Chemistry & Nature

Why Does Ice Float?

Most solids sink in their own liquid, but ice does the opposite. When water freezes, its molecules form an open structure that takes up more space.

The short answer

Almost every solid sinks in its own liquid. Ice does the opposite because of molecular structure. Liquid water molecules move constantly and pack fairly tightly. When water freezes, hydrogen bonds lock them into a rigid hexagonal lattice with open spaces. That makes ice about 9 percent less dense than liquid water. Less dense things float. This is unusual. Most substances contract when they solidify because the molecules pack closer together. Water expands. The same mass takes up more volume, so its density drops. The consequence is enormous. When lakes freeze, ice forms on the surface and acts as an insulating lid. The water underneath stays liquid, letting fish and other life survive. If ice sank, many bodies of water would freeze from the bottom up.

Large chunk of ice floating in dark polar water

How much less dense is ice

Ice is approximately 9 percent less dense than liquid water at 0°C. This is why roughly 90 percent of an iceberg sits below the water line.

Myth: all solids sink in their own liquids

Most solids do, but water is a famous exception. Bismuth and gallium also expand when they freeze.

Why this matters for life

Floating ice insulates the liquid water below. If ice sank, lakes and oceans would freeze from the bottom up.

Maximum density point

Liquid water is densest at 4°C, not at 0°C. As it cools from 4°C to freezing, it expands slightly.

Visual answer

How Water Molecules Rearrange When Water Freezes

The shift from liquid to solid water creates open space in the molecular lattice, reducing density.

Liquid water molecules move freely

In liquid water, hydrogen bonds form and break rapidly, allowing molecules to shift past each other and pack relatively tightly.

Cooling slows molecular movement

As temperature drops toward 0°C, molecular motion slows and hydrogen bonds begin to persist.

Freezing locks molecules into a hexagonal lattice

At 0°C, hydrogen bonds stabilize into a rigid six-sided structure with large gaps in the arrangement.

The lattice is less dense than liquid water

The same number of molecules occupy about 9 percent more volume in ice than in liquid water, so ice floats.

Diagram comparing liquid water molecules with the open hexagonal lattice of ice

Hydrogen bonds

It All Comes Down to How Water Molecules Bond

Water molecules are polar: the oxygen end is slightly negative and the hydrogen ends are slightly positive. This lets neighboring water molecules attract each other through hydrogen bonds.

In liquid water, these bonds constantly form and break as molecules tumble past each other. When water freezes, the molecules slow down and the bonds become stable. The geometry of the molecule naturally creates an open crystal lattice.

That open space is what makes ice less dense. The chemistry is the same as liquid water, but the arrangement is different.

Tiny note

Why 90 percent of an iceberg is underwater

Ice is close in density to seawater, so it barely floats. To float, it must displace a volume of water equal to its own mass, leaving only about 10 percent above the waterline.

Myth vs reality

Myth vs Reality

What people think

Ice floats because it is cold and cold things are lighter

Temperature itself does not determine whether something floats. Density does. The reason ice floats is the structural change during freezing.

What actually happens

Ice floats because its crystalline structure is less dense

When water freezes, the lattice its molecules form takes up more space than the same molecules in liquid form. More volume for the same mass means lower density.

Liquid vs solid water

Liquid Water vs Ice: The Key Differences

Molecular arrangement

Liquid water: constantly shifting, randomly packed. Ice: rigid hexagonal lattice with fixed gaps.

Density

Liquid water at 0°C: approximately 1.00 g/cm³. Ice at 0°C: approximately 0.917 g/cm³.

Hydrogen bonds

Liquid water: bonds constantly break and reform. Ice: bonds are stable and hold the lattice in place.

Volume for the same mass

Ice takes up about 9 percent more volume than an equal mass of liquid water.

Quick answers

Common questions

Why does ice float instead of sinking like most solids?

When water freezes, hydrogen bonds lock molecules into an open hexagonal lattice. This makes ice about 9 percent less dense than liquid water.

Why do pipes burst when water freezes?

Water expands about 9 percent when it freezes. If it is trapped in a rigid pipe, this expansion generates enough pressure to crack the pipe.

At what temperature is water most dense?

Liquid water is densest at 4°C. Below that temperature and above freezing, water becomes slightly less dense as it cools.

Why is only about 10 percent of an iceberg above water?

Ice is only slightly less dense than seawater, so most of its volume must stay submerged to displace enough water for it to float.

More ways to keep going

Jump back to this shelf, browse generated topics, or let TinyThat choose the next question.

Browse why questions

See every why question in this shelf.

Browse topics

Explore generated topic pages from article metadata.

Random Question

Jump to a random published article.

Why Does Ice Float?

How Water Molecules Rearrange When Water Freezes

Liquid water molecules move freely

Cooling slows molecular movement

Freezing locks molecules into a hexagonal lattice

The lattice is less dense than liquid water

It All Comes Down to How Water Molecules Bond

Why 90 percent of an iceberg is underwater

Myth vs Reality

Ice floats because it is cold and cold things are lighter

Ice floats because its crystalline structure is less dense

Liquid Water vs Ice: The Key Differences

Molecular arrangement

Density

Hydrogen bonds

Volume for the same mass

Common questions

Related why questions

Why Is Snow White?

Why Is Sea Water Salty?

Why Do Tides Happen?

Why Is the Human Body Made of Water?

Why Do We Cry When Cutting Onions?

Why Do Mosquito Bites Itch?

More ways to keep going