Crystals and Crystallisation (2/2) [Shankland]

Question 1

How is a super-saturated solution made?

Answer 1

Dissolve solid in solvent
Remove excess solid from saturated solution
Allow solution to cool or evaporate
= Super-saturated solution
Therefore seed crystals will grow

Question 2

What is the metastable zone?

Answer 2

The metastable zone is a region to the left of the saturation line between under-saturated and super-saturated

Question 3

Why must a solution be super-saturated before crystallisation can occur?

Answer 3

This is thermodynamically favourable

Question 4

Define: Nucleation

Answer 4

The first step in the formation of either a new thermodynamic phase or a new structure via self-assembly or self-organisation

Question 5

Define: Super-saturated solution

Answer 5

A solution which contains more solute than the solvent can dissolve

Question 6

How is a seed formed?

Answer 6

Collisions occur between molecules in solution
A critical cluster size is reached and a stable nucleus is formed
Further molecules attach and crystal growth ensues
Crystallisation from a super-saturated solution is thermodynamically favourable

Question 7

Why are amorphous forms not widely used?

Answer 7

Offer excellent solubility but do not possess good thermodynamic stability and have a tendency to revert to (lower solubility) crystalline forms