Many apparently dry crystalline materials contain water as part of their crystal structures.
The study of water loss or gain by solid materials provides an investigation that can last several days. Children will have the opportunity to track small changes in mass, as well as more obvious alterations in appearance. There are three key words linked to these experiments. Some crystals lose water vapour when exposed to the atmosphere. This first phenomenon is called efflorescence. The name originally referred to the slow "creeping" of various salts in solution when left in open containers. Crystals first form around the edges of the liquid surface and the solution can then rise up the sides by capillary action. In time, the crystal crust reaches the rim of the container and starts down the outside. This kind of chemical jailbreak can be thwarted with a film of grease along the upper surface of the container.
Washing soda crystals (hydrated sodium carbonate) are a good choice to study efflorescence. The ratio of water to washing soda is 10:1, so there is a lot of water to lose. Get pupils to carefully weigh about 10g of fresh crystals in a shallow dish. Monitor the changes in mass over several days, displaying the data graphically. The original glassy crystals turn into a dull white powder after efflorescence. You could also try hydrated sodium sulphate. When new houses are built, you often notice a white powdery deposit on the surface of the bricks. This is also efflorescence, from salts in the original clay.
The second key word is hygroscopic - these materials absorb moisture from the atmosphere but still look much the same. Black copper oxide works well.
Using a fresh or oven-dried sample, monitor the changes in mass over a week. The small sachets of drying crystals that are packed with new electronic equipment are also hygroscopic. Try reversing the water gain by heating in an oven at 60C. When so much moisture is absorbed that the material dissolves, we call it deliquescence. Test dried calcium chloride and watch it slowly transform into a solution when left in air.
Ray Oliver teaches science at St Albans Girls' School, Hertfordshire