Density of Water in Relation to Temperature
On cooling, water molecules come closer which makes water dense. It is most dense at 4°C (Figure shown below) but it is still a liquid and water molecules move freely. However, when temperature drops below 4°C the movement of water molecules ceases and the bonds between molecules become relatively less rigid and more open, this traps the air molecules in pockets fonned between water moieties which make the ice lighter than water. It is for this reason that ice floats above the liquid water. Water always freezes in a top down manner. Formation of an ice sheet on the surface of a water body acts as an insulator and prevents freezing of the water below. This is important for the survival of aquatic organisms in areas where surface i water freezes during the winter period.