Science

Why ice floats, and why almost nothing else does

Nearly every substance is denser as a solid than as a liquid. Water is not, and a great deal depends on the exception.

By 5 min read
An iceberg in Arctic water with part of its underside visible above the surface.
An iceberg. Photograph by AWeith, CC BY-SA 4.0, via Wikimedia Commons.

Key takeaways

  • Almost all substances contract on freezing; water expands, by roughly nine per cent.
  • Hydrogen bonds lock water molecules into an open, six-sided lattice with gaps in it.
  • In liquid water those bonds are constantly breaking, so molecules pack more closely together.
  • Water is at its densest at about 4 degrees C, not at freezing point.
  • Because ice floats, lakes freeze from the top down and stay liquid underneath.

Freeze almost anything and it shrinks. Cooling slows molecules down, they settle into an ordered arrangement, and that arrangement is usually tighter than the disordered jostle of the liquid. Drop the solid into a pool of its own liquid and it sinks.

Water does the opposite. It expands by around nine per cent on freezing, and ice floats. This is genuinely unusual, and an enormous amount depends on it.

The molecule is bent, and slightly lopsided

A water molecule is one oxygen with two hydrogens attached, and the three do not sit in a line. They form a V.

Oxygen pulls electrons more strongly than hydrogen does, so the oxygen end of the molecule carries a slight negative charge and the hydrogen ends a slight positive one. Because of the bent shape those charges do not cancel out, and the molecule as a whole is polar.

Polar molecules attract each other. The specific attraction here, between a hydrogen on one molecule and the oxygen on another, is a hydrogen bond. It is much weaker than the bonds holding a molecule together, but there are a great many of them, and they are directional: they pull molecules into particular orientations rather than simply pulling them together.

Freezing builds a lattice with holes in it

That directionality is the whole story.

In liquid water the hydrogen bonds are forming and breaking constantly, billions of times a second. At any instant a molecule is bonded to some neighbours and free of others, so molecules can slide past one another and fill in the gaps. The packing is untidy, but it is close.

Cool it to freezing and the molecules no longer have the energy to break those bonds. Every molecule settles into the orientation the hydrogen bonds want, bonded to four neighbours arranged around it. Lock that pattern in across a whole crystal and you get an open hexagonal framework, which is also why snowflakes have six sides.

The framework is rigid and regular, and it is full of empty space. The molecules are held apart by the very bonds that hold them together. Same number of molecules, more volume, lower density.

Liquid water against ice: why the solid takes up more roomOn the left, molecules in liquid water are packed closely and untidily, because hydrogen bonds are breaking and reforming constantly and molecules can slide into the gaps. On the right, every molecule has settled into the orientation its four hydrogen bonds require, producing a regular open six-sided framework with empty space inside it. The same number of molecules occupies more volume, so ice is about nine per cent less dense and floats.Same molecules, different packingLIQUID WATERbonds break and reform constantly; molecules fill the gapsICEevery molecule bonded to four neighboursliquid water at 4 °Cdensest stateice at 0 °C9 per cent less denseThe molecules are held apart by the very bonds that hold them together.Six-sided in the lattice, six-sided in a snowflake.
Almost every other substance packs tighter as it freezes and its solid sinks. Water builds a rigid frame with gaps in it, which is why a lake freezes from the top down and stays liquid underneath. Diagram by HRITU; molecular arrangement shown schematically.

Densest at four degrees

The anomaly starts before freezing. Water reaches its maximum density at about 4 degrees C, not at 0.

Two effects are competing as it cools. Losing thermal energy makes molecules move less and pack closer, which increases density in the ordinary way. At the same time, more and more molecules start adopting the open ice-like arrangement, which decreases it. Above 4 degrees the first effect is winning; below 4 the second takes over.

That crossover has a direct physical consequence in any deep body of water in winter. The densest water sinks, so the bottom of a cold lake sits at around 4 degrees, with colder water above it and ice on top. The lake stratifies in an order that would be impossible for a normal liquid.

What floats on top changes everything below

Imagine the alternative for a moment. If ice were denser than water, it would form at the surface and immediately sink. Fresh surface water would be exposed, freeze, and sink in turn. A lake in a cold winter would fill with ice from the bottom up and freeze solid.

Because ice floats, the opposite happens. The first ice stays at the surface and forms a lid, and ice is a poor conductor of heat, so that lid insulates the water beneath it from the air above. Below it the water stays liquid through the winter, and everything living in it survives.

At the largest scale, the same property gives the polar oceans a floating ice cover rather than a frozen sea bed, which shapes ocean circulation and climate.

And some more everyday consequences

  • Burst pipes. Water expanding inside a closed pipe generates enough pressure to split metal.
  • Frost shattering rock. Water seeps into cracks, freezes, expands and levers them wider. Repeated over seasons it breaks up mountainsides, and it is a major agent of the weathering that makes the sea salty.
  • Icebergs mostly underwater. Ice is only slightly less dense than seawater, so roughly nine tenths of any iceberg sits below the surface.
  • Food damaged by freezing. Ice crystals expanding inside cells rupture their walls, which is why some things are unpleasantly soft after thawing.

Elsewhere in the behaviour of water

The same polarity that builds the ice lattice is why water alone cannot pick up grease. And freezing is the one place bread is safe from the process described in why bread goes stale faster in the fridge.

Sources

  1. Properties of water, including the density anomaly
  2. Hydrogen bond
  3. Ice

Frequently asked questions

Two effects run against each other as water cools. Ordinary thermal contraction makes it denser, and the growing tendency of molecules to form the open hydrogen-bonded arrangement makes it less dense. Above 4 degrees the first effect dominates; below it the second takes over. The crossover is the density maximum, and it is why the coldest water in a deep lake in winter sits above water at about 4 degrees, not below it.

Because water expands as it turns to ice, and if it is enclosed it has nowhere to go. The pressure generated is considerable. Interestingly the split often appears somewhere other than the ice plug itself: as ice grows it pushes trapped liquid water along the pipe, and the failure happens where that pressurised water meets an obstruction.

Ordinary ice does. Under very high pressures water forms other crystalline structures, several of which are denser than liquid water, and these exist inside icy moons and in laboratories. The everyday ice you meet is the one anomalous form that happens to be less dense.

HRITU Editorial Desk is the byline for pieces produced in-house by the HRITU editorial team. We write about cooking, travel, science and everyday knowledge, and we explain the reasoning behind what we publish rather than just stating conclusions. Every article is checked against primary or reference sources before it goes out, dated on publication, and updated in the open when we learn something is wrong. Corrections and questions are welcome at info@hritu.com.

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