The chemical explanation for the liquid phase of any compound is that it is specific to that compound. Water has a specific melting and boiling point, as does iron, or oxygen in the air, or any other simple compound you name. The fact that a compound (H20) contains given elements from other compounds (O2 and H2) has no relation to the properties, such as boiling and freezing point. Properties of materials are generally related to physical qualities of the individual molecules, for example, molecular diameter, polarity, and geometric structure. Since the physical attributes of H20 are very different from O2 and H2, the properties, such as melting point, are also different.