Melting of a solid

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... 

It may be added here that Le Chatelier s principle is quite general in nature, and that its applicability is not restricted only to chemical equilibria. It can also be applied to physical equilibria, as for example, to explain qualitatively the effects of temperature and pressure on solubility or the effect of pressure on the melting of a solid. 

Further experimental work has been carried out on the rates of melting of a solid in a liquid, using a single component system, and Hixson and Baum express their results for the heat transfer coefficient as ... 

Partial melting of a solid solution controlled by diffusion in the solid... 

These examples involve partitioning of elements as liquids cooled and crystallized. Partial melting of a solid rock also results in partitioning of incompatible elements into the liquid phase, which contains no rigid crystalline sites. Separation of the melt then fractionates incompatible elements from the compatible elements left behind in the solid residue. 

Almost every change in a system involves either a gain or a loss of enthalpy. The change can be either physical, such as the melting of a solid to a liquid, or chemical, such as the burning of propane. Let s look briefly at examples of both kinds. 

Heat can also be pumped out from a calorimeter by other principles, e.g., by use of a cooling liquid, but such procedures are now only of historical interest in connection with laboratory calorimeters. Mainly of historical interest is also compensation of exothermic processes by use of melting of a solid, e.g., ice, surrounding the calorimeter vessel. Lavoisier used such an ice calorimeter (later often called a Bunsen calorimeter) in his pioneering biocalorimetric work (see Kleiber, 1961). For endothermic processes, compensation is easily achieved by release of electrical energy in the vessel. 

When the sample undergoes a transformation, it will either absorb (endothermic) or release (exothermic) heat. For example, the melting of a solid material will absorb heat, where that thermal energy is used to promote the phase transformation. The instrument will detect that the sample is cooler than the reference, and will indicate the transformation as an endotherm on a plot of differential temperature (AT) versus time.2 Figure 3.2 shows a typical DTA trace of the decomposition of dolomite. If the sample and reference are exposed to a constant heating rate, the x-axis is often denoted as tem-... 

Lennard-Jones suggested that the migration of surface atoms may be the first stage of melting of a solid this may be one of the preliminaries of large scale molting. It is very probable that migration of adsorbed... 

Rapid solidification 10 -10 Pulsed laser melting of a solid surface Amorphous and nanocrystaUine thin films... 

The dissolving of a solid in a liquid is similar to the melting of a solid. In each case, the energy of attraction which holds the crystal to-... 

Rgure 3.1 Melting of a solid involves a change from on ordered arrangement of molecules, represented by (a), to a more chaotic liquid, represented by (b). As a result, the melting process is accompanied by on increase in entropy. 

PHASE TRANSITIONS Another type of constant-temperature process is a phase transition such as the melting of a solid at constant pressure. This occurs reversibly at the fusion temperature, Tf, because an infinitesimal change in external conditions, such as reducing the temperature, can reverse the melting process. The reversible heat when 1 mol of substance melts is = AHfus, so... 

Reactions in melts, vitreous materials, polymers, etc. can justifiably be analyzed by equations based on a concentration dependence of rate. Some reactions proceeding in vitreous reactant phases have been shown to conform to second or even third-order rate equations. Progressive melting of a solid reactant during decomposition results in acceleratory behaviour [52,71-73] and comprehensive melting before dehydration was observed to result in an approximately constant rate of water evolution [74,75]. 

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. 

EXAMPLE 10 What is the name for the heat associated with the melting of a solid substance ... 

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