Why does ice float? The short answer
Ice floats because it is less dense than liquid water.1 A cubic centimetre of ice weighs about 0.92 grams, while the same cube of water weighs about 1.00 gram.61 Anything that is less dense than the liquid around it floats.7
The surprising part is why ice is lighter. When water freezes, hydrogen bonds lock its V-shaped molecules into an open hexagonal lattice, a honeycomb with empty space in every ring.34 The molecules end up farther apart than in the liquid, so the density drops by about 9 percent.12
Water is the odd one out
For most substances, the solid is denser than the liquid. As a liquid cools, its molecules slow down and pack more tightly, so the solid sinks in its own liquid.2
Water does the opposite. It expands when it freezes. The same water takes up about 9% more space as ice, which is why a full ice tray bulges and why frozen pipes can burst.112
What is the density of ice vs water?
Density is how much mass is packed into each bit of space: density = mass ÷ volume. Here is how ice compares with some common liquids:681
How deep a floating object sits tells you its density. By Archimedes' principle, the fraction underwater equals the object's density divided by the liquid's density.7
fraction under = 0.917 ÷ 1.00 ≈ 92%
Why is ice less dense than water?
1. A bent molecule with charged ends
A water molecule is one oxygen atom joined to two hydrogen atoms, bent at an angle of about 105°, like a V.3 The oxygen end carries a slight negative charge and the hydrogen ends carry slight positive charges.2
2. Hydrogen bonds: a weak grip between molecules
Opposite charges attract, so the hydrogen of one molecule is pulled toward the oxygen of a neighbour. This attraction is called a hydrogen bond.2 It is much weaker than the bonds inside a molecule, only about 5 to 10% as strong.5
3. In the liquid, bonds keep snapping
In liquid water, the molecules are moving so fast that hydrogen bonds constantly break and re-form.3 Because no bond lasts, molecules can tumble around and crowd close to each other.
4. In ice, the bonds lock into hexagons
As water freezes, the molecules slow down enough for the hydrogen bonds to hold. Each molecule links to four neighbours, and together they form an open lattice of six-sided rings.34 (The four neighbours are arranged in 3D, like the corners of a pyramid. From above, the pattern looks like a honeycomb.) That rigid, open structure holds the molecules farther apart than in the liquid, leaving empty space in the middle of every ring.23 More space for the same molecules means lower density, so ice floats.
Why is water densest at 4 °C?
Most liquids get denser all the way down to their freezing point. Water gets denser as it cools only until about 4 °C (39 °F), where it hits its maximum density.1 Between 4 °C and 0 °C, some molecules already start forming open, ice-like arrangements, so the water expands again and gets lighter.4
Why do lakes freeze from the top down?
In autumn, the surface of a lake cools first. Cold surface water is denser, so it sinks, and warmer water rises to take its place. This mixing goes on until the whole lake reaches about 4 °C.1
After that, the surface water keeps cooling, but now it gets lighter, so it stays on top. The surface reaches 0 °C and freezes first, while the bottom stays near 4 °C.1 The floating ice works like a lid that insulates the water below, so fish and plants survive the winter.23
If ice sank, or if water were densest at its freezing point, lakes could freeze from the bottom up.1 That's why the odd behaviour of one small molecule matters so much for life on Earth.
Why is most of an iceberg underwater?
Icebergs are chunks of freshwater ice that broke off glaciers, which formed from snow falling on land.11 They float in salty sea water, which is a bit denser than fresh water (about 1.025 g/cm³).6 Using the float rule, 0.917 ÷ 1.025 ≈ 0.89, so roughly nine-tenths of the ice sits below the waterline. NASA puts it as "as much as 90 percent of any iceberg lies below the sea surface."9 That hidden bulk is what makes icebergs so dangerous to ships. In April 1912, the Titanic hit one and sank.10
Why do frozen pipes burst?
Because water expands as it freezes, ice forming inside a closed pipe pushes hard on the walls. The American Red Cross warns that this pressure can break metal or plastic pipes, no matter how strong they are.12
Try it at home: float or sink?
- Drop an ice cube into a clear glass of water. Look from the side: only a thin slice pokes above the surface.
- Pour some olive oil into a second glass and add an ice cube. Olive oil (about 0.92 g/cm³) is almost exactly as dense as ice, so the cube sits almost completely under the surface.6 As it melts, watch the water drops sink through the oil: liquid water is denser than both.
- Dissolve lots of salt in a third glass of water. The cube floats a little higher, just like an iceberg in the sea.
Common misconceptions
"Ice floats because of trapped air bubbles." Clear, bubble-free ice floats too. The open hexagonal lattice itself is less dense than the liquid.
3
"Solids are always denser than their liquids." That's true for most substances, but not for water: ice is about 9% less dense than liquid water.
12
"The coldest water sinks to the bottom of a lake." Water is densest at about 4 °C. Water colder than that floats, which is why ice forms on top.
1
"Ice floats in any liquid." Ice sinks in pure alcohol, which is only about 0.79 g/cm³.
6
"What you see is most of the iceberg." As much as 90% of it is hidden below the surface.
9
Keep exploring
Salt lowers the temperature at which water freezes, which is why towns spread it on icy roads: how does salt melt ice. Snowflakes are ice too, grown from vapour into six-sided crystals high in the clouds: how does snow form. And icebergs float in sea water that is salty for a reason of its own: why is the ocean salty.
FAQ
Why does ice float on water, in simple words?
Ice is less dense than liquid water. When water freezes, its molecules lock into an open, honeycomb-shaped crystal with empty space in every ring. The same amount of water takes up about 9% more room, so a cube of ice weighs less than the same-sized cube of water, and it floats.
What are the two reasons why ice floats?
First, ice is less dense than liquid water (about 0.92 g/cm³ versus about 1.00 g/cm³), and anything less dense than the liquid around it floats. Second, the reason it is less dense: hydrogen bonds hold the molecules in ice in an open hexagonal pattern, farther apart than in the liquid.
Why is ice less dense than water?
In liquid water, hydrogen bonds keep breaking and re-forming, so molecules can crowd close together. In ice, every molecule is held by hydrogen bonds to four neighbours in a rigid, open lattice of six-sided rings. That lattice has more empty space, so the density drops by about 9%.
Why does ice float on water but sink in alcohol?
Ice (about 0.92 g/cm³) is less dense than water (about 1.00 g/cm³) but denser than pure alcohol (about 0.79 g/cm³). An object floats only in a liquid that is denser than it is, so ice floats in water and sinks in alcohol.
What does it mean if ice cubes sink?
The liquid is less dense than ice. That happens in pure alcohol, for example. Ordinary ice in ordinary water always floats. Special 'heavy ice' made from heavy water is denser than normal water and does sink.
Why is most of an iceberg underwater?
A floating object sinks until it pushes aside its own weight of liquid, so the fraction underwater equals its density divided by the liquid's density. Ice is about 90% as dense as sea water, so about nine-tenths of an iceberg sits below the surface.
Why do lakes freeze from the top down?
Water is densest at about 4 °C. As a lake cools below that, the colder water becomes lighter and stays at the surface, where it freezes. The floating ice then acts like a lid that helps keep the water below from freezing, so fish survive the winter.
Sources
- USGS Water Science School, Water Density
- OpenStax Biology 2e, 2.2 Water
- Encyclopaedia Britannica, Why Does Ice Float on Water?
- Encyclopaedia Britannica, Ice
- OpenStax Chemistry 2e, 10.1 Intermolecular Forces
- OpenStax College Physics 2e, 11.2 Density
- OpenStax College Physics 2e, 11.7 Archimedes' Principle
- OpenStax Chemistry 2e, 1.4 Measurements
- NASA Earth Observatory, Iceberg Goes With the Flow
- NASA Earthdata, What is Glacial Calving?
- National Snow and Ice Data Center, Science of Sea Ice
- American Red Cross, Frozen Pipes