There’s a 1.5L bottle of Skyy in our freezer, which is now about two-thirds full.
I never looked at this closely before, but it’s quite easy to see on a Skyy bottle, which is deep blue instead of colorless like most vodka bottles. The thin layer of frost on the outside of the bottle only goes up to the surface of the liquid inside the bottle.
Presumably this means that the air inside the bottle must have a different, i.e. warmer, temperature as compared to the vodka in the bottle. But the air, vodka, and the bottle containing them are all in the freezer, the same environment which caused the bottle to become frosted in the first place, and also imparted a noticeable increase of viscosity to the vodka. So how is this possible? Is the alcohol a kind of heat sink that can actually become colder than the freezer itself? What else besides temperature inside the bottle could influence the the behavior of water molecules on the outside of the bottle?
The term you’re looking for is heat capacity. Alcohol, and more especially water, can absorb large amounts of heat with only a small change in temperature. In contrast a gas like air can only absorb a tiny amount of heat before the temperature rises.
So you take your Vodka bottle out of the freezer. The outside of the glass is cold (-15oC), and it absorbs heat from the air, including the moisture in the air. In so doing it warms up. It then transfers that heat to the inside of the glass, and then to the fluid inside the glass. And that’s when you notice the difference.
As soon as the heat is transferred to the gas in the bottle, the gas temperature rises to near room temperature (low heat capacity, see). At that point the glass has nowhere else to put the excess heat it is absorbing from the air, and it also rises to air temperature. Since frost can’t possibly form at room temperature, it doesn’t.
However when the heat is transferred through the glass to the liquid, the liquid stays at nearly the same temperature, eg -13oC. Frost certainly can form at -14oC, and it does so. But the frost line can only go as high as the level of the liquid because above that the glass is too warm.
So yes, the alcohol acts as a heat sink, and a very good one. But no, it’s not colder than the freezer. It’s the same temperature as the freezer. The point is that it is a heat sink. A lot of heat can be dumped into it very fast before it becomes “full” and starts to warm up. In contrast the air can’t absorb much heat at all before it becomes full and starts to warm.
You will notice if you look carefully on a humid day that a thin layer of frost actually forms all over the bottle initially, but that it rapidly melts above the waterline. That’s because the act of condensing and freezing the water actually transfers heat from the air to the glass, and that heat has nowhere to go, to the glass then warms up to room tamp and the frost then melts. Below the waterline the heat is transferred to the liquid behind, so the glass remains below freezing point.
A quick guess: For water to condense on the outside of the bottle, it has to give up heat. The liquid in the bottle has more capacity to accept this heat than the air in the bottle, so that is the place where the condensation takes place.
Then, when I pour the vodka into a metal flask (for scientific purposes only, of course,) condensation immediately appears on the outside of the flask.
It depends how cold the vodka is and how warm and thick the metal is. If the vodka is very cold and the metal is either cold or very thin then you might well see frost forming as the water sublimes directly out of the air into ice, rather than seeing condensation.