Why does ice float?
Ice floats because water is one of the very few substances that expands when it freezes — about 9% — so a given mass of ice takes up more space than the same mass of liquid water and is therefore less dense.
The short answer
Density is mass divided by volume. Freeze water and the mass stays the same while the volume grows by roughly 9%, so ice is about 9% less dense than the water around it and floats with about a tenth of itself above the surface.
Why water expands when almost nothing else does
Most liquids pack more tightly as they cool because the molecules slow down and settle closer together. Water does that too, right up until about 4 °C. Below that, hydrogen bonding starts winning: each water molecule pulls three others into a fixed tetrahedral arrangement, and that arrangement has gaps in it. The crystal is a lattice with holes, and the holes are why it is bigger.
The 4 °C detail that surprises people
Water is at its densest at about 4 °C, not at 0 °C. That is why a deep lake in winter is 4 °C at the bottom and colder above it — the densest water sinks, and the coldest water is not the densest.
Why it matters more than it sounds
Because ice floats, a freezing lake forms a lid. The lid insulates what is beneath it and the water below stays liquid. If ice sank, lakes would freeze from the bottom up, and shallow bodies of water would freeze solid every winter — which would have made freshwater life as it exists impossible.
What this does not explain
It does not explain why ice is slippery. That is a separate and genuinely unsettled question involving a thin surface layer whose behaviour is still argued about, and the old explanation about pressure melting is now known to be mostly wrong at ordinary temperatures.
Why a frozen pipe bursts
The same 9% expansion, in a container that cannot give. Water freezes from the pipe walls inward and traps liquid in the middle; as that pocket freezes it has nowhere to expand to, and the pressure rises until the pipe fails. The split often appears somewhere other than the ice plug, because the pressure travels through the trapped water — which is why the burst and the blockage are frequently in different rooms.
Sources
NIST — Thermophysical properties of water; Petrenko & Whitworth, Physics of Ice (Oxford). Every claim above is drawn from these; where a claim is contested or unsettled the article says so rather than picking a side.
How this article was made
Written by a person. Reviewed by Editorial review, Leumas on 2026-08-30. 401 words.
Questions
Why does ice float?
Ice floats because water is one of the very few substances that expands when it freezes — about 9% — so a given mass of ice takes up more space than the same mass of liquid water and is therefore less dense.
How do I know this is right?
The sources are listed on the page and a person reviewed it on 2026-08-30. Where a question is genuinely open, the article says so instead of resolving it.