Freezing Point
Most recent answer: 10/22/2007
Q:
Having read several of the answers to other questions asked about freezing water, I am still a little unclear on one issue. Some of your answers say that water becomes a solid at Zero degrees Celsius. However, one I read said that water molecules stick together and form ice when the temperature falls below zero degrees Celsius.
My question is, can tap water at normal atmospheric pressure exist as a liquid at zero degrees C or will it immediately turn to ice? Does water freeze at zero or below zero?
- Peter (age 39)
St Laurence’s College, Brisbane, Australia
- Peter (age 39)
St Laurence’s College, Brisbane, Australia
A:
Right at the freezing point, both the liquid and the solid are stable.
They’re in equilibrium with each other, meaning that the free-energy
per molecule is the same. At any other temperature, one or the other
has lower free energy, and is more stable.
In addition to the issue of true long-term stability, there’s the question of stability for practical purposes. Pure water cooled a little below the freezing point won’t actually freeze any time soon. The reason is that a nucleus of ice has to form before the freezing really gets going. Just below the freezing point, those ice nuclei don’t become more stable than the liquid until they have a lot of molecules in them. Structures like that happen only very rarely by chance, so it takes a long time for the freezing to occur. A liquid that is still liquid even though its temperature is below its freezing point is called "supercooled".
Even if there is a little crystal of ice present to get things started, and even if the temperature is a tiny bit below the freezing point, water won’t ’immediately turn to ice’. The reason is that as it turns to ice, heat is released. That keeps the nearby liquid from freezing until enough time has passed for the heat to diffuse away and the temperature to drop down to a low enough value. In practice, of course, this is not really a start-and-stop process but rather a slow steady process limited by the rate a which heat diffuses away.
Going the other way, ice melts without a nucleation delay when heated above the freezing point. The reason is said to be a layer of liquid-like molecules on the surface, which serve as a nucleus for the liquid phase. However, melting soaks up heat, so the process still proceeds at a rate limited by how fast heat flows in.
Mike W.
In addition to the issue of true long-term stability, there’s the question of stability for practical purposes. Pure water cooled a little below the freezing point won’t actually freeze any time soon. The reason is that a nucleus of ice has to form before the freezing really gets going. Just below the freezing point, those ice nuclei don’t become more stable than the liquid until they have a lot of molecules in them. Structures like that happen only very rarely by chance, so it takes a long time for the freezing to occur. A liquid that is still liquid even though its temperature is below its freezing point is called "supercooled".
Even if there is a little crystal of ice present to get things started, and even if the temperature is a tiny bit below the freezing point, water won’t ’immediately turn to ice’. The reason is that as it turns to ice, heat is released. That keeps the nearby liquid from freezing until enough time has passed for the heat to diffuse away and the temperature to drop down to a low enough value. In practice, of course, this is not really a start-and-stop process but rather a slow steady process limited by the rate a which heat diffuses away.
Going the other way, ice melts without a nucleation delay when heated above the freezing point. The reason is said to be a layer of liquid-like molecules on the surface, which serve as a nucleus for the liquid phase. However, melting soaks up heat, so the process still proceeds at a rate limited by how fast heat flows in.
Mike W.
(published on 10/22/2007)