Calcium sulphate, commonly known as gypsum, is a soft mineral formed when calcium hydroxide (lime) reacts with sulphuric acid. It is moderately soluble but unlike most other salts it is more soluble in cold water than in hot water. Its chemical formula is CaSO4.2H2O – as it has two molecules of water associated with it in its hydrated form.
When it is heated it loses water and becomes plaster (anhydrous calcium sulphate). When the plaster is mixed with water a reaction occurs causing it to return to its hydrated form. This reaction consumes the water making it solidify back into gypsum. This is how plaster casts set. Plaster is often called plaster of Paris because of large gypsum deposits at Montmartre in Paris.
The process of hydrating plaster is, similarly to adding water to anhydrous copper sulfate, an exothermic process.
Gypsum occurs naturally as the translucent mineral selenite. It is also used to make the sticks of chalk used to write on blackboards. Real chalk (calcium carbonate) is too hard and would scratch the surface of the board. Gypsum is produced artificially as the by-product of removing sulphur dioxide from the waste fumes of factories burning fossil fuels. This process if known as flue-gas desulphurisation.
Questions…
- Suggest another substance that could be reacted with sulphuric acid to produce calcium sulphate.
- What type of reaction turns hydrated calcium sulphate into anhydrous calcium sulphate?
- As what is calcium hydroxide solution commonly known? For what is it used in the laboratory?
- Why is it necessary to desulphurise flue gases?
- As well as sulphur dioxide, suggest another waste product that might be produced by the burning of fossil fuels?
- How would you separate undissolved calcium sulphate from water?
- What would happen to the mass of calcium sulphate when it is converted to anhydrous calcium sulphate?
- Why does plaster become hot when it is mixed with water?
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