Effect of impurities upon the melting point

Effect of impurities upon the melting point. Let us take a specific example and examine the effect of the addition of a small quantity of naphthalene to an equilibrium mixture of pure solid and liquid a-naphthol at the temperature of the true melting point (95 5°) at atmospheric pressure. 

The naphthalene wUl dissolve in the liquid a-naphthol and, according to Raoult s law, the vapour pressure of the latter will be reduced. Hence a-naphthol will pass preferentially into the liquid phase and, if the external temperature is maintained at 95 5°, the ultimate result will be the complete melting of the solid a-naphthol since melting requires heat and no heat is imparted to the system, the temperature will fall. 

A somewhat different method of plotting the results will help the reader to appreciate the significance of the eutectic temperature. In Fig. 1,11, 2 melting points are plotted against composition. The curve AC portrays the decreasing melting point of a-naphthol as naphthalene is added up to a mol fraction of 0 605. The curve BG represents the 

The effect can easily be interpreted by reference to Fig. /, 11, 1. The initial state of the system is represented by M. Upon the addition of a smaU amount of the second substance, the vapour pressure of the liquid first falls to A. Now a soUd phase 

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Crystallization is governed by the usual thermodynamic variables of temperature, composition, and pressure. It is common to describe the thermodynamics in terms of dominant chemical species present, vhich in the majority of crystallization processes are the material to be crystallized and a small number of solvents. For all real systems, a further influence has to be taken into account, that is, of impurities. These are present in every system, in varying amounts. These impurities can have an effect on the solubility or melting point of the material to be crystallized, if small. The presence of additional components in the solution is often more noticeable in their effect upon the kinetics of crystallization and more specifically on growth rates of crystals. For the sake of clarity, additive is used as a collective noun for any minor component in a given system, whether this additive is in fact an impurity stemming from the raw materials employed or a by-product from the reaction stages required to manufacture the final product, or a true additive supplied to the system to achieve a specific effect. It is noted that the word impurity is even more widely used, as solvent or solvent mixture can also act as additives. 

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