Why does ice float in water?Ice has more density than water.

A simple property, but unique and very important for the
environment, is that in solid form of ice,it floats on the liquid water.The solid form
of water has a lower density than liquid water, because the geometry of the hydrogen
bridges  which are forming only at lower temperatures.

For
almost all substances and all other 11 unusual  forms of water,except ice-XI, the
solid state is denser than the liquid one. Fresh water is most dense at 4 ° C,
and it will sink by convection as it will cool to that temperature and if it's cooled
further, it will rise.

Because of this
property,depth water will be warmer than  surface ice water, so the ice
will be formed from the surface and it will extend down and most of the water underneath
will remain constant at 4 ° C. Thus, the bottom of a lake, sea or ocean is virtually
isolated from the cold, allowing the survival of animal
species.

Although this behavior may seem trivial, it should
be noted that almost all other chemicals are denser in solid state and they are freezing
from bottom to top.