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In the Feynman Lectures, Richard Feynman makes the following statement:

The particular crystal pattern of ice shown here has many “holes” in it, as does the true ice structure. When the organization breaks down, these holes can be occupied by molecules. Most simple substances, with the exception of water and type metal, expand upon melting, because the atoms are closely packed in the solid crystal and upon melting need more room to jiggle around, but an open structure collapses, as in the case of water.

It explains ice melting, but what does he refer to by saying "[...] these holes can be occupied by molecules. [...]"? Does it mean that when ice melts, the water molecules fill the empty spaces left by the atoms?

I am not sure if my question is properly formatted, if you need more information I can edit it. I would appreciate if someone explains this particular topic a bit deeper.

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    $\begingroup$ Yes, you interpret it correctly. $\endgroup$ – NickD May 7 '17 at 2:37
  • $\begingroup$ @Nick Could you expand on the topic? I interpreted it correctly but I am not sure to understand it completely, what does the fact that water molecules occupy space once they are in liquid state imply? Or yet, am I making a coherent question? $\endgroup$ – Pablo Ivan May 7 '17 at 2:52
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The water molecules in the ice form a ordered structure, but the average spacing is quite large. Imagine building a house of cards - the 52 cards occupy a large volume. If you "melt" the house of cards, the volume becomes smaller.

There are a handful of substances with a similar behavior - all with the same type of crystal structure.

See this answer for more details.

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