Everything that gets colder shrinks, but water that gets so cold it turns into ice expands, why is that?

Answer

Hi Jean,

What you describe is indeed the normal course of events … but there are of course exceptions. Water is one of those exceptions, luckily it is, and luckily we know exactly why.
What we measure as temperature is actually the thermal movement of atoms and molecules. When we cool down a material, that movement decreases, so the molecules can move closer together. In that sense, liquid water behaves normally in the temperature range between 100 and 4°C: if we make it colder, it shrinks.
But below 4°C something strange happens: the water expands again, and when it freezes it becomes ice, which has a much larger volume than liquid water.
Moving to a more condensed state of aggregation (i.e. from liquid to solid) implies an increase in order and a decrease in thermal motion, and so here too usually holds that a solid is more compact than the liquid of the same material. For example, if you freeze benzene, the benzene “ice” will sink into benzene because it is heavier, denser and more compact.
But not water, which means that water ice floats, and forms an insulating layer that protects the deeper water of 4°C against further cooling. Otherwise life on earth would be a lot more difficult!
And now the reason: water consists of molecules, which in themselves consist of two hydrogen atoms and an oxygen atom. The angle between the two hydrogen atoms sitting on the oxygen atom is about 107°. There is a special kind of intermolecular interaction that is not only very strong compared to the other forces between molecules, but also very focused on the bargain. We call this a hydrogen bond, and water happens to be ideally suited to form it now: oxygen has two free electron pairs in water that can serve as electron donors for such a hydrogen bond, as well as two hydrogen atoms that can form the other end of it. It is thus possible to form four hydrogen bonds per water molecule, all of which are at an angle of about 109° to each other and thus have the shape of an equilateral pyramid. This gives rise to a large energy stabilization by forming the hydrogen bonds, but it is not very space efficient, because large voids remain between adjacent water molecules. And that’s also why ice is lighter than water – it forms a “hollow” structure. At 4°C, water reaches its greatest density, because that is where the ideal balance is found between forming “ice-like” groups of molecules and decreasing thermal movement. Below 4°C, the “ice-like” chunks in the liquid water become so large that the amount of voids in the structure increases the volume of the water again.

I hope I have been able to give you an explanation for this strange behavior of water!
Via the added link you can view the structure of hexagonal ice and see the holes “with your own eyes”.