How long does 1 gallon of water take to freeze?

How long does 1 gallon of water take to freeze? It depends on how quickly you can remove the 1280 KJ (1213 BTU) of heat required to chill and freeze the 3800 grammes of water in a gallon.

A jug in a freezer has relatively slow heat transfer, so it takes a while, and the freezer can only move so much heat as the air inside the freezer heats up. Heat does not travel quickly through air. Immerse that jug in a cold brine of ice and salt, and it will freeze much faster because the brine conducts heat better. Put it in a 50 MPH wind at -40 and it freezes up quickly, especially if you shake the jug around (wear mittens).

It’s all about heat transfer, which increases as the area of the water increases, the heat capacity of the cooling fluid increases, and the rate at which cold fluid is brought in to replace the warmed up cooling fluid increases.

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A snow machine spraying water into 40-degree air can instantly freeze a gallon of water. The water has a large surface area and is sprayed at high speed through a very cold cooling medium (air). You’ve probably seen videos of people throwing pots of boiling water into the air on cold winter days and having it freeze before it hits the ground. Because it takes heat to evaporate, some of the water evaporates and cools what is left behind. If you want a gallon of ice, you’ll need to start with more than a gallon.

If I dump a gallon of water on the surface of a frozen lake, it will also freeze quickly due to the large surface area.

You can also take advantage of a phenomenon known as supercooling. It is possible to chill water below the freezing point by boiling it, placing it in a very clean bottle, covering it, and placing it in a freezer. Tapping the side of the glass, stirring, or shaking it will cause it to freeze almost instantly. This is awesome. Water requires a nucleation site to crystalize around in order to freeze, and it does not generate them spontaneously until it is much colder than freezing. Normally, the rough surface of the glass, a gas bubble, or a bit of dust falling in the glass will cause it to freeze at 0 C, but under the right conditions, it can be super cooled:

It is not instant, but it is very fast. The limiting factor in this case is not heat transfer, but how quickly the crystal can grow, which is related to how quickly the water molecules can move into position.