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In ice at 0°C the water molecules (H₂ O) are systematically arranged in a hexagonal crystal structure. The hydrogen atom in one molecule bonding to Oxygen atom in the neighboring molecule is relatively weak. So the molecules are relatively spaced.

When ice is heated from 0°C to 4°C, the molecules get some heat energy and kinetic energy. So they get some mobility. Around 4°C, the hydrogen bonds with oxygen atoms in the neighboring molecule are relatively stronger. The molecules come nearer and orient themselves in different directions than earlier.  So the packing of molecules is denser. The total volume of water molecules decrease.

The strange phenomenon of increasing density on melting is due to the Hydrogen bonds between neighboring molecules.