Why does ice float on water?
― Almost everything else sinks
Ice floating looks perfectly ordinary. But as far as matter goes, this is deeply strange. Most substances shrink and sink when they solidify. Only water gets bulkier when it freezes. Without this single exception, the lakes and oceans of Earth might freeze from the bottom up.
Drop an ice cube into a glass of water and it floats. Nobody finds that strange.
But melt some candle wax, then drop in a lump of solid wax. It sinks. Drop solid metal into the same metal melted down, and it usually sinks too.
The reason is simple. When something solidifies, its particles line up and pack together with no gaps. Packed tight, the same volume weighs more, so it's heavier than the liquid. So it sinks. That's the normal case.
Water doesn't play by that rule. When it freezes, its volume grows by about 9%. That's why a bottle of water left to freeze swells up tight.
A water molecule isn't straight but bent, and it has a habit of linking hands in four fixed directions. This fixed direction is the key to everything.
In ice, every molecule links hands in all four directions, forming a gap-riddled scaffold. As a liquid, molecules can break and remake bonds, letting them huddle together more tightly.
In short, water has the odd property that lining up actually takes up more room. Let's look at this step by step.
Why does ice have gaps?
A water molecule is one oxygen and two hydrogens, bent into a "V" shape. This shape creates an uneven distribution of electric charge, so the hydrogen of one molecule is drawn to the oxygen of another. It's like molecules holding hands.
What matters is that these "hands" have a preferred direction. They can't link up just anywhere — only at fixed angles does the connection work well. Each molecule can reach out in four directions.
Cool it down slowly, and the molecules stop moving, and all of them link hands in the direction that suits everyone best. The result is a scaffold like stacked tetrahedrons. And this scaffold is full of empty space.
In liquid water, by contrast, hands are constantly breaking and reforming. Without that fixed alignment, molecules can slip into the gaps and huddle more densely.
That's why water expands, rather than shrinks, when it freezes — that is, when it lines up. It's a rare substance where order costs you space.
Water swells when it lines up.
Normally, a liquid shrinks and gets heavier the colder it gets. But water is heaviest at 4°C, and cooling it further toward 0°C actually makes it lighter.
The explanation is that the groundwork for ice's scaffold starts forming even while the water is still liquid. The "packing tighter" effect and the "building a scaffold" effect pull against each other, and balance out at 4°C.
This property matters for lakes too. When the surface cools in winter, that water gets heavier and sinks, stirring the whole lake. But once it drops below 4°C it gets lighter, so it stops sinking. The colder water stays at the surface and freezes, while the bottom stays at 4°C.
If ice sank instead
Imagine it. In winter, ice forms at a lake's surface. If ice sank, it would drop to the bottom as fast as it formed.
The surface would then stay liquid indefinitely, exposed to cold air, and keep freezing over and over. Ice would pile up from the bottom, and eventually the whole lake would become a block of ice.
Even in summer, the ice at the bottom would get no sunlight and, with water above it, would be slow to warm. It might never fully melt.
In reality, because ice floats, a lid forms at the surface. Ice conducts heat poorly, so the water beneath cools slowly. That's why water remains at the bottom, letting fish and plants survive winter (bottom of Figure 1).
Without this property, aquatic life in cold regions might never have taken hold. The fact that the molecule happens to be bent is a precondition for whole ecosystems.
When ice already floating on the Arctic Ocean melts, sea level barely changes. Floating ice already displaces exactly its own weight in water. Once it melts, it fills exactly the volume it was displacing.
You can check this with a glass of ice water. Even after all the ice melts, the water doesn't overflow.
So where does sea-level rise actually come from? Ice sitting on land — the ice sheets of Greenland and Antarctica, mountain glaciers. These aren't floating in the ocean, so when they melt, that meltwater is added on top. It's essential to distinguish "floating ice" from "land ice" (the seawater itself expanding as it warms is also considered a major contributor).
The troublesome side of "fattening up" when frozen
This volume increase can turn into serious force. The pressure water exerts as it freezes is extremely strong.
- Water pipes burst ― water inside freezes, expands, and breaks the pipe from within. Leaving a trickle running on cold days is meant to stop it from freezing
- Rocks crack ― water seeps into cracks, freezes, then thaws, over and over, gradually shattering the rock. This is a major cause of mountain weathering and rockfall
- Roads develop potholes ― water under the pavement freezes and heaves it up, then sinks when it thaws. This is one reason roads deteriorate in cold regions
- Frozen food changes texture ― ice crystals grow inside cells and break them from the inside. Freezing quickly makes the crystals smaller, which is said to reduce the damage
The trick of salt lowering ice's temperature also comes from this same property of water.
Something you can check in your kitchen
- Fill a small container (like a plastic cup) with water right up to the brim
- Mark the water level with a permanent marker
- Freeze it overnight in the freezer
- The next morning, the surface of the ice will bulge above the mark. That's the extra volume
- Next, put water and ice in a glass, mark the water level, and leave it until the ice fully melts. The water level will barely change
The first half confirms that "volume increases on freezing," and the second confirms that "the water level doesn't change when floating ice melts." Never do this in a glass jar or a sealed container. It may crack.
Summary
Ice floats on water because water molecules are bent into a "V" shape and can only link hands in fixed directions. Lining up neatly creates a gap-riddled scaffold, making it lighter than the liquid. It's a rare substance where lining up costs you.
If the molecule had been straight,
lakes might freeze from the bottom up.
Once that ice grows into a large mass, it starts to "flow." That mechanism is explained in Why do glaciers flow like rivers even though ice is solid?
How much weight a sheet of ice on the surface can support (double the thickness supports four times the weight) is explained in How thick does ice on a frozen pond or lake need to be to hold a person?
Being heaviest at 4°C also drives major seasonal movement inside lakes. That mechanism is explained in Why do the top and bottom of a lake swap places in autumn?
Also, water doesn't necessarily freeze the instant it hits 0°C. Without a trigger for ice's scaffold to start forming, it can stay liquid below 0°C. That mystery is explained in I learned water freezes at 0°C — so why doesn't some water freeze even at -20°C?
Want to know more? ― Terms, numbers, and links to the textbookWe've labeled how advanced each part is, from middle-school science to open research questions
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Chemistry"
- High school+High-school "Chemistry," or advanced/sidebar content in textbooks
- UniversityNot covered in high school — university-level specialized content (physical chemistry)
- ResearchNot yet settled even at university level — questions researchers are actively investigating
Middle schoolTerms: words for water's mysteries
- Density: weight per unit volume. Ice is about 0.917 g/cm³, water (4°C) is 1.000 g/cm³.
- Hydrogen bond: the real identity of "linking hands" in this article. The attraction between a water molecule's hydrogen and another molecule's oxygen.
- Crystal: a state where particles are arranged in a regular pattern. Both ice and snowflakes are built from the same scaffold.
- Anomalies of water: the umbrella term for water's properties that are hard to explain compared with other substances. Ice floating is one of them.
- Frost weathering: rock breaking apart from the force of freezing water. Considered one of the main causes of mountain weathering.
High schoolWorking it out with a formula: how much of an iceberg is visible?
There's a saying, "the tip of the iceberg." So what fraction is that tip, exactly? If you know the densities, one division gets you there.
fraction submerged = density of ice ÷ density of liquid
| Density of ice | 0.917 g/cm³ |
| Density of water | 1.000 g/cm³ |
| Density of seawater | 1.025 g/cm³ |
A floating object settles at the point where it has displaced liquid equal to its own weight. So the submerged fraction is determined purely by the ratio of the two densities. Size and shape don't matter. An ice cube and an iceberg submerge by the same fraction.
| Fraction submerged in fresh water | 0.917 ÷ 1.000 = 0.917 (91.7%) |
| Fraction above the surface | (1 − 0.917) × 100 = 8.3% |
| Fraction submerged in seawater | 0.917 ÷ 1.025 ≈ 0.895 (89.5%) |
| Fraction above the surface, at sea | (1 − 0.895) × 100 ≈ 10.5% |
What you see is roughly a tenth. So "the tip of the iceberg" turns out to be an almost exact figure, not just a figure of speech.
Slightly more pokes above the surface in seawater, because salt makes seawater heavier. That single number, 0.917, gets you all of this.
Now flip the same number around. How much does volume grow when water becomes ice?
| Volume of 1 L of water once frozen | 1 ÷ 0.917 ≈ 1.09 L |
| Fraction of increase | (1.09 − 1) × 100 = 9% |
It swells by about 9%. That explains several things.
- Freezing a drink can crack its container. A capped bottle has nowhere for the expansion to go, so it bursts
- Water pipes burst in winter. The water inside has nowhere to grow that 9%, so it forces the metal apart
- Rocks crack and roads develop potholes. Water in a crevice freezes and pushes it wider. Repeated over and over, this reshapes the landscape
"Expanding on freezing" is only 9%. Yet it still breaks metal pipes. With nowhere to escape to, even a small volume change becomes this much force.
Water has one more odd property. Its density peaks not at 0°C, but at 4°C.
| Density of water at 4°C | 1.000 g/cm³ (maximum) |
| Density of water at 0°C | about 0.9999 g/cm³ |
| Density of ice at 0°C | 0.917 g/cm³ |
Cooled water sinks, but once it passes 4°C it gets lighter instead, so it stops sinking any further. As a result, a pond collects 4°C water at the bottom and freezes from the surface down.
If ice were a substance that sank, ice forming at the surface would drop to the bottom, and the pond would freeze from the bottom up. That number, 0.917, decides whether life there can survive the winter.
High schoolLooking at the numbers: just how light is it?
| Density of ice (0°C) | about 0.917 g/cm³ |
| Density of water (4°C) | 1.000 g/cm³ |
| Volume increase on freezing | 1 ÷ 0.917 ≈ 1.09 → about a 9% increase |
| Fraction above the surface, ice in fresh water | (1 − 0.917) × 100 = about 8% |
| In seawater (about 1.025) | (1 − 0.917 ÷ 1.025) × 100 ≈ about 11% |
※ These vary with temperature and impurities. Figures here are representative estimates.
The phrase "the tip of the iceberg" comes from this calculation. What you see is roughly a tenth of the whole; the rest lies below the surface.
High schoolHigh school+Why does having a "fixed direction" matter so much?
A water molecule is bent at about 104.5 degrees, with a negative charge on the oxygen side and a positive charge on the hydrogen side. Each molecule offers two hands via its hydrogens, and accepts two hands via the oxygen's lone electron pairs, giving it four bonding directions in total.
These four directions are most stable when pointing at the vertices of a tetrahedron. When every molecule tries to satisfy this arrangement, the inevitable result is a structure riddled with hexagonal tunnels. Snowflakes being hexagonal is this same scaffold showing itself on the outside.
The contrast is telling. The attractive force between most molecules (van der Waals force) is direction-independent. So when they solidify, they pack like spheres crammed in as tightly as possible, and density rises. Water sacrifices tight packing because its bonds have a preferred direction.
UniversityThe phase diagram slopes the wrong way
On a diagram plotting a substance's state against pressure and temperature (a phase diagram), water has a distinctive shape. The boundary line between solid and liquid slopes to the left. Most substances slope to the right.
This means that applying pressure melts ice. In the Clausius–Clapeyron relation dP/dT = ΔH / (T ΔV), the volume change on melting, ΔV, is negative. For ordinary substances it's positive, which flips the sign of the slope.
One caveat, though. The widely known claim that "ice skates glide because pressure melts the ice" doesn't hold up: calculation shows the melting-point drop from a skate blade's pressure is far too small to explain it on its own (this point is also discussed in the article on salting ice).
At even higher pressures, forms of ice with entirely different structures appear, distinct from the ice we know. These are numbered, and nearly 20 kinds have been reported so far. Some of them are denser than liquid water. "Ice floats" is a statement that only holds under surface conditions on Earth.
ResearchWater still isn't fully explained
- Water is said to have more than 70 "anomalies." Its density peaking at 4°C, its large heat capacity, its high surface tension, becoming easier to flow under pressure. No single theory has yet managed to explain all of these together. The most familiar liquid may also be the hardest one to understand.
- There's a hypothesis that water might be a mixture of two kinds of liquid. The idea is that a low-density state and a high-density state coexist, with the proportions shifting depending on conditions. If true, it's hoped this could explain many of the anomalies at once. But the temperature range where this needs testing (roughly -90°C to -40°C) is a region where water freezes almost instantly, making experiments extremely difficult — researchers have called it "no man's land." Recent ultrafast measurement techniques have produced results supporting the idea, but it remains unresolved.
- How far supercooled water can stay liquid is also unsettled. Under the right conditions, pure water is known to stay unfrozen down to around -38°C. But where the true limit lies, and what happens there, is an open question tied directly to the debate above.
- The search for new ice structures also continues. New forms of ice keep turning up under high pressure and unusual conditions, and the number of reported kinds keeps growing. Even how many kinds of "ice" there are is still not settled.
Links to the textbook (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: density / states of matter / buoyancy | Volume increasing on freezing, why it floats, the lake story |
| High school | Basic Chemistry: molecular shape and polarity / intermolecular forces | The "V" shape, hydrogen bonds, four-direction bonding |
| High school+ | Chemistry: crystal structure / phase diagrams | The gap-riddled scaffold, snowflakes' hexagons |
| University | Physical chemistry: phase equilibrium | The phase diagram's slope, Clausius–Clapeyron, high-pressure ice |
| Research | Physics of liquids (unresolved) | A unified explanation for water's anomalies, the two-state hypothesis, the limit of supercooling |
| ― | Earth science / biology | Frost weathering, overwintering in lakes, the sea-level misconception |
- Atkins, P. & de Paula, J., Physical Chemistry (phase diagrams, hydrogen bonding, phase transitions).
- Petrenko, V. F. & Whitworth, R. W., Physics of Ice (the standard textbook on ice structure and properties).
- Gallo, P. et al., Water: A Tale of Two Liquids, Chemical Reviews 116(13), 7463–7500, 2016 (a review on the two-state hypothesis).
- Kim, K. H. et al., Maxima in the thermodynamic response and correlation functions of deeply supercooled water, Science 358, 1589–1593, 2017 (measurements of supercooled water).
- IPCC Sixth Assessment Report (assessment of factors driving sea-level rise).
※ Density and other figures vary with temperature and impurities. This article gives commonly used approximate values.
※This article is a general-audience science explainer. The figures given are approximations meant to aid understanding and may vary with conditions. When trying the observations, do not freeze water in a sealed container or glass jar. It may break.