Crystals dissolving in water

Class practical

This apparently trivial experiment can lead young students into interesting discussions.

Apparatus and Materials

For each student group

• Common salt
• Beaker of water (400 ml is convenient)
• Microscope (e.g. Junior type, x20), optional
• Evaporating basin
• Bunsen burner
• Tripod
• Pipe-clay triangle

Health & Safety and Technical Notes

Eye protection is required if the evaporation stage is done. (See CLEAPSS Laboratory Handbook for further information).

Read our standard health & safety guidance

Procedure

1. Students put some of the salt into the water and watch it dissolve. They then add more and more of the salt and watch it dissolving.
2. A faster group might do this experiment under a microscope. Some crystals of salt are put on a microscope slide and observed through the microscope. Then a few drops of water are added and the crystals are observed dissolving.

Teaching Notes

• Have students already seen the crystalline structure of common salt? If so, watching the salt dissolve should raise questions of what has happened to the crystals. If such questions do not come from the students, ask them what they think is happening to make the crystals appear to vanish.
• Analogies may be drawn with the washing away or removal of polystyrene spheres from a pyramid model.
• If it helps the discussion, put some water from one of the beakers in an evaporating dish. Heat it to drive off the water, revealing the salt that had been dissolved.

This experiment was safety-tested in June 2004