BrightKidz Library
Subjects
Sixteen water molecules fit in a box as liquid, but only six fit once they lock into ice Three panels read from left to right. The first box holds sixteen small circles crowded close together with only narrow gaps between them, standing for water molecules in liquid water, tumbling past one another and packing in as tightly as they can get. The second box is exactly the same size, but it holds only six circles, joined by six straight bonds into a wide six sided ring with a large empty hole through the middle. Once the molecules lock together they hold each other at arm's length, so far fewer of them fit in the same amount of room. An arrow leads from that ring to a tumbler of water, where a square ice cube floats with a small part of it showing above the waterline and much the greater part below.

Why Ice Floats When Almost Nothing Else Does

About 11 minutes

Drop an ice cube into a glass of water and it bobs up. You have seen it so many times that it looks like the ordinary way for things to behave.

It is not. It is close to unique, and the reason it happens is worth knowing.

The rule that everything else obeys

Cool almost anything down and it shrinks. The particles inside slow down, stop barging about so much, and settle closer together. Freeze it and it shrinks again as those particles lock into place, packing in as tightly as they can manage.

Something that has shrunk holds the same amount of stuff in less room. That makes it heavier for its size, and heavier-for-its-size things sink.

So the ordinary result is this: solid candle wax dropped into melted wax goes straight to the bottom. A lump of solid metal sinks in a pool of the same metal, molten. Almost every solid on Earth sinks in a puddle of itself.

Water refuses.

Water molecules hold each other at arm's length

A water molecule is not a smooth ball. It has a definite shape, with two arms, and it has sticky patches that pull on the sticky patches of its neighbours.

In liquid water the molecules are moving too fast for any of that to hold. They tumble, slide, grab a neighbour for an instant and let go. Nothing keeps its place, so they simply crowd in, and crowding is efficient. Liquid water is water packed as tightly as jostling molecules can pack.

Freezing is what happens when they slow down enough for the sticky patches to win.

And here is the whole answer: the arrangement they lock into is an open six-sided framework. Each molecule takes hold of its neighbours and holds them at a set distance, out at arm's length. It is a scaffold, and like any scaffold it is mostly empty space.

Holding hands in a ring takes up more room than huddling. So the same water, frozen, needs a bigger box.

Six things happen to a jug of water as it cools from warm to frozen solid. Two of them run in opposite directions, which is the part worth getting right.

  • Warm water. The molecules move fast, tumble past each other, and take up a fair amount of room.
  • The water cools. The molecules slow down, barge about less, and settle closer together. The water shrinks, exactly as the rule says it should.
  • At about four degrees the water is as tightly packed as it will ever get. This is the heaviest, densest water there is.
  • Cooling further, the sticky patches start to win, and molecules begin taking up their positions in the framework. The water now starts to expand again, though it is still getting colder.
  • At zero degrees the framework locks solid all through. The water is now ice, and it has swollen to about a tenth bigger than it was.
  • The ice sits in the water it came from with about nine tenths of itself below the surface, because that is how much lighter for its size it has become.

Look again at stages 2 and 4. Cooling water shrinks, and then, past a certain point, cooling water swells. That turn is where water stops behaving like everything else, and everything on this page follows from it.

You can measure the swelling yourself

The framework makes ice about a tenth bigger than the water that made it. That is not a small effect, and you do not need any equipment to see it.

  1. Fill the plastic cup a little over halfway with water.
  2. Mark the water level on the outside of the cup with the pen, or lay a strip of tape with its top edge exactly at the surface. Get it accurate. The whole result is a comparison with this line.
  3. Stand the cup upright in the freezer and leave it for about four hours, until it is frozen right through.
  4. Take it out and look at the mark. The top of the ice is above it.
  5. Leave the cup on a draining board to melt, then check the level again. It comes back down to your mark, because the amount of water never changed at any point.

Nothing was added. The rise you measured is the same water, holding itself apart.

Why a pond in winter is not a solid block

This is the part that matters most, and it is easy to miss because it depends on something not happening.

As a pond cools, the coldest water sinks and warmer water rises to take its place, so the whole pond turns over and cools evenly. That continues until the water reaches about four degrees, its densest point. After that the coldest water is no longer the heaviest, so it stops sinking. It stays on top, freezes there, and the ice floats.

Now the pond has a lid. Ice is a poor carrier of heat, so that lid slows down the cooling of everything beneath it, and the water underneath stays liquid at around four degrees all winter. Fish, frogs and insects live down there until spring.

Run it the other way for a moment. If ice behaved like everything else, it would form at the surface and sink. Fresh water would freeze at the top and sink too, and the pond would fill from the bottom upwards until it was solid, with everything in it. Deep lakes would never fully thaw in one summer, and after enough winters much of the water on Earth would be locked up as ice at the bottom of things.

An open six-sided framework, in a molecule too small to see, is what stands between us and that.

An ice cube floats with about a tenth of itself above the water. What does that tenth actually tell you?

What to try next

Try the freezer test again, but fill the cup with cooking oil instead of water and mark the level the same way. Oil follows the ordinary rule, so watch which way the line moves this time.

Then look for the swelling where it does damage. A pothole in a road usually starts as a crack that filled with rainwater and then froze. The frost that lifts paving slabs, splits rocks on a mountainside and breaks garden pots over the winter is all the same six-sided framework, pushing outwards, one tenth at a time.