Water at 4⁰C is at it densest state, later if heated or cooled it will expand 
required conditions:
1. low temperature 
2. high pressure
Water is an amazing solvent. It is able to retain large amounts of salts and other materials in solution. When this occurs, the salts change the properties of water. For example, when salt is dissolved in water the freezing point of water is lowered. Thus, fresh water freezes at 0° C., whereas normal seawater freezes at –1.9 degrees C. As ice forms in salt water, there is no room in the crystal for salt. Most of the salt is squeezed out of the ice structure and the resulting ice is less salty than when it began to freeze. In the polar regions, where seawater freezes to form sea ice, the ice is not as salty as the seawater from which it formed. Sea ice, which is different from icebergs, looks like flat ice “plates” floating at the surface in polar seas. In the Antarctic Ocean, most (perhaps 90%) of the sea ice forms anew each winter and melts the next summer, and it is seldom more than about one meter thick. In the Arctic Ocean, however, sea ice often does not melt each summer, but instead a new layer of ice is added each winter, producing multiyear sea ice that can become many meters thick. Icebergs, on the other hand, are formed on land, and thus glaciers are formed entirely of fresh water. Most icebergs are pieces of glaciers, which have broken off and floated away when the glaciers reached the sea. They are jagged chunks of ice that can take many shapes. Most of an iceberg is below the surface of the sea.