Drop an ice cube into a glass of water and it floats. It seems completely ordinary, something we see every day. Yet it is actually very unusual. For almost every other substance, the solid form is denser than the liquid, so a solid piece sinks in its own melt. Solid wax sinks in melted wax; solid iron sinks in molten iron. Water is one of the rare exceptions. This odd behavior has enormous consequences: it allows lakes to freeze from the top down, protects aquatic life in winter and helps shape Earth's climate. This explainer describes why ice floats and why water is such a remarkable substance.
Floating and density
Whether something floats depends on density, the amount of mass in a given volume. An object floats in a liquid if it is less dense than the liquid. It sinks if it is denser.
This principle was described more than 2,000 years ago by the Greek scientist Archimedes. He found that an object in a fluid is pushed upward by a force equal to the weight of the fluid it displaces, called the buoyant force. If an object weighs less than the fluid it displaces before it is fully submerged, it floats.
Liquid water at room temperature has a density of about 1.00 gram per cubic centimeter. Ice has a density of about 0.92 grams per cubic centimeter, roughly 9 percent less. Because ice is less dense, it floats, with about 90 percent of its volume below the surface. That is the origin of the phrase "the tip of the iceberg."
Why most solids are denser than liquids
In most substances, molecules in a liquid move freely and are loosely packed. When the liquid cools and freezes, the molecules slow down and settle into a tightly packed, orderly arrangement. Packed more closely together, they take up less space, so the solid is denser than the liquid.
Water does the opposite. When water freezes, its molecules arrange themselves into a structure that takes up more space than in the liquid. To understand why, we need to look at the water molecule itself.
The water molecule
A water molecule, H₂O, consists of one oxygen atom bonded to two hydrogen atoms. The molecule is bent, with an angle of about 104.5 degrees between the two hydrogen atoms, rather than in a straight line.
Oxygen attracts electrons more strongly than hydrogen. As a result, the shared electrons spend more time near the oxygen atom. The oxygen end of the molecule has a slight negative charge, and the hydrogen ends have slight positive charges. Molecules like this, with uneven charge distribution, are called polar.
Hydrogen bonds
Because water molecules are polar, they attract each other. The slightly positive hydrogen atom of one molecule is attracted to the slightly negative oxygen atom of a neighboring molecule. This attraction is called a hydrogen bond. It is much weaker than the chemical bonds within a molecule, but strong enough to have major effects on water's properties.
In liquid water, hydrogen bonds constantly form, break and reform as molecules move around, trillions of times per second. Molecules can move close together, sometimes squeezing into gaps between other molecules.
The structure of ice
When water freezes, the molecules slow down enough that hydrogen bonds hold them in fixed positions. Each water molecule forms hydrogen bonds with four neighbors, arranged in a regular, three-dimensional pattern. The result is a hexagonal crystal structure, with an open, honeycomb-like arrangement containing a lot of empty space.
Because the molecules are held farther apart in this open structure than in the liquid, ice takes up more volume than the same mass of liquid water. Water expands by about 9 percent when it freezes. That is why ice is less dense and floats.
The hexagonal structure of ice is also why snowflakes have six-fold symmetry. The arrangement of water molecules in the crystal determines the six-sided shape of every snowflake.
Water is densest at 4 °C
Water's unusual behavior begins even before it freezes. Most liquids become steadily denser as they cool. Water also becomes denser as it cools, until it reaches about 39 °F (4 °C). Below that temperature, water starts to become less dense again, because hydrogen bonds begin to form more ice-like, open structures, even before freezing.
This means that water at 39 °F (4 °C) is denser than water at 32 °F (0 °C), the freezing point. This property, called the density anomaly of water, has important consequences in nature.
Why lakes freeze from the top down
In autumn and winter, as the air cools, the surface water of a lake cools too. Cooler water becomes denser and sinks, while warmer water rises. This mixing continues until the whole lake reaches about 39 °F (4 °C).
When the surface water cools further, below 39 °F, it becomes less dense than the water below and stays at the top. Eventually, the surface freezes, forming a layer of ice. The ice, being even less dense, floats on top.
The ice layer acts like an insulating blanket, slowing further heat loss from the water below. The bottom of a deep lake often remains at about 39 °F (4 °C) throughout the winter. This allows fish, plants and other aquatic life to survive under the ice.
If ice were denser than water, it would sink to the bottom as it formed. Lakes and even parts of oceans could freeze solid from the bottom up, and much of the life in them would struggle to survive. Earth's climate and the history of life could have been very different.
Floating ice and climate
Floating ice also plays an important role in Earth's climate. Sea ice in the Arctic and Antarctic reflects much of the sunlight that falls on it back into space, helping keep polar regions cold. When sea ice melts, darker ocean water is exposed, which absorbs more sunlight and warms further, a process called the ice-albedo feedback.
Interestingly, melting sea ice does not raise sea levels much, because floating ice already displaces its own weight in water. When an ice cube melts in a glass of water, the water level stays about the same. But melting land ice, such as glaciers and ice sheets on Greenland and Antarctica, does raise sea levels, because it adds water that was not already in the ocean.
Expansion when freezing
The expansion of water when it freezes has many practical effects:
- Burst pipes: Water in pipes can freeze and expand in cold weather, splitting the pipes. That is why people insulate pipes or let taps drip during very cold periods.
- Frost weathering: Water seeping into cracks in rocks freezes and expands, gradually breaking the rocks apart. This is a major force in shaping mountains and creating soil.
- Potholes: Water seeping into road cracks freezes and expands, breaking up the pavement.
- Frozen bottles: A sealed glass bottle full of water can crack in the freezer as the water expands.
Other unusual properties of water
Hydrogen bonds give water other remarkable properties:
- High heat capacity: Water can absorb a lot of heat before its temperature rises. Oceans and lakes moderate climates and help keep Earth's temperatures stable.
- High boiling point: For such a small molecule, water has a surprisingly high boiling point, 212 °F (100 °C) at sea level, because hydrogen bonds must be broken for it to evaporate.
- Surface tension: Water molecules at the surface cling together, forming a "skin" that allows small insects to walk on water.
- Excellent solvent: Because it is polar, water dissolves many substances, such as salts and sugars, which is essential for chemistry in living cells.
Is water the only substance whose solid floats?
Water is not completely alone. A few other substances, including silicon, gallium, germanium and bismuth, also expand when they solidify. But water is by far the most common and important example, and its behavior has shaped our planet.
Questions readers ask
Why does ice float? When water freezes, hydrogen bonds arrange the molecules into an open crystal structure that takes up more space, making ice about 9 percent less dense than liquid water.
At what temperature is water densest? At about 39 °F (4 °C).
Why do lakes not freeze solid in winter? Ice forms at the surface and insulates the water below, which often stays at about 39 °F (4 °C).
Does melting sea ice raise sea levels? Very little, because floating ice already displaces its weight in water. Melting land ice does raise sea levels.
Why do snowflakes have six sides? Because the water molecules in ice form a hexagonal crystal structure.
In short
Ice floats because water behaves unusually when it freezes. Polar water molecules attract one another through hydrogen bonds, and as water freezes, these bonds lock the molecules into an open, hexagonal structure that takes up more space than liquid water. Ice is therefore about 9 percent less dense and floats. Water is also densest at 39 °F (4 °C), so lakes freeze from the top down, protecting life below. This simple fact about ice has shaped Earth's landscapes, climate and the survival of countless living things.