It is said that when water is frozen, it changes to ice,while the density is the same...So i wanna ask this question.I have with me,a glass of water,with ice cubes in it...The question is "Why do ice cubes float on water" An d we know that ice and water have the same density

A block of ice doesn't have the same density as liquid water.

Water reaches its minimum density at approximately four degrees centrigrade. As the temperature falls below this the density rises. There is a slight volume increase upon freezing. This accounts for the bursting of uninsulated water pipes in cold climates.

Ice floats 'on' water because of gravitation. :)

As you know water is made up of H₂O, both Hydrogen and Oxygen are of course gases at room temperature, only when in the molecule for water are they actually liquid unless adversely frozen individually.

Oxygen's boiling point is -183 ºC, Hydrogens boiling point is -252.87 ºC. The boiling point refers to when the gas in a liquid form turns to the gas form we all know of. Water's boiling point is 100 ºC

I mention these as an inquisative mind might ask why Water is a liquid at the temperature it's at normally, when the atoms that make it up would normally be a gas. Simply the density and entanglement of the molecules generate waters apparent look and feel, where the gases frozen individually to liquidform is using the inert nature of freezing to increase the atomic density and appear as a liquid.

The above is just how Liquid is, however when these forms (either elements or molecular substance) are frozen down further, their density increases further, generating the Solid state.

Again due to water being Molecular it takes less freezing to produce a solid state (0 ºC), where as Oxygen (-218.4 º C) and Hydrogen (-259.14 º C ) take yet further freezing on their own.

Why does ice float?
It's very simple the molecules do not compact like frozen hydrogen would do, this gives reason as to why ice has a greater volume while also having an "inert" density. It's floatation is caused by it's displacement of volume which more information can be found by looking into Archimedes (http://en.wikipedia.org/wiki/Archimedes).

Spurious is right to suggest "Gravity", as while gases are in an ertia state, their atoms bounce/collide off one another defying gravity, when reduced to a liquid state the speeds are restricted and reduced yet further (to stop/nearstop) as a solid state. Without "Gravity", freezing gases to generate liquid or solids wouldn't necessarily constitute density.

What constitutes density can be suggested by experiments and observations of space. In space it's known that a large mass will enduce smaller masses to be drawn to it, the greater this mass becomes the higher the density would become. (The greater the Gravity)

(My overall post probably won't help you one bit with understanding, however hopefully you can pull some key pieces from it to do a search with for more information, heck if you find anything wrong with what I suggest reply back with your findings :) .)

Ophiolite:

Water reaches its MAXIMUM density at about 4 degrees C. Hence, colder waters sink, warmer waters rise. Below about 4 degrees (actually, I believe it's closer to 3 degrees, but will have to look it up again), water begins expanding, leading to a lesser density, and it will float atop water at about 4 degrees. Thus, when it gets to zero, the coldest water is at the surface, which is the water that then freezes, and the expansion continues, with solid water at a density of about 0.9 compared to 1.0 for liquid water.

Water is just about the only important chemical compound with this property. Virtually every other material is denser in its solid state than its liquid state.

It is this unique property that makes aquatic life possible in cold climates. In winter, when bodies of water begin to freeze, the frozen water floats to the top instead of sinking to the bottom. Eventually there is enough ice at the top to create a complete cover. This insulates the water underneath the ice sheet from the frigid air above it, so it remains at above-freezing temperature. As a result, the rate of new freezing drops precipitously once the body of water is completely covered by ice. The ice sheet does not become thicker very fast so most of the water underneath it remains liquid all winter long, allowing the aquatic lifeforms to continue moving and eating instead of being frozen into fish-cicles.

If water behaved like all other compounds, many lakes and rivers in subarctic regions and even in the colder temperate regions like the Great Lakes would freeze solid every winter, destroying all life in them. The same would happen to the polar oceans where the temperature is always below freezing. Over the years the ice would continue sinking to the bottom until it piled up so high that it reached the surface and the water stopped flowing. There would be nothing for the polar bears and penguins to eat, not to mention what this double blockage would do to the world's ocean currents.

It's no coincidence that virtually all life on earth is built around a chemistry and physics in which water is the vital component. Life had to start out in a liquid medium in order to provide the most primitive organisms with mobility. Our bodies still carry the ocean inside us. Our blood is salty because it provides our cells with the oceanic environment that they can never forget and still require.

When we start discovering other worlds that host life, it's difficult to predict whether that life will be built around DNA like ours, or if it will even be carbon-based at all. Extraterrestrial life based on silicon, which is chemically very similar to carbon, is a sci-fi cliche. But it is very likely that most of the life in the universe will be fundamentally water-based, like ours. There is nothing else anywhere in the universe like water because the laws of physics are universal.

Things "float" because of density gradients due to gravity, right? So in zero g (or microgravity on orbit) an ice cube would wander around in a spherical blob of water. Si?

Hmm...
What effect would the surface tension have? Obviously the surface tension acts to minimise the surface area of the blob, but is it sufficient to expel the ice cube, or are there other factors here I'm missing?

Hmm...
What effect would the surface tension have? Obviously the surface tension acts to minimise the surface area of the blob, but is it sufficient to expel the ice cube, or are there other factors here I'm missing?

I think if you push the cube through the surface, there are no density gradients, and therefore no preferred direction for the cube to go. The surface tension might actually act to hold the cube inside...

True. But I'd have thought that once the cube broke the surface, the water would push the cube out. Or do oyu mean that the surface tension could act to prevent the ice cube breaking the surface?

True. But I'd have thought that once the cube broke the surface, the water would push the cube out. Or do oyu mean that the surface tension could act to prevent the ice cube breaking the surface?
Yes. I can see the surface tension acting like a weak membrane, keeping the cube from breaking the surface at all.

I guess it would depend on how water behaves at the surface of the ice cube, and also the curvature of the water sphere in relation to the ice cube.

I guess it would depend on how water behaves at the surface of the ice cube, and also the curvature of the water sphere in relation to the ice cube.
True. And I really don't know how surface tension behaves from underneath (inside). Is it somehow biased at the air-water interface? Interesting.

bracess:

Just to be clear, the density of ice is less than the density of water. Less dense objects float in more dense fluids, so ice cubes float in water.

For the same reason, a helium balloon floats in air because helium is less dense than air.

thanks to all who reply...but most of the long post i do not understand...I onli understand the water at 4 degree.thanks anyway

People have danced around this, but I don't think anyone has actually stated it outright.

Water's density decreases when it gets cold enough because the water molecules form a hexagonal network that's held together by hydrogen bonds at low temperatures. The spacing between the water molecules in the network is actually larger than the spacing between the molecules in liquid water. The hydrogen bonds that hold the network together are very weak, and unless the temperature is very low the thermal motion of the water molecules will prevent the network from forming. Once the temperature gets low enough (around 4 C), the molecules begin to order and therefore spread apart slightly. As the temperature gets lower they order more and more until eventually the water goes through a phase change and freezes.

It is actually possible to make dense ice that will sink by freezing water extremely quickly, so that it doesn't have time to organize into a network before it undergoes a phase change.

Nasor:

Your last comment is interesting. Any references?

http://www.lsbu.ac.uk/water/amorph.html
It intrigued me as well, so here's the link I found.
Also, as mentioned in the article, you can supercool water to ~ -42C without it freezing, if you're careful. However, if you then disturb the water, it will freeze almost instantly. Apparently, this method is used by some insects to survive winter:
http://www.mountainnature.com/Articles/winter_insects.htm

It is actually possible to make dense ice that will sink by freezing water extremely quickly, so that it doesn't have time to organize into a network before it undergoes a phase change.

Just to add to this: there are approximately 7 or 8 (I forget) distinct arrangements of water molecules in ice, meaning there are 7 or 8 different types of ice. These are usually labelled as Ice I, Ice II, Ice III, Ice IV etc. Some types form only under rather odd conditions of temperature and pressure, and so are quite uncommon.