When describing change of phase, it is very commonly said that molecules in hotter materials vibrate more, weakening the intermolecular bonds within them. My question is "How can increased vibrational velocity weaken these bonds?" It certainly makes sense that greater vibration increases intermolecular spacing, and the strength of intermolecular forces decreases with spacing, but when you get around to analyzing the situation, the numbers are not compelling. Take ice, for example. Average inter-molecular spacing is proportional to the inverse cube root of density. Heating ice from -10C to 0C decreases its density by about 1/10 of one percent. This means the average inter-molecular spacing goes down by about 1/30th of one percent. I believe Hydrogen bonding decreases in strength as the inverse cube, but even if we were using weaker forces, with an inverse 6th, inverse 7th, or even inverse 12th power law, an increase of 1/30 of 1 percent in spacing leads to a decrease of less than 1 percent in the inter-molecular forces...hardly anything to hang a phase change on... With that obvious answer out of the way, can someone explain how exactly a small increase in the average vibrational speed decreases the intermolecular forces significantly? [For example, the increase in average molecular velocity between -10C and 0C on average is less than 2%] Note that nothing changes much even if I had picked -45C instead of -10C. The difference in densities is about 1/3 of one percent between ice at -45C and ice at 0, meaning the average spacing has only changed by a factor of 1.001 or so.