Melting or fusion

Thermal Properties. Because all limestone is converted to an oxide before fusion or melting occurs, the only melting point appHcable is that of quicklime. These values are 2570°C for CaO and 2800°C for MgO. Boiling point values for CaO are 2850°C and for MgO 3600°C. The mean specific heats for limestones and limes gradually ascend as temperatures increase from 0 to 1000°C. The ranges are as follows high calcium limestone, 0.19—0.26 dolomitic quicklime, 0.19—0.294 dolomitic limestone, 0.206—0.264 magnesium oxide, 0.199—0.303 and calcium oxide, 0.175—0.286. 

Figure 3.2 Change of entropy of a substance with temperature - the substance has undergone an allotropic change (t), fusion or melting (/) and vapourization (v) over the entire temperature range.

Figure 12.2 Phase changes and their enthalpy changes. Each type of phase change is shown with its associated enthalpy change. Fusion (or melting), vaporization, and sublimation are endothermic changes (positive AH°), whereas freezing, condensation, and deposition are exothermic changes (negative AH°).

Changing from the solid state to the liquid state is called fusion or melting, and from liquid to solid is called freezing. Because both liquids and solids are less energetic than liquids and gases, the amount of heat required to melt a solid is normally less than the amount of heat required to evaporate a liquid. This state transition is similar to the liquid-to-gas transition in that it occurs at a constant temperature for a pure substance. 

A first-order transition normally has a discontinuity in the volume-temperature dependence, as well as a heat of transition, AHf, also called the enthalpy of fusion or melting. The most important second-order transition is the glass transition, Chapter 8, in which the volume-temperature dependence undergoes a change in slope, and only the derivative of the expansion coefficient, dVIdT, undergoes a discontinuity. There is no heat of transition at Tg, but rather a change in the heat capacity, ACp. 

Another common phase transition is fusion, or melting, as when ice melts to water. The change in molar enthalpy that accompanies fusion under standard conditions (pure solid at 1 bar changing to pure liquid at 1 bar) is called the standard enthalpy of fusion, A H. Its value for water at 0°C is +6.01 kJ mol . As for enthalpies of vaporization, all enthalpies of fusion are positive, and the sign is not written explicitly in tables. Notice that the enthalpy of fusion of water is much less than its enthalpy of vaporization. In vaporization the molecules become completely separated from each other, whereas in melting the molecules are merely loosened without separating completely (Fig. 1.21). 

Fusion, or melting, is the transition from solid to liquid. The opposite process is freezing. 

In the case of polymer single crystals, the situation is not very different from that of regular crystals these entities undergo fusion or melting at a rather well-defined temperature because the dimensions of the crystalline domains are relatively large. The only structural defects are those corresponding to chain folding, hairpin turns, loops, and chain ends. 

Problem What is the relation between temperature and pressure expected for fusion or melting What is the form of the phase diagram line for the boundary between a liquid... 

Solution The Clapeyron equation. Equation 4.28, provides the relation between temperature and pressure for a phase transition. We need to consider how it ap>plies in the case of a solid-to-liquid (or liquid-to-solid) phase transition. The approximation of Equation 4.32 to the molar volume for a phase transition is based in part on the fact that for a solid or a liquid, the molar volume is largely independent of temperature, as well as being a value much less than the molar volume of a gas. This means that for fusion or melting, the change in the molar transition volume in Equation 4.28 is usually well approximated as a constant (independent of T). The following steps are carried out with this approximation, as well as with the assumption that the transition enthalpy is independent of temperature. 

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