I read the following statement in my textbook

While a liquid is boiling, the energy supplied to the liquid goes to breaking the bonds between neighbouring molecules.

Can anyone explain the process behind how bonds are broken. Is breaking of the bonds solely due to increasing the intermolecular distance?

  • $\begingroup$ I think it's talking about hydrogen bonds. $\endgroup$ – PM 2Ring Oct 12 '18 at 8:10
  • $\begingroup$ @PM2Ring I thought this too at first, but do all liquids have hydrogen bonds? $\endgroup$ – Aaron Stevens Oct 12 '18 at 11:39

The following is too long for a comment.

There are two frameworks in current day physics:

a) the classical framework with classical mechanics, classical electrodynamics from which emerge thermodynamics explained by classical statistical mechanics. The term liquid, belongs to this classical framework of thermodynamics. There exist classical molecules in this framework, obeying classical statistical mechanics . Boiling belongs to the emergent thermodynamic description.

b)The quantum mechanical framework, with quantum electrodynamics . Elementary particles, atoms and molecules are described in this frame differently than the classical frame, and the classical frame emerges from this underlying quantum model. The term "bond" belongs to this framework.

Confusions and sometimes paradoxes arise when terminology belonging to one framework is mixed with the terminology of the other.

The hand waving quantum picture of the condition of molecules in a liquid is based on the orbitals of the electrons of the molecules, which allow attractive forces to appear between molecules and quantum mechanical states to exist where the molecules are caught in a shallow collective potential well. Depending on the particular energy the quantum mechanical states may collectively show a liquid phase, but it is not bonding, as the energy levels are practically continuous.

Yes, with the boiling ( classical phase transition) the average temperature is such that the molecules acquire energy and exit the shallow effective collective potential well into the classical gas phase.


I find this to be odd terminology... Typically bond breaking refers to breaking actual chemical bonds. This is not the case for liquids boiling, but the idea is still the same.

Yes when you add energy to the liquid the molecules become more separated as they transition to the gaseous phase. More specifically, the increase in energy increases the vapor pressure within the liquid until it becomes equal to the atmospheric pressure.

I wouldn't say bonds are being broken though. Adding energy to molecules can break chemical bonds, but the process is different from boiling.

  • $\begingroup$ For clarification, is it the word “breaking” which you object to and what is an “actual chemical bond”? $\endgroup$ – Farcher Oct 12 '18 at 7:07

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