Looking at crystals

Class practical

Looking for crystalline structures in each specimen.

Apparatus and Materials

• Hand lenses
• Microscope, with illuminants and power supply
• Granit specimens, with unweathered surfaces
• Cast bismuth specimens
• Granulated sugar
• Demerara sugar
• Caster sugar
• Icing sugar

Health & Safety and Technical Notes

Do not use the sub-stage mirror on a microscope to reflect the Sun through the instrument.

Read our standard health & safety guidance

It might be worth looking also at flour or talc to see something which is not visibly crystalline.

Broken slabs of cast zinc can also be tried, but they are not as effective as bismuth.

Procedure

1. Ask the students to look for crystalline structures in each specimen: flat faces with angles between them characteristic of that material, regardless of their size. Ask students also to note similarities and differences between the crystals they see.
2. All specimens should be examined first with the naked eye, then with a hand lens and finally, in some cases, with a microscope.
3. Point out that strong illumination at a low angle to the surface is helpful.

Teaching Notes

• Sugars: Demerara sugar is seen to be crystalline even to the unaided eye. Granulated sugar is clearly seen to be crystalline when viewed with the hand lens, the caster sugar with the microscope. It will probably not be possible to see the crystalline nature of icing sugar even with a microscope, though this will depend on both the grade and the microscope.
• Students should learn that the basic shapes of sugar crystals appear regardless of the size of the crystal. They should realize that crystals do not just appear separately but sometimes as a fused mass and sometimes with others.
• Other specimens: Specimens of granite may be examined with hand lenses. Three main constituent materials will be observed: clear, glass-like quartz, pink or white, opaque feldspar, and shiny, flaky mica. Traces of other minerals can often be distinguished. It may be possible to observe the perfect cleavage lines of the mica crystals, less perfect on the feldspar and none on the quartz.
• Cast bismuth can be examined either with the naked eye or with a hand lens. The crystal structure is clearly visible.
• This is a chance to enjoy using the microscope and wall displays of crystals. Exotic ones like diamond and snowflakes, and common ones such as table salt, are helpful for discussion.

This experiment was safety-tested in May 2004