It’s weird that heat rises, but ice floats.
Lily Clarke, Physics consultant for Innovia Technology
"Whether something rises or sinks is to do with the relative density of matter. Water is lot more dense than air, because the molecules are closer together. Intuitively, water will be heavier than air.
From a physics perspective, we actually say that “hot air” rises, rather than heat. Heat energy causes molecules in air to vibrate, and so the average distance between molecules increases, decreasing the density. This is why hot air rises, because it has become less dense than ambient air and so ‘floats’ to the top.
The reason ice floats is due to a strange quirk of water. Most solids, such as rocks, are more dense than liquids, so they sink. In fact, most substances become more dense when they freeze.
One water molecule contains 1 positively-charged hydrogen atom and 2 negatively-charged oxygen atoms, held together by both covalent bonds and weaker hydrogen bonds. When water freezes, these hydrogen bonds adjust slightly to keep the 2 negatively charged oxygen atoms apart, creating a honeycomb structure that actually causes ice to become less dense than liquid water."