Why is the volume of water increasing when it is freezing?

Most liquids have a quite simple behavior when they are cooled (at a fixed pressure): they shrink. The liquid contract as it is cooled; because the molecules are moving slower they are less able to overcome the attractive inter-molecular forces drawing them closer to each other. Then the freezing temperature is reached, and the substance solidifies, which causes it to contract some more because crystalline solids are usually tightly packed. Water is one of the few exceptions to this behavior. When liquid water is cooled, it contracts like one would expect until a temperature of approximately 4 degrees Celsius is reached. After that, it expands slightly until it reaches the freezing point, and then when it freezes it expands by approximately 9%. This unusual behavior has its origin in the structure of the water molecule. There is a strong tendency to form a network of hydrogen bonds, where each hydrogen atom is in a line between two oxygen atoms. This hydrogen bonding tendency gets stronger as the temperature gets lower (because there is less thermal energy to shake the hydrogen bonds out of position). The ice structure is completely hydrogen bonded, and these bonds force the crystalline structure to be very "open"

Dear Abdalla Hassan Mihdy Jassim,

The expansion of water is scientifically anomalous (abnormal) but to answer this, "Why is the volume of water increasing when it is freezing?", I would refer you to mother nature. Nature created water for several reasons which majorly include habitation of aquatic lives (fishes and co.).

As natural habitat, water should offer conducive conditions for survival of aquatic organisms. At the temperature range of the anomalous expansion, on cooling, water expands, decreases in density and floats (over the warmer, less dense layers). This segregation of layers of different temperature and density is possible because water is a poor conductor of heat. As a result, at the anomalous temperature range, the upper layer of water is cooler than the bottom. This makes it naturally possible and easy for living organisms to survive at extremely low temperature. Cooling of a lake (or every other natural water habitat) is effected from the upper water surface and the surface layer may get frozen and sub-cooled without significantly affecting the bottom water layers because heat transfer by convection (due to density differences of water molecules or layers) is zero and transfer by conduction is very poor. This means that it is virtually impossible for a lake to freeze solid. No matter how cold it is, there will always be water at the bottom of a frozen lake, at a temperature close to 4°C where life can be sustained.

If the ultimate reason had to do with molecular structure of water and inter-molecular H-bonding then, why is this behavior restricted to temperature < 4 deg. C? That would refer you back to nature.

Kind regards.

Water molecule is held together by hydrogen bonds H-O-H. Also, water is a polar molecule and when the temperature decreases, these bonds align is such a way that the volume of the water get increases when frozen.