Ice floats on water because water is less dense as a solid than as a liquid, due to its unique hydrogen-bonded structure.

Quick Scoop: What’s Going On?

Most solids sink in their own liquid because the solid form is denser (its particles are packed more tightly).
Water is weird: when it freezes, its molecules lock into an open, lattice-like structure that actually takes up more space than the same amount of liquid water. That makes ice less dense than liquid water, so it rises to the surface instead of sinking. So, the property that explains this is:

Water’s unusual density behavior caused by hydrogen bonding, which makes solid water (ice) less dense than liquid water.

In many school answers, this is often summed up as:

  • “Water has lower density in the solid state than in the liquid state because of hydrogen bonding.”