Why Does Ice Float ?
A substance will float in water or other liquids if it is less dense, or has less mass per unit volume.An unusual property of ice is that the solid is approximately 9% less dense than liquid water. Ice is the only known non-metallic substance to expand when it freezes. This interesting phenomenon is what causes bottled water to explode when frozen.
One very important consequence of this to life on earth is that lakes and rivers freeze from top to bottom, allowing fish to survive and rivers to flow even when the surface of a lake has frozen over. If ice sank, the water would be displaced to the top and exposed to the colder temperature, forcing rivers and lakes to fill with ice and freeze solid. This would dramatically disrupt marine life and water supplies to humans.
It also causes ice bergs to float, which may not have been so great for the Titanic! As these pictures show, ice is 9% less dense than water, so only 9% of the ice berg will be above the surface at any one time!