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Can someone explain me why is ice less dense than water?

As I know, all solids are usually denser than the liquids (correct me if I am wrong).

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Due to the crystal structure of the solid phase of water, the molecules arrange themselves in a rigid, ordered fashion and end up being, on average, farther apart from each other (than they are in the liquid phase), and thus less dense. Less dense things float because of buoyancy.

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I don't think that this question is still fully resolved, water is a fascinating molecule! But here are some thoughts.

Clearly, if ice is lighter than liquid water it is because it doesn't pack as well. Its an example of how a random-ish packing can be more efficient than an (non-closed packed) ordered packing of a "weirdly" shaped molecule. Imagine throwing LEGOs into a box, vs placing them very neatly, and orderly, two inches apart.

Why is this? To solidify the water should enter into a crystalline phase, but that requires accommodating the weird bonding angle of water (which is not quite tetragonal, which is easily packed. See C, Si, ect) in a way that can be made infinitely periodic, as compact as possible, and still satisfies the energy requirements. Largely due to this weird bonding angle, the resulting packing needs volume.

By contrast, in the liquid phase, the molecules do not have to be in specific sites. Rather, there is an attempt to maximize the hydrogen bonds. This leads (for example) to water chains. Meanwhile the thermal energy tries to scramble everything up, which breaks these chains up (average length of 7, if I recall).

Overall effect is that the water molecules in the liquid can put themselves into tight spaces, and stay long enough to make a difference, they can't in the ice phase.

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    $\begingroup$ I up-voted this because it's the only answer to mentions hydrogen bonding and the H-O-H angle of water. The other answers read like tautologies to me. They pretty much say the density of ice is lower because the molecules pack less densely. They don't explain the mechanism for why the molecules pack less densely. $\endgroup$ – Syntax Junkie Sep 16 '19 at 19:31
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ice is less denser than water because in ice the molecules arrange themselves in a rigid tetrahedral structure due to which cage like spaces remain in their bonding. But water molecules remain in linear bonding form. As the volume of ice becomes greater, it is less denser.

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ice is a crystal pure made up of water after frozen on a certain temperature,as it is lighter than water hence its density is less than water............this can be taken as in simple words

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    $\begingroup$ This does not really answer the question, since the question already implies that the density of ice would be lower that that of water, but your answer does not explain why the crystal form of ice would have a lower density. $\endgroup$ – fibonatic Dec 3 '14 at 13:27
  • $\begingroup$ This does not fully answer the question. The assumed question behind the question of the OP is why solid water is less dense than liquid water, which is not true for most substances. $\endgroup$ – Sean Dec 3 '14 at 14:24
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    $\begingroup$ Just stating for the record that this does not qualify for deletion as a non-answer. $\endgroup$ – David Z Dec 3 '14 at 14:41

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