Why does ice float and cold water sink?

Because temperature affects density. The molecular structure of the ice causes it to be less dense than liquid water, so it floats.

Water is one of very few compounds that is less dense as a solid than it is as a liquid. Less dense things float, so ice floats.

EDIT: Ah, ninja’d.

In general, as temperature decreases, density increases. A denser material will sink in a material of lower density. Water is somewhat atypical in that it is a polar molecule. It’s polarity makes water less dense as a solid than as a liquid, so it floats in water.

Edit: Double ninja’d

When water freezes, it adopts a crystalline structure that is less dense than liquid water (molecules are spaced further apart than they are in liquid form). Therefore ice floats. Also, cold water is denser than hot water. Molecules move around a lot faster in hot water thereby decreasing the density with respect to cold water. See this for a visual comparison of water and ice on the molecular level.

ice has expanded—less dense

@lilikoi oops, leapt b4 i lookd

This property of water makes it a very unusual substance. Almost all compounds are denser as solids than liquids. Water molecules, however, as they lose kinetic energy through cooling form ice crystals in which the molecules link up to contain more empty space than what’s typically seen among molecules of liquid water.

The only other substance I know of with this solid-floats-on-liquid property is the metal bismuth.

That water floats has important implications for life. Bodies of water form an ice crust that helps keep the remainder of the volume liquid. Any other kind of liquid would crystallize from the bottom up, eventually freezing totally solid every time air temperature fell. Living organisms must have liquid water to survive.

Because penguins need something to stand on…but sometimes a piece has to break off because God loves the Killer Whales too. :)

Water expanding as Ice is why pipes burst in cold spells, and why it is hard to get ice cubes out of the trays.

Ice is one of the few materials that does expand when cooling. Most materials expand when they are heated!

Thank you all you’ve been really helpful