Melting point The temperature

It is instructive for the student to construct a rough melting point diagram (compare Section 1,13 and Fig. 1,12, 1) for mixtures of cinnamic acid and urea. Weigh out 1 00 g. each of the two finely powdered components, and divide each into ten approximately equal portions on a sheet of clean, smooth paper. Mix 4 portions of cinnamic acid (A) with 1 portion of urea B) intimately with the aid of a spatula on a glass slide, and determine the melting point (the temperature at which the mixture just becomes completely fluid is noted). Repeat the procedure for 3 parts of A and 2 parts oiB 2 parts of A and 3 parts of B and 1 part of A and 4 parts of B. Tabulate your results as follows —... 

Melting point The temperature at which a solid liquefies. The same as freezing point. 

Melting point The temperature at which the solid and liquid phases of a substance are in equilibrium with each other, 13 effects of pressure on, 235 liquids, 235 low, 235 metals, 245... 

Chemistry is concerned with the properties of matter, its distinguishing characteristics. A physical property of a substance is a characteristic that we can observe or measure without changing the identity of the substance. For example, a physical property of a sample of water is its mass another is its temperature. Physical properties include characteristics such as melting point (the temperature at which a solid turns into a liquid), hardness, color, state of matter (solid, liquid, or gas), and density. A chemical property refers to the ability of a substance to change into another substance. For example, a chemical property of the gas hydrogen is that it reacts with (burns in) oxygen to produce water a chemical property of the metal zinc is that it reacts with acids to produce hydrogen gas. The rest of the book is concerned primarily with chemical properties here we shall review some important physical properties. 

Melting point the temperature at which a thermotropic liquid crystal passes from the solid to the mesophase (or to an isotropic liquid). 

FIGURE 6.24 The temperature of a solid rises as heat is supplied. At the melting point, the temperature remains constant and the heat is used to melt the sample. When enough heat has been supplied to melt all the solid, the temperature begins to rise again. A similar pause occurs at the boiling point. The data are for water. 

Normal boiling point the temperature at which the vapor pressure of a liquid is exactly one atmosphere. (16.10) Normal melting point the temperature at which the solid and liquid states have the same vapor pressure under conditions where the total pressure on the system is one atmosphere. (16.10) Normality the number of equivalents of a substance dissolved in a liter of solution. (17.1)... 

There are other properties besides density and solubility that can be measured precisely and expressied in terms of numbers. Such another property is the melting point, the temperature at which a crystalline substance melts to form a liquid. The electrical conductivity and the thermal conductivity are similar properties. On the other hand, there are also interesting physical properties of a substance that are not so simple in nature. One such property is the malleability of a substaqce—the ease with which the substance can be hammered out into thin sheets. A related property is the ductility—the ease with which the substance can be drawn into a wire. Hardness is a similar property we say that one substance is less hard than a second substance when it is scratched by the second substance, but this test provides only qualitative information about the hardness. A discussion of hardness is piesented in Chapter 6. 

The most critical factor in determining an accurate melting point is the rate of heating. At the melting point the temperature rise should not be greater than 1°C per minute. This may seem extraordinarily slow, but it is necessary in order that heat from the bath be transferred equally to the sample and to the glass and mercury of the thermometer. 

There are six sublimation studies and two vaporization studies from which an enthalpy of formation for Hf(g) could be derived. A plot of these vapor pressure data, log p vs 1/T, reveals considerable discrepancy between the various studies. In the liquid region, the vapor pressure values of Koch et al. (1 ) and Ackermann and Rauh (2) are in close agreement. The pressures of the former study are 5% higher at 2500 K but become 10% lower at 2800 K. The measured temperatures in the former study ( ) were adjusted (i.e., the pyrometer reading) by Koch et al. ( ) so that at the observed melting point the temperature would be 2500 K. The sample purity was not stated. In the latter study (2) the reported vapor pressures were corrected pressures (assuming ideal solution) from a eutectic of Hf and W. The Hf sample had a measured melting point of 2464 K the purity was not reported. 

Melting point. The temperature at which solid and liquid phases coexist in equi-hbrium. (11.8)... 

Crystalline Melting Point - The temperature of melting of the crystalline phase of a crystalline polymer. It is higher than the temperature of melting of the surrounding amorphous phase. 

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