Text 18 A The Nature of a Liquid - раздел Образование, Part One THE PREDICATE When Iodine Crystals Arc Heated To 114°C, They Melt Forming Liquid Iodine. Th...
When iodine crystals arc heated to 114°C, they melt forming liquid iodine. The temperature at which the crystals and the liquid arc in equilibrium — that is, at which crystals have no tendency to melt or the liquid has no tendency to freeze—is called the melting point of the crystals, and the freezing point of the liquid. This temperature is 114°С for iodine.
Liquid iodine differs from solid iodine (crystals) mainly in its fluidity. Like the solid, and unlike the gas, it has a definite volume (1 g occupics about 0.2 cm3), but it docs not have a definite shape: instead, it fits itself to the shape of the bottom part of its container.
From the molccular viewpoint the proccss of melting can be described in the following way. As a crystal is heated, its molecules are increasingly agitated, and move about more and more vigorously, but at lower temperature, this thermal agitation docs not carry any one molcculc any significant distance away from the position fixed for it by the arrangement of its neighbours in the crystal. At the melting point the agitation finally becomes so great that it causcs the molecules to slip by one another and to change somewhat their location relative to one anothcr. Thcy continue to stay close together, but do not continue to retain a regular fixed arrangement. Instead, the grouping of molcculcs around a given molcculc changes continually, sometimes being much like the close packing of the crystal, in which cach iodine molcculc has twelve near neighbours, and sometimes considerably different, the molecule has only ten or nine or eight near neighbours. Thus, a liquid, like a crystal, is a condensed phase, as contrasted with a gas, the molcculcs being piled rather closcly together; but whereas a crystal is characterized by regularity of atomic or molecular arrangement, a liquid is characterized by randomness of structure. The randomness of structure is usually the reason why the density of a liquid is somewhat less than that of the corresponding crystal.
