Matter and the particle model · Contrast
Why ice floats
Solids sink in their own melt. Iron does, wax does, aluminium does. Water does not — and if it did, ponds would freeze from the bottom and take everything in them with it.
Start here
Every ice cube in the glass is sitting on top.
A glass of water with four ice cubes in it. All four float, most of each cube under the surface, a small dome above. The cubes were made from that same water.
Why does the ice float on the water it came from?
Less dense than the water it came from — and that is a genuinely strange thing for a solid to be. Freeze almost anything else and the solid sinks in its own melt, because cooling packs particles closer together. Water does the opposite: as it freezes, each molecule is locked into an open hexagonal cage that holds its neighbours further apart than they were in the liquid. The ice expands by about 9%, its density drops to 0.92 g/cm³, and it floats.
Cooling a substance normally makes it denser. The particles slow down, they no longer need as much room to move about in, and they settle closer together — so the same mass occupies less volume. Freeze it and it contracts again as the particles lock into a tight regular pattern. A lump of the solid dropped into its own melt sinks.
Water breaks this rule. Liquid water is at its densest at about 4 °C. Cool it further and it starts to expand, and at 0 °C, as it freezes, it expands sharply — by roughly 9%. The mass has not changed, so the density falls from 1.00 g/cm³ to 0.92, and the ice floats.
The cause is the shape of the water molecule and the way it bonds. Each molecule can hold hands with four others at fixed angles, and in the solid those bonds lock into an open hexagonal cage with a gap in the middle. In the liquid the same molecules jostle and slip past one another and, on average, sit closer together than the cage allows. Freezing water builds the cage, and the cage takes up more room.
At the bench · four substances, each weighed as a solid and as its own melt
Solid on the left, its own liquid on the right.
Change a control to begin
Pick a substance and switch between its solid and its liquid. Watch which of the two bars is longer — and notice that one substance out of the four disagrees with the other three.
Commit first. Molten iron is poured, and a lump of solid iron is dropped into it. What happens to the lump?
The substance
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On the balance
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Density
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Freezing changes the density by
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Solid on its own melt
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Key fact
Almost every substance is denser as a solid than as its own liquid, so the solid sinks in the melt. Water is the exception: freezing expands it by about 9%, so ice at 0.92 g/cm³ floats on water at 1.00 — which is why ponds freeze from the top down.
Think again
“Ice floats because it is lighter than water.”
A block of ice the size of a car is far heavier than a teaspoon of water and it still floats. What matters is not the weight of the object but its density against the density of the water it has to push aside. Ice is 0.92 g/cm³ against water’s 1.00, so any lump of ice of any size floats — and it sits with about 92% of itself below the surface, which is where the phrase “tip of the iceberg” comes from.
“Water expands when it freezes, so it must expand when it is heated too.”
It does, above 4 °C — and between 0 °C and 4 °C it does the opposite, contracting as it warms. That narrow band is called the anomalous expansion of water, and it is why the densest water in a pond sits at 4 °C at the bottom, with colder water above it and ice on top. Everything about how a pond freezes, and about what survives the winter in it, follows from those four degrees.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Read the comparison
Ice has a density of 0.92 g/cm³ and liquid water 1.00 g/cm³. What fraction of a floating iceberg is above the surface?
Rung 2 · The one that catches people
A student says ice floats because it is cold, and cold things float. What is wrong?
Rung 3 · Explain
Explain why a bottle of water left in the freezer can split, and why the same bottle full of cooking oil does not.
Rung 4 · Take it somewhere new
Explain why a pond freezes from the top down, and what would happen to life in it if ice were denser than water.
Key note
When almost any substance freezes, its particles pack closer together, the solid is denser than the liquid, and a lump of the solid sinks in its own melt. Water is the exception. Its molecules lock into an open hexagonal structure on freezing, so it expands by about 9%: ice is 0.92 g/cm³ against liquid water’s 1.00, and it floats with roughly 92% of its volume below the surface. This is why a pond freezes from the top down, why the ice layer insulates the water beneath it, and why life in fresh water survives a winter at all.
Going further
The expansion is powerful enough to be a geological force. Water that seeps into a crack in a rock and freezes pushes outwards with a pressure of tens of megapascals — far more than the rock can take — and the crack widens a little every time it thaws and refreezes. Repeated over enough winters this is freeze–thaw weathering, and it is a major reason mountain roads need resurfacing and mountains themselves fall apart.
The same property runs through biology and engineering. Cells burst when frozen because the water in them expands, which is why frozen fruit goes soft and why organs for transplant cannot simply be put in a freezer. Water pipes split for the same reason, and always at the weakest point rather than where the ice formed. Cryobiologists get round it with antifreeze compounds that stop the hexagonal cage forming in the first place.
Before this lesson
Connects to
- Density
- Changes of state
What melting and freezing do to the arrangement of the particles — this lesson measures what that does to a density.
- Solids, liquids and gases
The general rule this lesson finds the exception to.
At GCSE this becomes
- Density changes at changes of state, the particle model of solids and liquids, and hydrogen bonding as the reason water behaves unlike other small molecules.
Where to next
Ask Mr Badmus AI
Want to know why water is the one that breaks the rule?
The bench is a teaching model. Densities are quoted just below and just above each substance’s melting point, to two decimal places: water 0.92 solid and 1.00 liquid; candle wax about 0.93 and 0.90; aluminium 2.70 and 2.38; iron 7.87 and 6.98 g/cm³. Candle wax is a mixture rather than a single compound and its figures vary between blends. The 1.00 g/cm³ comparison line is liquid water at 4 °C. Percentage changes, and the fraction of a floating lump above the surface, are calculated from the quoted pairs.
Lesson content © MrBadmusAI.