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Why does ice float?

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What else makes you wonder?

Would ice from salty ocean water float the same way?

Salt sneaks in between the water bits and changes how tightly they pack β€” so the weigh-off might not come out quite the same.

Do other liquids spread out and get lighter when they freeze too?

Most don't β€” they pack tighter and their solid version sinks in the puddle. Water is the rebel. What could make it break the rule?

How much of a giant iceberg hides underwater where ships can't see?

If only a tenth pokes above, where is all the rest β€” and how huge would the hidden part of a mountain-sized berg be?

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After you watchWhy does ice float?

The short answer

Ice floats because it is the same water spread into a roomier crystal: when water freezes, its molecules lock into open six-sided rings with empty space inside, so fewer molecules fit in the same amount of room. That makes ice lighter than the water around it, and the lighter stuff always rides on top.

Try this next

  • What if you froze salty ocean water instead of plain water? Freeze a cup of plain water and a cup of very salty water side by side, then drop each ice piece into its own glass of the same water and predict which one floats higher before you watch.
  • What if you pushed a whole ice cube under and let go? In the pond, hold the cube under the surface in your mind, then predict how much pops back up before you drop it in and watch about a tenth settle above the line.
The whole story

How it works

Floating is really a weigh-off between two equal-size cups: the lighter cup always rises above the heavier one. Liquid water molecules huddle close and slide around, so a lot of them pack into one cup. When water freezes, those very same molecules link into open hexagonal rings with a hole in the middle, so fewer of them fit in a cup that's the same size. None are added or removed and none get heavier, so a cup of ice weighs about 9% less than a cup of water, which is why ice floats.

What people get wrong

Almost everyone believes a solid is always heavier and denser than its liquid, because freezing makes water feel hard and many other things sink when they turn solid. Water is the famous rebel: it actually spreads out and takes up more room when it freezes, so a cup of ice weighs less than the same cup of liquid water.

The catch

Spreading out is both handy and dangerous. Because ice is lighter it floats as a frozen lid on a pond, keeping the water below liquid so fish and frogs survive the winter, and only about a tenth of an iceberg shows above the surface while nine-tenths hides below. But that same expansion means freezing water pushes outward hard enough to crack pipes, split rocks, and burst a soda can left in the freezer.

Questions kids ask

Does the ice weigh less because some water disappeared when it froze?

No. Freezing doesn't add or remove a single molecule and doesn't make any of them heavier. The exact same molecules just spread into a roomier ring with empty space, so fewer of them fit in the same-size cup, which makes that cup lighter.

How much of an ice cube sticks out above the water?

About a tenth, or roughly 9%. That's the very same amount the cup of ice was lighter than the cup of water, so the other nine-tenths stays hidden below the surface.

If water expands when it freezes, why is that dangerous?

Because the spread-out ice crystal takes up more room than the liquid did, freezing water pushes outward with real force. That's strong enough to crack pipes, split rocks, and burst a sealed can left in the freezer.

Do other things float because they freeze too?

Most don't. Water is unusual: most substances pack tighter and get denser when they turn solid, so their solid form sinks in their own liquid. Water's open ice crystal is the rebel that makes it lighter and able to float.

Talk about it

  • Before we freeze it, guess: will a cup of ice weigh more or less than the same cup of water, and why?
  • Ice floats but a rock sinks β€” what do you think makes water so unusual?
  • Why might it actually be a good thing that ponds freeze from the top down instead of the bottom up?

For grown-ups

Liquid water near 0 C is unusually dense because hydrogen bonds let molecules sit close together. On freezing, those same hydrogen bonds lock each molecule to four neighbours in an open hexagonal lattice (ice Ih) with empty channels, lowering the density by about 9% (roughly 1.00 g/cm3 for water vs 0.917 g/cm3 for ice at 0 C). Lower density means it floats, and by Archimedes' principle about 1 - 0.917 = 9% of an iceberg's volume rides above the surface. This negative volume change on freezing is water's anomalous expansion, and it's why pipes burst and why lakes freeze top-down, insulating the life below.