normal boiling point (each compound in the mixture has a Tb above 350 K so the mixture will be a liquid), normal melting point (each compound in the mixture has a Tm below 250 K so the mixture will be a liquid), the Hildebrand solubility parameters of each of the compounds should be between 18-22 MPa172 (so the two compounds are mutually miscible). 

Apparently, the direct transition from vapor to solid is less common than the double transition vapor — liquid — solid, see, e.g., Refs.158-160). From the rate of solidification of metal droplets (average diameter near 0.005 cm) at temperatures 60° to 370° below their normal melting points, the 7sl was concluded158) to be, for instance, 24 for mercury, 54 for tin, and 177 erg/cm2 for copper. For this calculation it was necessary to assume that each crystal nucleus was a perfect sphere embedded in the melt droplet the improbability of this model was emphasized above. 

Which point on the diagram below might represent the normal melting point ... 

White tetragonal crystals refractive index 1.973 hardness 1.5 Mohs density 7.16 g/cm3 does not have a normal melting point triple point 525°C sublimes at 383°C insoluble in water, ethanol and ether. 

Aggregate State and Phase Diagram Normal Melting Point (rm), Normal Boiling Point (rb), and Critical Points (Tc, P r)... 

Aggregate State and Phase Diagram Normal Melting Point (Tf),... 

First we inspect the normal melting points (Tm) of the compounds. Note that because Tm, Th and Tc already have a subscript denoting that they are compound specific parameters, we omit the subscript i. Tm is the temperature at which the solid and the liquid phase are in equilibrium at 1.013 bar (= 1 atm) total external pressure. At 1 bar total pressure, we would refer to Tm as standard melting point. As a first approximation, we assume that small changes in pressure do not have a significant impact on the melting point. Extending this, we also assume that Tm is equal to the triple point temperature (Tt). This triple point temperature occurs at only one set of pressure/temperature conditions under which the solid, liquid, and gas phase of a pure substance all simultaneously coexist in equilibrium. 

Table 4.1 Normal Melting Points (Tm), Normal Boiling Points (Th), and Critical Points (7),p ) of some n-Alkanes. Note that temperatures are given in °C and not in Kfl...

Forms of Boric Acid. Orthoboric acid, B(OH)3, formula wt, 61.83, crystallizes from aqueous solutions as white, waxy plates that are triclinic in nature sp gr144, 1.5172. Its normal melting point is 170.9°C, however, when heated slowly it loses water to form metaboric acid, HB02, formula wt, 43.82, which may exist in one of three crystal modifications. Orthorhombic HB02-III or a-form (d = 1.784 g/mL, mp = 176° C) forms first around 130°C and gradually changes to monoclinic HB02-II or 3-fomi (d = 2.045 g/mL, mp = 200.9° C). Water-vapor pressures associated with these decompositions follow. To convert kPa to mm Hg, multiply by 7.5. 

---