Why Ice Floats on Water

Most substances become more tightly packed when they solidify, so their solid form is denser than their liquid form. Water is an important exception. An ice cube floats not because it has no weight, but because a given volume of ice has less mass than the same volume of liquid water.

Water molecules form hydrogen bonds with one another. In liquid water, these bonds continually break and reform, allowing the molecules to move and pack relatively closely. When ordinary hexagonal ice forms, hydrogen bonds hold the molecules in a more open crystal lattice with additional space between them. Water therefore expands by about nine per cent when it freezes, and its density falls. Because buoyancy equals the weight of displaced liquid, ice needs to submerge only about nine-tenths of its volume to displace its own weight in water. The rest remains above the surface.

This property matters far beyond a glass of iced water. Liquid water is densest at about 4°C and sinks, while colder water and ice remain near the surface. Lakes therefore freeze from the top. The floating ice then slows heat loss from the water below, leaving a liquid habitat for aquatic life. If ice sank like most solids, many lakes would behave very differently in winter.

References
U.S. Geological Survey, Water Density
https://www.usgs.gov/water-science-school/science/water-density
Royal Society of Chemistry, The Density of Ice
https://edu.rsc.org/experiments/the-density-of-ice/1776.article