Question

Why is density of water maximum at 4 degree celsius?

Answer 1

I don't think there is a "simple explanation" as this is actually quite a complex phenomenon. The vast majority of solids sink in their molten counterparts. Water (ice) floats and is very unusual in this - silicon, germanium and one of the allot ropes of tin do it too, but those are the only ones I know. In those non-aqueous substances, hydrogen bonding obviously plays no part as there's no hydrogen!

It's down to the way the V-shape of the water molecule packs together as it freezes to make crystalline ice - the molecules are (on average) a little further away from each other than in the liquid state, so it's less dense and hence floats. 

As a terrible analogy, think of a Lego model. The assembled model is bigger than the box it came in because of the enforced structure. You can, of course, pack the bricks together tightly to made a solid lump which WILL go back in the box but if you stick to the plan, it won't. For most substances, the "plan" (minimum energy crystalline structure) is nearer to "solid brick" than "Millennium Falcon" and the solid sinks. For a few, like water, the "plan" gives us a floating solid and, in the case of water/ice, lakes which don't freeze all the way to the bottom in winter!

Answer 2

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I don't think there is a "simple explanation" as this is actually quite a complex phenomenon. The vast majority of solids sink in their molten counterparts. Water (ice) floats and is very unusual in this - silicon, germanium and one of the allot ropes of tin do it too, but those are the only ones I know. In those non-aqueous substances, hydrogen bonding obviously plays no part as there's no hydrogen!

It's down to the way the V-shape of the water molecule packs together as it freezes to make crystalline ice - the molecules are (on average) a little further away from each other than in the liquid state, so it's less dense and hence floats. 

As a terrible analogy, think of a Lego model. The assembled model is bigger than the box it came in because of the enforced structure. You can, of course, pack the bricks together tightly to made a solid lump which WILL go back in the box but if you stick to the plan, it won't. For most substances, the "plan" (minimum energy crystalline structure) is nearer to "solid brick" than "Millennium Falcon" and the solid sinks. For a few, like water, the "plan" gives us a floating solid and, in the case of water/ice, lakes which don't freeze all the way to the bottom in winter!