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APPLIED FIZZICS · Field Notes · Edition No. 11
A whimsical look at the universe through a glass of Champagne (or your favorite cocktail).
September 29 · Seattle, Washington
Dear Reader,
Put an ice cube in a glass of water and it floats. Hardly breaking news.
But that familiar little observation is one of water’s most consequential peculiarities. It helps fish survive winter. It explains the tip of an iceberg, and played a role in the wreck of the Titanic. And it lets us fill a glass right to the brim, leave a sizable chunk of ice sticking out of it, and ask a question:
When that ice melts, where does the water go?
Water does things differently
Most substances become denser when they freeze. Water expands instead.
In ordinary ice, water molecules arrange themselves into an open crystal structure that takes up more space than they occupied as a liquid. The same mass occupies a larger volume, so its density decreases.
That’s why ice floats—even perfectly clear ice without a single air bubble.
We see it so often that it hardly seems remarkable. But your ice cube is doing something distinctly unusual in the material world.
Good news for fish
Floating ice also changes what winter looks like beneath a lake’s surface.
Because ice floats, lakes freeze from the top down. The layer of ice at the surface acts as insulation, while water beneath it can remain liquid.
If ice were denser than water, newly formed ice would sink, exposing more water to the freezing air. Lakes in cold climates could freeze solid from the bottom up, profoundly changing the environment in which aquatic life survives.
So this apparently trivial peculiarity of the ice in your cocktail has consequences that extend far beyond the cocktail hour.
And that raises a much bigger question.
What happens when seemingly small properties of the physical world turn out to be essential to the kind of universe we live in?
That question took this week's Field Notes somewhere I wasn't expecting to go.
From an ice cube in a cocktail, we somehow ended up talking about frozen lakes, atomic nuclei, the fundamental constants of physics—and why the universe has properties that allow us to be here asking questions about it in the first place.
All because ice floats.
Watch Field Notes #11 below: What If Ice Sank?
Meanwhile, back at the experiment...
For this week’s demonstration, I made an ice “cube” approximately ten inches tall.
All right, it’s a pillar. But I’m calling it a cube.
I floated it inside a cylinder filled right to the brim with water. A substantial portion of the ice stuck up above the waterline, looking very much like a spill waiting to happen.
So what happened when it melted?
Nothing.
The water stayed at the brim.
Archimedes explained why more than 2,000 years ago. A floating object displaces an amount of water equal in weight to the object itself. When our ice melts, it becomes exactly the amount of water it had already displaced while it was floating.
Or, more intuitively:
The ice simply melts into the hole it had already made in the water.
No overflow required.
Want to see Archimedes' Principle derived from first principles? It takes nothing but simple algebra. Click the link below.
