Crystalline arrangements ordered

Solids lacking an ordered, crystalline arrangement are termed amorphous materials, and resemble rigid liquids in structure and properties. Glass (SiO 2) is the classic example of an amorphous solid. Such materials typically soften on heating, rather than showing a sharp melting point. 

If you analyze the four spontaneous endothermic processes mentioned previously, you ll see that each involves an increase in the randomness of the system. When ice melts, for example, randomness increases because the highly ordered crystalline arrangement of tightly held water molecules collapses and the molecules become free to move about in the liquid. When liquid water vaporizes, randomness further increases because the molecules can now move independently in the much larger volume of the gas. In general, processes that convert a solid to a liquid or a liquid to a gas involve an increase in randomness and thus an increase in entropy (Figure 17.3). 

Two types of liquid crystalline arrangement are recognized. One is smectic and the other one is nematic. Both are parallel arrangements of long molecular axes. The smectic liquid crystals are more ordered, however, than the nematic ones. This is a result of differences in the orientations of the chain ends. In smectic liquid crystals, the chain ends are lined up next to each other. In nematic ones, however, they lack any particular orientation. Also, the smectic liquid crystals are layered while the nematic ones are not. Microscopic observations can help distinguish between the two forms. 

All of these ordered arrangements, we call crystalline. The last arrangement, order of chains only, is observed in colloidal solutions of chain molecirles in a poor solvent. Chains are collapsed and exert attractions to each other that may lead to colloidal crystals. It is order of chains but not of segments. These phenomena are outside the scope of the following discussion. 

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