A second interesting perspective: with the transition from a liquid to a gas, the entropy of vaporization (which equals the ratio of the heat of vaporization to the temperature) is nearly constant across most substances. Higher temperature boiling substances require the addition of more heat to boil, because the boiling point is a measure of the amount of molecular agitation necessary to overcome intermolecular force; the boiling point is an indication of the magnitude of these forces. At vaporization, the kinetic energy of the particles, measured by the temperature, has grown sufficient for the random interplay of energy among its possible forms within the substance to increase the electrostatic potential energy along lines of intermolecular force to a degree sufficient to lead to molecular separation. This relative balance between the electrostatic potential energy increase (the heat flow required from the surroundings) and the amount of kinetic energy present in the particles (gauged by the temperature), equals the entropy change (ΔH/T)).
A third interesting fact is that symmetry of the molecules has a pronounced effect on the melting point, but not the boiling point (the greater the symmetry, the higher the melting point). This is an advanced concept. If molecules are symmetrical, then there will not be as great an increase in disorder when they move from solid to liquid. Flipping over does not change anything, so there is not as much order bound up in the crystal, so there is not so great a loss of the order of crystallization in the melting process, which makes melting somewhat less favorable. Think about how much more easily a molecule can go in the reverse direction (back to crystal) if it does not have to hit it just right.
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