Part 1 - Ostwald's rule of stages and Ostwald ripening

Question 1

What does Ostwald’s rule of stages tell us?

Answer 1

That in general it is not the most stable, but the least stable, polymorph that crystallizes first.

Question 2

According to Ostwald’s rule of stages, what would be the order of polymorph formation of CaCO3?

Answer 2

First: ACC
Second: Vaterite
Third: Aragonite
Fourth: Calcite

(This is temperature dependent of course)

Question 3

If less stable polymorphs first crystallize, how does the more stable polymorphs form?

Answer 3

The less stable polymorph will dissolve and re-precipitate in the thermodynamically more stable state.

Question 4

What is Ostwald ripening?

Answer 4

The formation of bigger particles at the expense of the smaller particles. The chemical potential of the smaller particles is bigger than for the larger particles, so there is a driving force towards larger particles.