Why does ice float on water?
The arrangement of water molecules changes when the liquid becomes an ordinary ice crystal.
The unusual behaviour of water
Most substances become denser when they solidify. Water behaves differently near its freezing point. As ordinary ice forms, hydrogen bonds help arrange molecules into an open crystal structure. That structure occupies more space for roughly the same mass, so the resulting solid has a lower density than the liquid.
A floating protective lid
Because ice is less dense, it rises to the surface of lakes and ponds. A surface layer of ice can then slow heat loss from the water below. This helps many aquatic ecosystems persist through winter even when the air is very cold. If ice routinely sank, freezing conditions would affect bodies of water in profoundly different ways.
Why frozen pipes can burst
The same expansion can create pressure in confined spaces. When water freezes inside a pipe or bottle, its increased volume may strain the container. This familiar winter problem is a practical consequence of molecular geometry. A small change in how neighbouring molecules are arranged can have enormous effects at the scale of lakes, buildings and living organisms.
External references are provided for exploration; Curio Hour’s explanation is written separately.