Why Freezing Liquids Need Nucleation Sites
Date: Winter 2012-2013
Why do liquids need something to freeze onto?
The quick answer to your question is "they don't! While it is true that a
solid impurity or other solid surface, will help initiate the freezing
process of a liquid, it is readily possible for a droplet of a liquid,
suspended in air by (for example) electrostatic forces, and in a dust
free environment, to freeze to a solid.
What typically happens in this case is that the liquid will remain liquid,
even as temperature drops well below its normal freezing point, then
suddenly it will freeze. This effect is called "supercooling". Normally,
the presence of an impurity such as a tiny grain of sand, a scratch in
the wall of a container, or even a tiny bubble of air will be present, and
this will act as a nucleus to initiate freezing as soon as the liquid's
temperature drops to its freezing point. From this point on, the liquid
will slowly continue freezing, as heat is removed from the liquid, until
all the liquid is frozen.
But in the absence of any impurity, a pure liquid will still freeze. All that
is needed is to supercool the liquid sufficiently and at some
temperature sufficiently below its normal freezing point, the liquid will
suddenly freeze, often completely.
I do not think it is a matter of liquids needing something to freeze onto as more of that conditions for crystal formation are better at nucleation points.
In order to form crystals, substances need to release energy and align themselves to each other. Nucleation points provide both of these since nucleation points are better places for releasing energy because they have a lot of surface area and have a lot of contact with the substance. Nucleation points also support the alignment of molecules by having the molecules adhere to the nucleation site which then limits the possible orientations molecules might have.
Freezing can occur without nucleation sites. However, because this so called "homogeneous nucleation" or simply - crystallization within the body of the liquid - occur without the aid of a nucleation site, the liquid has to be supercooled first before sufficient energy has been released for spontaneous nucleation to happen within the body of the liquid. Thus, in most conditions, since nucleation sites are very common, crystallization will happen through heterogeneous nucleation rather than homogeneous nucleation.
Greg (Roberto Gregorius)
Hi Sandra, technically, they do not need something to freeze onto. A
nucleus of crystal can form without a nucleation site, but it happens
more slowly than when there are nucleation sites.
Hope this helps,
In principle, liquids do not need something to freeze on. The molecules form
their own seeds to initiate freezing from super-- cooled liquids .
Practically, without extremely pure liquids some freezing sites take over at
higher temperatures -- even the container has such sites.
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Update: November 2011