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Or, to ask this another way - "Why does water expand when it freezes?" For something to float, its density must be less than that of the environment. Helium, having less mass per volume than air, floats. Ice, although it is a solid, is actually less dense than water. Water is one of the few liquids that has this particular property. This is due to the fact that water spontaneously forms ordered crystals when frozen.

First, lets mention an important aspect of the physical nature of water. Water has two hydrogens and one oxygen. Oxygen is a great donor and acceptor for forming hydrogen bonds. Hydrogen bonds are formed by sharing one hydrogen between two oxygens: O...H-O. This bond is strong enough to affect the physical properties of water. In the liquid state, many waters are involved in large transient networks of these hydrogen bonds. This is why water has such a high boiling point relative to heavier liquids. Gasoline is heavier on a per molecule basis, but it doesn't form extensive hydrogen bonds so it is more volatile.

These hydrogen bonds, which give the water strength also have an effect on the structure of ice. The hydrogen bonds between individual waters forms a crystal lattice in the solid state. This crystal is arranged in such a way that there is more space between water molecules than in the liquid state. As a result, the bulk water on a whole, increases in volume. The denisty is less, ergo the ice floats.

This is also why they say that ice skaters are not skating on ice, but a thin layer of water. By squeezing the ice with the skate blade, you are forcing the water molecules closer than they like to be in ice, causing it to melt. Of course, once the skate passes, the water quickly expands and refreezes. I suspect, although I haven't tested this, that water which has been flash-frozen (nearly instantaneously) will not expand as much, because ice hasn't had the chance to form ordered crystals.

This node ignores the second half of the issue which is the origin of buoyancy. Why does something lighter float in something heavier. See the buoancy node for a description of that phenomenon.

One important thing about ice being lighter than liquid water is that this allows the oceans (and any other water masses such as lakes) to remain largely in liquid form.

If ice were denser than water, it would sink as it formed and a temperature gradient would exist from the ice at the bottom of the water to the surface of the water. Lakes would freeze from the bottom up and completely freeze over. Similarly the oceans might do that, if not for geothermal energy.

Instead we have the situation where water is densest at 4 degrees centigrade/celsius. As a body of water cools, the main avenue of heat loss is through the water surface. As it cools to 4 degrees centigrade, the temperature gradient is such that the warmest water is at the top, as the warmer water is less dense than the colder water below, facilitating heat loss. As the temperature of the water cools below 4 degrees, the densest water sticks to the bottom, with progressively colder water above it. No convection currents mix the water as the coldest part of the water is now at the surface. As the water at the surface freezes, it insulates the rest of the water below, slowing freezing down so that liquid water can exist at the bottom in the coldest winter, if the body of water is deep enough.

Why is this significant? Being frozen in ice is much more detrimental to life than cold water at 4 degrees Centigrade (the temperature at which water is densest). Lots of marine and freshwater life would perish every winter where the temperature dropped below the freezing point of water. Isn't the world we live in wonderful?

The fact that ice floats is also the reason why there is a lake in the middle of Antartica that is under more than 2 km of ice, that has been isolated from the rest of the world the thousands of years it took for the kilometres of ice to accumulate above it.

Urgh ... Some network glitch originally accidentally created this as a blank writeup from me. It got down to reputation: -3. :-(

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