Like every Friday, the weekly physics tidbit is back — number 28.
This time: supercooled water, Gibbs free energy, and a surprising connection to hand warmers.
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The bottle in the video is filled with water that hasn't frozen, even though it's been sitting in a freezer.
A gentle squeeze causes the water to freeze right before our eyes, in a remarkably rapid process.
How does it work?
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First, we need to understand what happens when water freezes.
Water is made up of molecules in constant motion.
When they move fast, water becomes steam; when they move slowly, it remains liquid; and when they bond together, it becomes solid ice.
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Water begins to freeze at 0°C — but only if it contains small particles on which ice crystals can nucleate.
The water in the video is pure, with no foreign particles whatsoever.
This allows it to drop several degrees below zero without freezing.
The gentle squeeze briefly pushes the molecules toward one another, close enough for them to bond and form an ice nucleus.
Once ice begins to form, it serves as a crystallization nucleus for neighboring molecules, and the process cascades from there.
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For those who want to go deeper, here's another layer of understanding:
Gibbs free energy deals with states of thermal equilibrium, and it tells us that water tends to crystallize into ice because doing so makes it more stable.
The transition from water to ice involves an energy barrier — a certain amount of energy must be invested to move water from its liquid state to a crystalline one.
That energy comes from the gentle squeeze on the bottle, which imparts kinetic energy to the molecules.
As the water begins to freeze, it releases energy, because the speed of its molecules decreases.
This released energy feeds the continued freezing of neighboring molecules in a chain reaction, analogous to nuclear fission.
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One of the most interesting applications of this phenomenon is active hand warmers, which become hot all at once with a single press.
Inside these warmers is a supercooled sodium acetate solution and a small metal disc.
Pressing the disc causes the solution to crystallize rapidly, releasing a significant amount of heat into the warmer in the process.
Shabbat Shalom 😊
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