Intermolecular forces and melting/boiling point

One thing that tends to make confusion to my students is the relationship between non-covalent intermolecular forces and the melting/boiling point of a substance. In fact, how one can explain in a molecular point of view, the existence of the liquid and solid states? The answer lies precisely by the existence of intermolecular forces. Both states are characterized by a higher order spatial distribution of the molecules compared to the gaseous state. This order is a consequence of the attractions between the different molecules that make up a substance. Thus, in the liquid state, each molecule establishes various interactions (non-covalent!) with neighboring molecules, and in the solid state the amount of interactions is further increased.

When it is reached the melting/boiling point, which is going on is a transition from a state in which there are more intermolecular forces to one where there are fewer. In other words, what is being done is to provide energy to break the non-covalent forces established between the molecules, by increasing their kinetic energy.

Therefore, the greater the intensity of the non-covalent forces existing between the molecules, the greater the amount of energy required to break these interactions, and thus the higher the melting/boiling point.

I often ask to my students what happens when it is reached the boiling point of a substance, and the answer that I invariably get is: "You are breaking the bonds." The problem comes when I ask what kind of bonds, because usually their tendency is to say the covalent bonds. I, playfully, then tell them that if so, the water in the gaseous state is no longer H₂O, and then they realize that indeed that what it is cleaved are not the covalent bonds but the non-covalent ones.

In short, there is a direct relationship between intermolecular forces and melting/boiling point, and the greater the sum of these forces, the higher the melting/boiling point of a substance.

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