Plate 17 · Matter

Why Ice Floats

Freeze most liquids and the solid sinks. Water does the opposite — and that is why lakes freeze from the top down, and fish survive the winter underneath.

Try it

Chapters · 6

01The Tip of the Iceberg
02Crowded, or Holding Hands
03Heaviest Cupful at 4°C
04A Lake That Freezes From the Top
05A Rare Exception
06Your Turn

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01

The Tip of the Iceberg

Ice is a little less dense than water — a cupful weighs a bit less — so it floats, but only a small part shows. In the sea, about nine-tenths of an iceberg hides below the surface. That is where “the tip of the iceberg” comes from.

In a glass of fresh water, an ice cube sits more than nine-tenths under, too.

An iceberg · about nine-tenths below the waterline

02

Crowded, or Holding Hands

Water is made of countless tiny water molecules. In liquid water they jostle and crowd close together.

When water freezes, each molecule links hands with four neighbors, and they line up in rings of six, with gaps in the middle. The same molecules take up about 9% more room as ice, so a cupful of ice weighs less than a cupful of water.

Water molecules, schematic · red is oxygen, white is hydrogen

03

Heaviest Cupful at 4°C

Water has another odd habit: a cupful weighs the most at about 4°C. Cool it further and more and more molecules briefly link up in small, open, ice-like patches, so the same cupful gets a little lighter.

Below is the density of water from 0°C to 12°C. The peak sits near 4°C.

Density of water · Kell (1975)

04

A Lake That Freezes From the Top

In autumn, water at the surface of a lake cools, grows heavier and sinks, and warmer water rises in its place. Once the whole lake has cooled to 4°C, colder water no longer sinks. It stays at the top, and there it freezes.

The ice works like a lid, shutting out the cold air and the wind. Underneath, the bottom of the lake stays near 4°C, and that is where the fish spend the winter.

A lake in winter · 0°C under the ice, about 4°C at the bottom

05

A Rare Exception

If ice sank like most solids, lakes would freeze from the bottom up, layer by layer, and many deep lakes might not thaw all the way through even in summer.

A solid lighter than its liquid is rare — only a handful of substances, such as silicon and gallium, share the trick. And because water swells as it freezes, pipes left full in winter can burst.

06

Your Turn

Change the temperature and watch the molecules go from crowded to lined up in hexagons. Keep an eye on how the density of water changes near 4°C.

Try thisCool it below 0°C: when the water turns to ice, does its density go up or down?

What the model leaves out

The molecules are drawn far larger than life and their arrangement is simplified: in liquid water, molecules also hold hands briefly, but the links keep breaking and re-forming. The ice shown is the most common kind (hexagonal ice). The lake’s winter temperatures are schematic; a real lake also depends on its depth, the wind and the sun, and in shallow lakes the bottom is often a little colder.

A common misreading

Ice does not float because of air bubbles. Perfectly clear ice with no bubbles floats just as well, because its molecules are packed more loosely than in liquid water. Salty seawater lifts ice a little higher, but ice floats in fresh water too.