Why does water expand when it freezes?

Have you ever wondered why you use antifreeze for filling the car radiator?

There are two main reason for that: One is to protect your engine from corrosion, and the second is to avoid water freezing. We need to avoid water freezing because it will cause engine overheating when you start it in the morning, and the most important it is because water expand when it freezes and this will cause cracks in the engine cooling system causing a loot of damages to your engine.

And what the heck! I remember from chemistry classes that liquids and other materials shrink when temperatures fall!

Ya! But this is not the case of water, because water have a type of chemical bound that is called the hydrogen bounding. Hydrogen bonding usually occur between molecules (intermolecular bonding). It is an electrostatic attraction between two polar groups that occurs when a hydrogen atom, covalently bound to a highly electronegative atom such as oxygen (in case of water), experiences the electrostatic field of another highly electronegative oxygen atom nearby.

When water molecules are in the liquid state, hydrogen bonds are continuously being formed and reformed in a disordered manner. But when it come to freezing, water molecules lose energy and do not vibrate or move around as powerfully. This allows more stable hydrogen-bonds to form between molecules, as there is less energy to break the bonds. Hydrogen bonds form a crystalline structure that causes density decreasing because each water molecule is held away from its neighbours at a distance equal to the length of the hydrogen bonds. Thus water expands as it freezes, and as we know ice float on water because it is less denser than the liquid water.