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Why do some matter expand while others contract when they change their state from solid to liquid and vice versa?

For example, ice, brass, bismuth, antimony etc. expand when they turn from liquid to solid(solidification) and contract or shrink when turn from solid to liquid(fusion)

On the other hand, paraffin wax, nickel etc. shrink during solidification and expand during fusion.

What does this change in volume depend on? (And I also read that, most matters are of the second kind, meaning they shrink during solidification and expand during fusion. Why are most matter like this?)

ACRafi
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1 Answers1

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Jon Custer should have posted his comment (+1 from niels) as an answer, so I will not claim credit here but just enlarge upon it a bit.

When a bunch of atoms or molecules solidify into a crystalline solid with long-range order, the particular form of that resulting crystal lattice is going to depend sensitively upon the structure of the outermost electron orbitals in those atoms or molecules. For metals, this will yield hexagonal or face-centered cubic or body-centered cubic, etc. crystal structures, each of which will have a slightly different packing density (in terms of atoms per unit cell) at the end of the day when the atoms are all schnuggled up against one another to minimize the free energy.

So it is possible for the solid to be less dense than the liquid or more dense, depending on how those orbitals settle in with their neighbors during solidification.

niels nielsen
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