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I have read in a paper that the for formation of new crystals in a solution the energy released due to formation of solid from solution should be more than the energy needed to overcome solid-liquid interactions. As in the meta-stable this is not favorable, no new crystals form in the meta-stable region i.e. low supersaturation state. My doubt is, if it is thermodynamically unfavorable to form new crystals in meta-stable region then why does crystals grow in meta-stable region. In a way, it is also the formation of new crystals.

The exact lines from the text is: "The formation of stable nuclei only happens if the increase in interfacial energy resulting from the formation of a solid{liquid interface is offset by a decrease in energy released by the formation of a (thermodynamically favored) solid. At low supersaturations, the energy required to form a solid{liquid interface is bigger than the energy released upon formation of a cluster which are consequently unstable resulting in a very low nucleation rate. Only when the supersaturation is high enough, i.e. when we enter the labile region, stable solid particles will form."

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What the text is saying is that there are two different processes at work.

  1. The creation of a solid-liquid interface. In other words, going from not having a crystal to having a crystal at all.

  2. The growth of a crystal when there is already something solid for it to grow on.

It is possible (the book says) for 2 to be thermodynamically favoured while 1 is not. In this régime new crystals will not spontaneously form out of the liquid but if a solid surface or particle is present then a crystal will grow on it.

In other words: nucleation.

There are further distinctions to be made based on the nature of the solid-liquid interface. A speck of dust is better than the smooth wall of a vessel, and a tiny crystal of the substance in question is better than a random speck of dust. The principle is the same, though: it is easier to add molecules to something that is already the right shape than to create “the right shape” out of nothing.

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