Liquids freeze faster in open containers?

General Questions
#1

Hello all!

Yesterday we were cleaning out the fridge, and while doing so we put all the fridge content on our balcony, to keep it cold. It’s about -10 degrees Celcius outside.

The content included, among other things, three packages of milk (“paper-boxes”), and two packages of tomato soup, same kind of “paper-box” (Tetrapak?). One of the milk packages had been opened, as well as one of the soup packages.

Today when I went to pick up our food from the balcony (yes, we were lazy!) I was not surprised to find that most items were now frozen solid, but what surprised me was that:

  1. The milk that had been opened (but was allmost full) was frozen solid, while both (identical) unopened milk packages was still in liquid form.
  2. The soup that was opened (and almost full) was also frozen solid, while its identical unopened package also was still in liquid form.

How do I explain this? My theories are:

  1. The opened containers had slightly less content
  2. The opened containers had slightly more air movement inside them
  3. The unopened containers might have a tiny bit of underpressure in them

But even theory 1+2+3 does not explain the big difference between frozen solid and “practically liquid”.

Please help me explain this!

Ola / Sweden

#2

My guess: #1 would help, #2 understates the situation and there is a great deal more movement inside that includes exchanging air and vapor with the outdoors, #2 also allows for evaporation to help lower the temperature, and #3 does not matter.

#3

Combine 1 and 2. The open containers, even if totally unused, will have less content, as some evaporates. More importantly just a little evaporation cools the liquid a lot. In the closed containers what little air there is has all the vapor it can take, in the open containers the vapor can escape.

3 is backwards. The packages you reference don’t have a pressure difference when sealed and have very little or no air in them. The liquid will be at its densest in your fridge at -4 C, and putting them outside will cause the liquid to expand, creating higher pressure, and keeping them, to some extent, from freezing.

Opening the containers though would release that pressure and the content would be free to crystallise, which is what you can get with a bottle of water left for just the right time in the freezer. So I’m actually just reiterating what Napier said: an open container freezes a lot faster because of evaporation. Even with a close to freezing liquid evaporation carries away a lot of heat.

Of course, none of this takes into consideration how you define “almost full”.

Oh, and I think the word you were looking for, for “paper-boxes”, is “carton”. As in milk carton.