The Exceptional Case of Ice Floating

That ice blocks float in water might seem extremely obvious, but it’s actually a very unusual phenomenon!

Why does ANYTHING Float?
Something called the ‘Archimedes Principle’ underpins buoyancy. Basically, it explains how an object put in a body of liquid will push the liquid out of that space. The water then wants to push the object out so it can take its old position back, so will exert its weight on the object to try to move it. This is where density comes in, since the volume of both liquid and object is the same, the only thing that making a difference is the weight each thing can pack into that space. So if the object is more dense than water, it sinks, but if it is less dense, it floats.
This means is that ice must be less dense than water.

Tall glass of water [7/365] by Alvin Trusty via Flickr (CC BY-NC-SA 2.0)

Solids are Packed
The difference between solids and liquids is how restricted the atoms in them are. Atoms in liquids can move around a bit, while in solids they can only vibrate. This is because solids are usually made of more tightly packed atoms than their liquid counterparts. More tightly packed atoms (or in this case molecules) implies higher density. As you might’ve already guessed, most solids would sink in when placed in their liquid.

Water’s Special Bond
I hope I’ve convinced you that ice’s density is special. The reason ice has a density lower than water is entirely related to the molecule which makes up the substance, H2O. There are only two molecules in this arrangement, hydrogen and oxygen, but they happen to have a special relationship. Hydrogen can form something called a ‘hydrogen bond’ (imaginative, I know) with only a few other molecules, among which happens to be oxygen. This hydrogen bond is much stronger than the regular bonds which hold most materials together, so when the water molecule starts to freeze, rather than compacting, like most materials, it rearranges itself. The H2O molecule is a V-shape, with the two hydrogens attached to the central oxygen, so this rearrangement points each hydrogen so that it can connect with the oxygen on a different water molecule.

Hydrogen Bonding In Water by Louis Shackleton via Flickr (CC BY-NC-SA 2.0)

The arrangement of these molecules means that they actually take up more space than the liquid form of water, where molecules can press closer together since they are not held by the hydrogen bond. Water is actually at its most dense at 4°C, the lowest energy it can reach before hydrogen bonding kicks in to start separating molecules.

Iceberg by gregpoo via Flickr (CC BY-SA 2.0)

Since hydrogen bonding is so specific and restricted, water is really the only substance we encounter which has a less dense solid than liquid state, allowing ice the unusual property of floating in water.