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Anomalous expansion of water is because the maximum density of water is attained at 4C.  That means this is the point where we can find the highest mass of the highest number of melecules per unit of volume. Liquid water contains by far the densest hydrogen bonding of any solvent with almost as many hydrogen bonds than covalent bonds. This is most probably because the energy of water molecules are relatively low while hydrogen bonding is still very strong compacting water molecules together. Lowering the temperatures have water molecules rearranging less frequently.

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