Watching crystal growth under a microscope

Class practical

This is a fascinating experiment. The sight of the edges of the crystal moving across the slide will never be forgotten.

Apparatus and Materials

• Microscope, with illuminants and power supply
• Microsope slide
• Beaker, 400ml
• Common salt, saturated solution
• Bunsen burner
• Salol
• Sodium thoisulfate

Health & Safety and Technical Notes

Students should be shown how to gently and safely heat the microscope slide over the Bunsen flame. They need to know that glass does conduct, even if poorly.

Also, that if a cool slide is heated too fiercely it may crack and shatter.

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

Read our standard health & safety guidance

If possible have a microscope per four students and a microscope slide per pupil.

There should be one Bunsen burner and beaker for every microscope.

Prepare an almost saturated solution of common salt the day before. Decant this into the beakers.

Salol is often easier to use than common salt to achieve good crystals. Sodium thiosulfate also provides good crystals to view.

Procedure

1. Adjust the microscope so that it is focused on a plane just above the slide. It may be helpful to focus on some mark on a piece of paper placed on top of the slide.
2. Warm the slide over a low Bunsen flame, place a drop of the salt solution on the slide and then immediately put the slide under the objective - without moving the objective. The slide must be warm enough to evaporate the liquid rapidly, but not so hot as to boil the liquid.

Teaching Notes

Students should see the straight edges and angles typical of common salt crystals. They should also realize that the solid grows because of what comes out of the liquid.

This experiment was safety-tested in January 2005