## A model of growing crystals

Practical Activity for 14-16

**Demonstration**

This model shows crystal growing by the addition of more polystyrene spheres.

Apparatus and Materials

- Foamed polystyrene spheres (3.8cm diameter), 55 approx
- Wooden base with ridges
- Large alum crystal, if possible

Health & Safety and Technical Notes

Read our standard health & safety guidance

The tray needs to be made as in the diagram.

The outer walls must be able to contain rows of 5 spheres.

The inner walls must not only be able to contain rows of three spheres, but also be thin enough to allow the outer layer of spheres in the final model to touch the spheres inside.

Procedure

- Form a square of 9 balls inside the inner walls. Then put 4 balls on this base and 1 on top to form a pyramid.
- Now let the
crystal

grow by the addition of more balls, one at a time. Add balls carefully until there is a pyramid with a 4 x 4 base. - Then add more balls to make a 5 x 5 base.

Teaching Notes

- At each stage, get the students to observe the shape and to look at the angles between the faces.
- You might ask students if they can identify any real crystals with the same shape or angles between faces as their model. A large alum crystal could be shown, if available.
- You could also use this model to draw students' attention to the possible planes that should be used to
cut a slice

through the model crystal, in readiness for cleaving a real crystal.

*This experiment was safety-tested in September 2004*