How does water freeze into ice?
Water doesn't freeze all at once. Ice creeps in from the outside, the temperature sticks at 0 °C for hours, and clean water can stay liquid at −10 °C until you tap the bottle. X-ray an ice cube, lock molecules into a honeycomb, grow a snowflake and freeze kulfi with salt.
▶ Play with it · Read the 60-second explainer · Watch the 40-second video
- An ice cube freezes from the outside in. Heat can only leave through the cube's skin, so ice grows inward from the walls and the top. The temperature sits flat at 0 °C until the last drop is solid. The middle goes last: dissolved air is squeezed into it and clouds it, and the last water swells into a bump.
- Molecules lock into an open, six-sided lattice. As water molecules slow down, hydrogen bonds hold them at fixed angles in six-sided rings with empty space inside. So ice takes about 9% more room than water: it floats, and it bursts pipes. Water is densest at 4 °C, which is why lakes freeze from the top and fish survive below.
- Freezing needs a seed. 0 °C is where ice melts, not where water must freeze. A first speck of crystal has to form, and tiny specks melt again. Clean, still water can stay liquid far below zero: cloud droplets down to about −38 °C. Dust, bacteria or silver iodide give it a start, and a tap sets off a supercooled bottle.
- Water must give up 334 kJ a kilogram. Freezing releases the latent heat of fusion: as much heat as cooling the same water by 80 °C. Carrying that away is why a tray takes about three hours. Fruit growers use it too, spraying blossom on frosty nights so the freezing water keeps buds near 0 °C. Whether hot water can freeze faster (the Mpemba effect) is disputed.
- Six arms, and no two alike. A snow crystal grows from vapour, and its six arms come from the six-sided lattice. Its shape follows the weather: plates near −2 °C, needles near −5 °C, feathery stars near −15 °C, as Ukichiro Nakaya mapped in the 1930s. Every crystal takes its own path through the cloud.
- Kulfi, frozen peas and lake ice. Salt lowers the freezing point, so a matka of ice and salt drops to −15 °C and freezes kulfi. Food frozen fast gets small crystals that spare its cells. Lake and river ice thickens more and more slowly, and is never certainly safe.
| Term | Meaning |
|---|---|
| Latent heat of fusion | The heat water gives up as it freezes without getting colder: 334 kJ for every kilogram. |
| Hydrogen bond | The pull between the slightly positive hydrogen of one water molecule and the slightly negative oxygen of another. |
| Ice Ih | Ordinary ice. Its molecules sit in an open lattice of six-sided rings, so it is about 9% less dense than water. |
| Density maximum | Liquid water is heaviest at about 4 °C. Colder than that, it gets lighter again. |
| Nucleation | The first step of freezing: a tiny cluster of molecules forms a crystal big enough to keep growing. |
| Supercooling | A liquid staying liquid below its freezing point because no crystal has started. |
| Nakaya diagram | A chart of which snow-crystal shape grows at each temperature and humidity. |
| Freezing-point depression | Dissolved salt or sugar lowers the temperature at which water freezes. Salt can take it to −21 °C. |
| Mpemba effect | The disputed claim that hot water can sometimes freeze sooner than cold. |
2,400 years from ice made under a desert sky to the twenty-second kind of ice.
- 1590 · Saltpetre, mountain ice and kulfi (Mughal court, Delhi and Agra, India)
- 1611 · Why always six? (Johannes Kepler, Prague, Bohemia (today Czechia))
- 1724 · Water that stays liquid below freezing (Daniel Gabriel Fahrenheit, Amsterdam, Netherlands)
- 1762 · The hidden heat of ice (Joseph Black, Glasgow, Scotland)
- 1775 · Ice from the night sky at Allahabad (Sir Robert Barker and Indian ice makers, Allahabad (Prayagraj), India)
- 1885 · The first photograph of a snow crystal (Wilson Bentley, Jericho, Vermont, USA)
- 1936 · A snow crystal made in the lab (Ukichiro Nakaya, Sapporo, Japan)
- 1946 · Seeding a cloud (Vincent Schaefer and Bernard Vonnegut, Schenectady, New York, USA)
The full story, with 24 moments, charts, people and 27 sources: glassbox.how/e/freezeclear/history. The data lives in history.json.
Made with the Glassbox studio from this box's storyboard (window.glassbox.director). Free to reuse under CC BY 4.0.
| File | What | Size |
|---|---|---|
glassbox/reel.mp4 |
Reel / Short, with captions and soundtrack | 1080×1920 |
glassbox/video.mp4 |
YouTube video, with captions and soundtrack | 1920×1080 |
glassbox/slide-1…10.jpg |
Instagram carousel | 1080×1350 |
glassbox/thumb.jpg |
YouTube thumbnail | 1280×720 |
glassbox/cover.jpg |
Share card and repo social preview | 1200×630 |
glassbox/history-reel.mp4 |
“History in 10 moments” Reel / Short | 1080×1920 |
glassbox/history-slide-*.jpg |
History carousel | 1080×1350 |
glassbox/post.json |
Post copy and schedule used by the publish kit |
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It remembers a few things in your own browser only, and never sends them anywhere:
| Browser storage key | What it holds |
|---|---|
freezeclear.v1 |
Which chapters you have opened, your best quiz scores, and sound on or off. |
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python3 -m http.server 8000Three.js and the fonts ship in vendor/ and fonts/, so it also works offline.
Then open http://localhost:8000.
| File | What |
|---|---|
index.html, css/app.css |
The page and its styles |
js/app.js, js/stage.js, js/ui.js, js/kit.js |
The shared Glassbox 3D engine: chapters, 3D stage, controls, quiz, video director |
js/physics.js |
The numbers and working models: the freezing ice cube, Mercedes-Benz water, the lake, nucleation, the snowflake automaton, Plank's freezing time. No 3D in it, so node can run it |
js/freeze.js |
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js/chapters/*.js |
One file per chapter: the 3D model, controls, text, key terms, quiz and video scenes |
glassbox.json |
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fonts/, vendor/three/ |
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