Why does ice float on water?
Freezing forces water molecules into an open framework with gaps in it, so the same molecules take up about a tenth more room as ice than as liquid.
Simple intuition
The plain reason, in everyday words
Most things shrink when they freeze, because cooling slows the molecules down and they settle closer together. Water does the opposite, and the reason is that its molecules do not simply pack — they hold hands. Each water molecule has a slightly positive end and a slightly negative end, and it attaches to its neighbours in specific directions rather than in whatever arrangement is tightest. In liquid water those attachments constantly break and re-form, so molecules can slip past one another and fill the gaps. When it freezes, every molecule takes up its allotted position holding four neighbours at fixed angles, and that arrangement has open space built into it, like a scaffold rather than a stack of bricks. The same molecules now occupy about a tenth more volume, so ice is lighter for its size and floats.
Ice floats because it contains trapped air bubbles.
Bubble-free ice made from degassed water still floats. The density difference comes from the spacing of the molecules themselves, not from anything trapped between them.
Every substance expands when it freezes.
Almost none do. Water is the well-known exception, along with a handful of others such as silicon, gallium and bismuth, which share the feature of forming open, directionally bonded solids.
Water is densest at zero degrees.
It is densest at about 4 degrees Celsius. Below that the open ice-like structure starts assembling and the density falls again, which is what drives the seasonal mixing of lakes.
Ice skates work because pressure melts the ice.
The pressure from a blade lowers the melting point by only a fraction of a degree — nowhere near enough. Frictional heating and a naturally disordered surface layer are the accepted explanations.
It is the clearest case of a molecule's shape and bonding having consequences you can see from a boat. The directional hydrogen bond that makes ice float is the same feature that gives water its high boiling point, its high heat capacity and its ability to dissolve so much — the properties that make it work as the medium for life. And the floating itself is not a minor detail: it is why bodies of water do not freeze solid from the bottom.
Who worked it out
That ice floats was never in doubt, but why became answerable only once molecules could be located. The Braggs' development of X-ray crystallography from 1913 opened the door.
What problem forced it
Linus Pauling's work on the chemical bond in the 1930s identified the hydrogen bond as the organising feature of water's structure and explained the tetrahedral coordination.
How it changed since
Later work mapped water's remarkably complex phase diagram, revealing many high-pressure ices, most of which are denser than liquid water — showing that the familiar floating behaviour is specific to the form that happens to exist at ordinary pressure.
Why lakes turn over twice a year
The density maximum at 4 degrees drives a seasonal mixing cycle that controls oxygen and nutrients in the whole lake.
What else hydrogen bonding explains about water
The same bond accounts for its boiling point, its heat capacity and its behaviour as a solvent.
Written for Curio rather than collected from a forum — it is part of the curated corpus that ships with the platform. The references it draws on are listed under Sources.