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The Wikipedia says about water:

Water also differs from most liquids in that it becomes less dense as it freezes.

and

The details of the exact chemical nature of liquid water are not well understood; some theories suggest that the unusual behavior of water is due to the existence of 2 liquid states

The fact that a substance becomes less dense when it freezes is kind of self-contradictory to me. I mean when a liquid is becoming a solid then by default its molecules must be gathered together at a higher-level density, thus, making the substance denser and harder.

I don't quite get it how one thing can be harder and yet less dense at the same time.

Can my confusion here be easily explained in simple words or is it one of those things that are "not well understood"?

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  • $\begingroup$ Hey! What do you mean by “harder”? $\endgroup$ – user249212 Dec 28 '19 at 10:42
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    $\begingroup$ @LeibnizRib - I mean less soft, stiffer, impossible to be poured out, etc. $\endgroup$ – brilliant Dec 28 '19 at 10:52
  • $\begingroup$ There's some info about it here: chemistry.stackexchange.com/q/51278 which also mentions several other substances that expand when they solidify under standard pressure. $\endgroup$ – PM 2Ring Dec 28 '19 at 11:20
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    $\begingroup$ Does this answer your question? Why is ice less dense than water? Or the links given in the question? $\endgroup$ – BioPhysicist Dec 28 '19 at 12:23
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    $\begingroup$ ...when a liquid is becoming a solid then by default its molecules must be gathered together at a higher-level density... Not necessarily. Solid just means "more organized and more stationary particles". This doesn't need to include high density though. $\endgroup$ – BioPhysicist Dec 28 '19 at 12:26
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When some substance is suffering a phase transition, the particles are rearranging their “configuration”.

Our intuition says that when we have gas, particles are more separated from each other and when we have something solid they are really compacted.

In water something different happens, when freezing water change the configuration of the particles that tend to occupy more volume, so there is much more “empty” space and so the density of ice is less than water.

I recommend you to visit this site: http://scienceline.ucsb.edu/getkey.php?key=4169

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