Here you will learn how to modify the shape and colour of crystals by purposely adding "contaminants" to the recrystallizing solution.

It should be noted that the technique of recrystallization has been used for many years to obtain "pure" materials in the presence of contaminants.

When crystals form, they do so by the laying down of the individual particles (atoms, ions, or molecules) in a specific pattern (called crystal lattice). The only way different particles can be included in the lattice is if they can somehow "mimic" the proper particle, either by being of a similar shape or size.

Thus, addition of food colouring would not likely make colourless crystals coloured.

EXAMPLE 1

For an illustrated example of how to change a colour of a crystal, compare the pictures of pure Alum crystals with that of Alum and added chromium. Chromium ions are about the same size as some of the ions in Alum and thus do not affect the crystal lattice and shape of the resulting crystal. Only the colour of the crystal is altered. The resulting intensity is dependent upon the chromium concentration.

EXAMPLE 2

In the second illustrated example, addition of copper ions to a solution of Rochelle salt changes both the colour AND the shape (sometimes called "habit"). The pure Rochelle Salt crystal is colourless; the copper "contaminated" crystal can have different shades of blue and a variety of habits. For the crystals illustrated here, the recrystallizing solutions were nominally 0.1M with respect to copper (II) ions. This concentration also created many technical problems for our students. Should you wish to experiment with this phenomenon, it would be better to use lower concentrations of about say 0.01M copper ion.