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Drop an ice cube into a glass of water and it floats. It seems too ordinary to notice, but it’s actually strange. For almost every other substance, the solid is denser than the liquid, so solid chunks sink in their own melt. Solid wax sinks in molten wax; a lump of solid iron sinks in liquid iron. Water is one of very few exceptions — and it’s an exception with enormous consequences for life on Earth. This article explains why, using the chemistry of hydrogen bonding.
The numbers
| State of water | Density (g cm⁻³, approx.) |
|---|---|
| Ice at 0 °C | 0.917 |
| Liquid water at 0 °C | 0.9998 |
| Liquid water at 4 °C | 1.0000 (maximum) |
| Liquid water at 20 °C | 0.998 |
| Liquid water at 100 °C | 0.958 |
Ice is about 9 % less dense than liquid water at the same temperature. That’s why about 90 % of an iceberg sits below the waterline and only about 10 % shows above it.
It also means water expands by about 9 % when it freezes. Anyone who has left a full glass bottle of water in a freezer has seen what that does.
Why most solids are denser
In most substances, particles in the solid are packed as closely as possible in a regular arrangement. When the solid melts, particles gain energy and move around, taking up a little more space on average. So the liquid is slightly less dense than the solid.
For water, the arrangement in the solid is not the closest possible packing. Something forces the molecules apart when they freeze. That something is hydrogen bonding.
Water’s hydrogen bonds
A water molecule has two δ+ hydrogen atoms and two lone pairs on its δ− oxygen atom (see the shape of a water molecule). Each hydrogen can form a hydrogen bond to a lone pair on a neighbouring molecule, and each lone pair can accept one. So each molecule can form up to four hydrogen bonds, pointing roughly towards the corners of a tetrahedron (see hydrogen bonding).
Hydrogen bonds are directional: they’re strongest when the O–H···O atoms line up in a nearly straight line. This is the key to understanding ice.
The structure of ice: an open lattice
When water freezes, molecules slow down enough for hydrogen bonds to lock them into a regular arrangement. In ordinary ice (called ice Ih, “hexagonal ice”):
- Every water molecule forms four hydrogen bonds.
- Each molecule sits at the centre of a tetrahedron of four neighbours.
- The molecules link into puckered hexagonal rings, stacked in layers — the same hexagonal symmetry you see in the six-fold shape of snowflakes.
To keep all the hydrogen bonds straight and all the molecules in tetrahedral positions, the structure has to be open, with a lot of empty space inside the hexagonal channels. The molecules are held further apart on average than they are in the liquid.
A useful comparison: think of people holding hands in a large ring dance — everyone at arm’s length. That arrangement takes more floor space than the same people standing in a crowd.
What happens when ice melts
When ice melts, about 10–15 % of the hydrogen bonds break (estimates vary), and the rigid lattice partly collapses. Molecules can then move into the empty spaces, packing more closely than in the ice. The liquid is therefore denser than the solid.
Liquid water still contains many hydrogen bonds — they’re constantly breaking and re-forming in a flickering, shifting network — but it lacks the long-range order of ice.
The strange maximum at 4 °C
As liquid water warms from 0 °C, two effects compete:
- More hydrogen bonds break, and leftover ice-like clusters collapse → molecules pack closer → density increases.
- Molecules move faster and take up more room (normal thermal expansion) → density decreases.
Between 0 °C and 4 °C, the first effect wins, so water gets denser as it warms. Above 4 °C, thermal expansion wins, and water behaves normally, getting less dense as it warms. So liquid water has its maximum density at about 4 °C.
Why it matters for life
Lakes freeze from the top down
In winter, as a lake cools, the surface water cools first. Down to 4 °C, cooling water becomes denser and sinks, mixing the lake. Once the whole lake reaches about 4 °C, further cooling of the surface makes it less dense, so the cold water stays on top. The surface freezes, and because ice floats, it forms an insulating layer.
The water beneath stays liquid — often around 4 °C at the bottom — so fish, plants and other organisms can survive the winter. If ice sank, lakes would freeze from the bottom up, and many would freeze solid, killing much of the life in them.
Climate
Floating sea ice reflects sunlight back into space, helping regulate Earth’s temperature. If ice sank, polar oceans would behave completely differently.
Weathering and damage
Water seeping into cracks in rock expands by about 9 % when it freezes, prying the rock apart — a process called freeze–thaw weathering, which breaks down mountains and creates scree slopes. The same process damages roads (causing potholes), and freezing water can burst pipes in cold weather.
Biology
Freezing can damage living cells, because ice crystals can pierce membranes as water expands. Some fish, insects and plants produce antifreeze proteins that bind to tiny ice crystals and stop them growing. In cryopreservation, chemicals such as glycerol are added to cells to prevent damaging ice formation.
Is water unique?
Water is the most important example, but not the only substance that expands on freezing. Silicon, gallium, germanium, bismuth and antimony are also less dense as solids than as liquids. In these, the solid has an open structure due to directional bonding, similar in principle to ice. Gallium famously melts in your hand (at about 30 °C), and its expansion on freezing means it can crack glass containers.
Ice under pressure
Ordinary ice isn’t the only form. Under high pressures, water forms more than a dozen other crystal structures (ice II, ice III and so on up to ice XX and beyond), many of them denser than liquid water — pressure forces the molecules into more compact arrangements. Some of these exotic forms may exist deep inside icy moons such as Europa and Ganymede.
Common misconceptions
- “Ice floats because it contains air bubbles.” Even perfectly clear, bubble-free ice floats — it’s the open lattice, not trapped air.
- “Water is densest at 0 °C.” Its maximum density is at about 4 °C.
- “Hydrogen bonds break when ice forms.” Ice has more complete hydrogen bonding (four per molecule) than liquid water.
- “All solids sink in their liquids.” Water, silicon, gallium and a few others are exceptions.
Key takeaways
- Ice (0.917 g cm⁻³) is about 9 % less dense than liquid water, so it floats.
- In ice, each molecule forms four directional hydrogen bonds in a tetrahedral, hexagonal lattice with lots of empty space.
- Melting breaks some hydrogen bonds; the lattice partly collapses and molecules pack closer.
- Liquid water is densest at about 4 °C.
- Floating ice insulates lakes and seas, allowing aquatic life to survive winter.
For more on water’s unusual properties, see why water has such a high boiling point.
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