You learned a remarkable property of H2O back in High School chemistry. Remember?
Normally, the density of compounds decreases as temperatures increase and molecules spread out. When temperatures fall, density increases as molecules become more tightly packed. Not true for ice – in fact, the exact opposite occurs!
In liquid form, each water molecule’s hydrogen is bonded to 3 other water molecules. In ice form, each molecule’s hydrogen bonded to 4 others. These hydrogen bonds form an open arrangement that is less compact than liquid water.