There is a group of compounds containing the metal element calcium that includes the word “lime” in their common name. They can be neatly demonstrated through the limestone cycle. Limestone (marble, chalk) has the chemical name calcium carbonate. When it is heated it decomposes (like all carbonates) releasing carbon dioxide and leaving calcium oxide behind. Calcium oxide is also known as quicklime. If a small quantity of water is added to the quicklime, an exothermic reaction results, and slaked lime (calcium hydroxide) is formed. If more water is added a solution will form called limewater (calcium hydroxide solution). When carbon dioxide is bubbled through the limewater, it becomes chalky as insoluble limestone forms again and precipitates out of the solution.
Limestone has many uses, primarily as a building material, but also as an important industrial chemical e.g. in iron smelting. Quicklime is used to make cement, as well as plaster, paper and bio-diesel. Slaked lime (often just called lime) is used as a flocculant in water treatment plants – it encourages particles of dirt to settle out. It is also used to reduce acidity in fields, to remove carbon dioxide from industrial fumes and in steel making.
Questions…
- What do you see when calcium is added to water?
- Name a food with high calcium content.
- What is produced when sulphuric acid is added to calcium carbonate?
- How would you remove the limestone from some chalky limewater?
- How would you get pure water from a sample of limewater?
- What would you expect the pH of limewater to be?
- What is a flocculant?
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