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.
Water is most dense at 4°C and becomes less dense as its molecules arrange into hexagonalicecrystals. Ice is 9% less dense than water and that’s why ice floats!
That’s a darn good thing because otherwise rivers, ponds and lakes would freeze - some permanently - from the bottom up with a resulting loss of aquatic plant and animal life. Thin, sheets of surface ice allow sunlight to penetrate while protecting life under ice from weather extremes.
The same expanded molecular matrix that allows ice to float occupies more volume than liquid water. Meteorologists sometimes call sub-zero temperatures "pipe cracking cold" when household plumbing bursts due to the volume expansion when water freezes.
Hopefully, you won't experience this property of ice yourself! Preferably, you’ll experience ice expansion as a loud "pop" of freezing sapwood in a winter birch forest or the deep resonant boom and cracking of expanding lake ice on a clear, cold, January night!
