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When iodine crystals are heated to 114º C. they melt forming liquid iodine. The temperature at which the crystals and the liquid are 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 114º C 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 (1g occupies about 0.2 cm³), but it does not have a definite shape: instead, it fits itself to the shape of the bottom part of its container.

From the molecular viewpoint the process 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 does not carry any one molecule 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 causes the molecules to slip by one another and to change somewhat their location relative to one another. They continue to stay close together, but do not continue to retain a regular fixed arrangement. Instead, the grouping of molecules around a given molecule changes continually, sometimes being much like the close packing of the crystal, in which each iodine molecule has twelve 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 molecules being piled rather closely 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.

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1. What is the melting point of a substance? 2. What are the characteristic features of a gas? 3. What does the physical state of a substance depend on?

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