# Is there a limit to how fast water can freeze?

See query.

I thought of it today sweating away, watching, waiting, waiting…agonizing seconds for my room temperature soda to get colder (please hurry) in my glass full of ice.

I say a tentative yes there is a limit.

Once a layer of ice forms between the water and whatever is cooling it.

The ice is acting as a solid insulator/thermal conductor. For a given temperature between the two sides, only so much heat will flow through, this value determined by the thermal properties of the ice.

The cold side has a limit. Absolute zero.

Therefore you can only “pump” heat through the ice so fast to cool the water side. And as the ice gets thicker, that value will continue to decrease.

The actual speed of a near absolute zero freezer and an ice cube tray would be interesting to know…

I was actually thinking of something like maybe approaching an exothermic reaction when atoms do their phase change stuff.

Mental image (obviously of layman): everybody (atoms) shuffling into alignment like in musical chairs, and all that elbowing must itself create some heat.

Crystallization is neat, let alone phase change.

Speaking of which…I managed to super cool a whole case of water bottles recently. People were wondering why I was out in the garage laughing with joy like a school kid yelling “cool as shit!” as each one magically turned solid over about 3 seconds when I picked it up.

I’d imagine that the fastest possible freezing would occur in supercooled liquid water upon nucleation. Water can remain in liquid form to -48degC at standard pressure.

https://en.wikipedia.org/wiki/Supercooling

Freezing involves the water molecules actually locking onto each other to form a solid lattice. Water temperature can go a long way below 0C, yet still not freeze. A large volume of super cooled water can freeze almost instantly, once the first ice crystals form.
So I suppose the fastest water can freeze, is limited by how fast the molecules can physically move into the ice crystal formation. Very fast under the right conditions.

Best way to make fast ice, is a shallow metal tray. Like a cookie sheet. Heat will conduct fast off the large surface area above and below. When done, you can shatter the sheet of ice into nice thin pieces with large surface area to cool your drink.

In a practical sense … no, water can appear to freeze instantly with the human senses. Obviously it’s not instant in the strictest definition, but close enough to make no difference.

I would offer freezing rain as an example. As it is falling through the air column, the liquid water temperature drops just below the freezing point. When it strikes an object, it pretty much all turns to ice upon this contact and sticks to whatever it hit.

Yup, freezing is exothermic, albeit not because of elbow friction! If you are trying to reduce the temperature of water, the freezing process slows that down temporarily. If you keep steadily extracting heat energy from liquid water, the temperature of the water drops steadily down to zero C, then stays constant for a while at zero C while it freezes, because the freezing process releases energy. Once freezing is complete, the temperature resumes dropping.

https://en.wikipedia.org/wiki/Enthalpy_of_fusion

This.

However, this limit doesn’t necessarily have a limit.

I mean, if you get to control the geometry of the problem, you can spread the water very thin, and do so on an object that is practically completely cold, i.e. near absolute zero.

They make (or made) some superconductor out of amorphous metal this way, IIRC. They have a very cold copper disk spinning fast, and spray the molten metal on the edge of the disk. It spreads out quite thin. The thermal path is therefore very short and freezing happens very fast.

What happens to a single H20 molecule when it drops to 0C? Is it just colder than it was before, or does something else about it change?

Basically nothing. The individual atoms might be vibrating within the molecule a little less than they were before, but that’s a continuous process — it’s not like there’s a qualitative change in this behavior at 0C. The phase transition is inherently a phenomenon that happens because you have a large number of molecules that can assemble themselves into a more ordered state.