Page 31 - Curriculum Visions Dynamic Book

dehydration: the removal of water from a substance.
hydration: the absorption of water by a substance. Hydrated materials are not “wet” but remain firm, apparently dry solids. In some cases hydration makes the substance change colour, in many other cases there is no colour change, simply a change in volume.
 This pictures shows a piece of crystalline gypsum, calcium sulphate. Gypsum is soft enough to be scratched by a nail.
Also...
Plaster of Paris is a special form of the
salt calcium sulphate. It is made by heating crushed gypsum to drive off some of the water. It is then crushed to a fine powder. When Plaster of Paris is mixed with water, the calcium sulphate takes up water again (it hydrates) and then sets.
 Plaster of Paris is so fine grained that it can be used to make intricate casts. It is, for example, used to make the
casts of teeth for people who need to be fitted with dentures.
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