Ice is less dense than liquid water because, when water freezes, its molecules lock into an open, hexagonal crystal structure that spreads them farther apart, so the same mass takes up more volume and the density decreases.

In liquid water, the molecules are constantly moving, and hydrogen bonds are continually breaking and reforming, allowing molecules to pack relatively close together. As water cools and freezes, hydrogen bonds “freeze” into a rigid lattice that maximizes bonding but creates large empty spaces between molecules, like a molecular honeycomb. Because density is mass divided by volume, those extra spaces mean fewer molecules in the same volume, so ice is less dense and floats on liquid water.