A new point of view on freezing point.

1. Jul 7, 2009

kelvin490

It is very hard to have homogeneous nucleation in common situation. Pure water with no nuclei at all will not freeze above -40oC. The presence of impurity particles allow the so-called heterogeneous nucleation to occur so that we can see water freeze at 0oC. It may be reasonable to guess at what temperature between -40oC and 0oC will water freeze depends on the amount of impurities in water. But we never see water freeze above 0oC at atmospheric pressure. Can the freezing point be defined in the following way? Freezing point is the temperature at which further increase of impurities cannot increase the temperature for the substance to freeze.

2. Jul 8, 2009

Mapes

This point (which also corresponds to the melting point) is already defined as the temperature at which the Gibbs free energy of each phase, liquid and solid, is equal. Your definition is a byproduct of this and kinetic theory.

3. Jul 8, 2009

kelvin490

Thanks.But why the melting point coincides with the "maximum" freezing point?

4. Jul 8, 2009

Mapes

Melting and freezing are, of course, two sides of the same coin: a phase transition. Liquids can be supercooled because of the energy barrier associated with nucleating the first bit of solid, as you know; sometimes cooling is necessary to provide the driving force for nucleation.

In contrast, the melting point is relatively constant because there's little or no activation energy involved with melting; nucleation occurs with ease. (An equivalent way of saying this is that for all or nearly all materials, the liquid phase wets the solid phase, so it's not necessary to nucleate a tiny droplet.)

So the melting point and the freezing point are the same, given that there are plenty of nucleation sites for the freezing case, and both values are set by the equivalence of energies for the two phases. Since we're assuming constant temperature and pressure, the energy is specifically the Gibbs free energy.

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