How does ice melt?
Ice does not give out cold. It soaks up heat, 334 joules for every gram, and stays at 0 °C while it does. Drop cubes in nimbu pani, shake a crystal of 128 water molecules apart, race crushed ice against a block, make kulfi with salt, pull a wire through ice and see why melting land ice raises the sea.
▶ Play with it · Read the 60-second explainer · Watch the 40-second video
- Heat flows into the ice. Ice does not send out cold. Heat flows from the warm drink and the air into the ice. A stirred mix of ice and water stays at 0 °C until the last ice has gone, and the level in the glass does not change as floating ice melts.
- An open honeycomb that falls in. In ice every water molecule holds four neighbours by hydrogen bonds, in rings of six with empty channels. Heat makes the molecules shake until the lattice breaks. The jumble packs closer, so water (1,000 kg/m³) is denser than ice (917 kg/m³) and ice floats.
- The hidden heat. Ice warms at 2.09 kJ per kg per degree. At 0 °C the temperature stops while 334 kJ per kg goes into melting, as much as warming that water to 80 °C. That is why a 30 g cube cools a drink about four times more than 30 g of ice-cold water.
- What makes it melt faster. Heat gets in at U × area × temperature difference. Crushed ice has about ten times the surface of a block. Water carries heat far better than still air. Sawdust or jute slows it all down, which is how ice sailed from Boston to Calcutta in 1833.
- Salt and pressure move the melting point. Salt lowers it, down to −21.1 °C at 23.3% salt: that is road salt, and the kulfi-wala's matka of ice and salt. Pressure lowers it by only 0.0074 °C per atmosphere, enough for a wire to pass through ice but not to explain skating. A loose surface layer and friction do that.
- A melting world. Melting floating sea ice barely raises the sea; melting land ice does. Greenland holds about 7.4 m of sea-level rise and Antarctica about 58 m. Himalayan glaciers lost ice 65% faster in 2011 to 2020 than the decade before. Less ice also means a darker surface that absorbs more sunlight.
| Term | Meaning |
|---|---|
| Melting point | The temperature at which a solid turns to liquid: 0 °C for pure ice at normal air pressure. |
| Latent heat of fusion | The heat needed to melt a solid without warming it: 334 kJ for each kilogram of ice. |
| Specific heat | The heat needed to warm 1 kg by 1 °C: 2.09 kJ for ice, 4.186 kJ for water. |
| Hydrogen bond | The pull between a hydrogen of one water molecule and the oxygen of the next. Four of them hold each molecule in ice. |
| Heating curve | A graph of temperature against heat added. It is flat while something melts or boils. |
| Freezing-point depression | The drop in freezing and melting point when something is dissolved in a liquid. Salt takes water down to −21.1 °C at most. |
| Eutectic | The mix with the lowest melting point. For salt and water it is 23.3% salt, at −21.1 °C. |
| Regelation | Ice melting under pressure and freezing again when the pressure is taken away. |
| Albedo | The share of sunlight a surface reflects: about 0.85 for snow, about 0.06 for open sea. |
Nearly 4,000 years of keeping ice, shipping it, measuring it, and finally working out why it melts and why it is slippery.
- 400 BCE · Ice in the desert (Persian builders, Persia (today Iran))
- 1586 · Himalayan ice for the Mughal court (Akbar's court; recorded by Abu'l-Fazl, Lahore (today Pakistan))
- 1762 · Latent heat (Joseph Black, Glasgow, Scotland)
- 1833 · Boston ice reaches Calcutta (Frederic Tudor, Samuel Austin and William Rogers, Boston, USA, to Calcutta, India)
- 1849 · Pressure lowers the melting point (James Thomson and William Thomson, Glasgow, Scotland)
- 1850 · Regelation (Michael Faraday, London, England)
- 1878 · Ice factories end the ice ships (Bengal Ice Company and Crystal Ice Company, Calcutta, India)
- 1935 · The structure of ice (John Desmond Bernal, Ralph Fowler and Linus Pauling, Cambridge, England, and Pasadena, USA)
The full story, with 30 moments, charts, people and 39 sources: glassbox.how/e/meltclear/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 |
This box has no accounts and no ads, and it ships its own fonts and libraries. When you run it yourself it sends nothing anywhere. On glassbox.how, the site's /bar.js also loads Glassbox's analytics: Google Analytics to count visits (it asks first in the EU, UK and Switzerland, and stays off when your browser sends Global Privacy Control or Do Not Track) and ClickTrust to detect bots.
It remembers a few things in your own browser only, and never sends them anywhere:
| Browser storage key | What it holds |
|---|---|
meltclear.v1 |
Which chapters you have opened, your best quiz scores, and sound on or off. |
Exactly what each one sees is at glassbox.how/privacy.
- Code: MIT. Use it, change it, ship it.
- Explanations, text, images and videos (
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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/chapters/*.js |
One file per chapter: the 3D model, controls, text, key terms, quiz and video scenes |
js/melt.js |
MeltClear's shared physics and parts: the heating curve, the drink-mixing sum, the salt and pressure melting curves, the melt-rate model, chart boards, and the glass, tank and ice-cube models |
glassbox.json |
Title, question, explainer beats, key terms, browser storage and credits shown on glassbox.how |
reel in each chapter |
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glassbox/ |
The published video, slides, thumbnail and post copy |
fonts/, vendor/three/ |
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