Why water at 4°C is most dense ?
In ice, the water molecules are in a crystal lattice that has a lot of empty space.
When the ice melts to liquid water, the structure collapses and the density of the liquid increases.
It's because of two opposing forces: thermal kinetic expansion and H-bonding.
The thermal properties are not enough to break all the h-bonds apart, but the h-bonds have not formed enough to widen the distance between water molecules to be as great as in ice which is why ice is lighter than water.
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