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Quicklime and slaked lime

For your exam, you need to know how quicklime and slaked lime are obtained from limestone.

Making quicklime

If limestone is heated strongly, it breaks down to form calcium oxide and carbon dioxide. Calcium oxide is also called quicklime. It is yellow when hot, but white when cold.

Here are the equations for this reaction:

calcium carbonate right facing arrow with heat calcium oxide + carbon dioxide

CaCO3right facing arrow with heat CaO + CO2

This is a thermal decomposition reaction.

Making slaked lime

Calcium oxide reacts with water to form calcium hydroxide, also called slaked lime.

Here are the equations for this reaction:

calcium oxide + water → calcium hydroxide

CaO + H2O → Ca(OH)2

A lot of heat is produced in the reaction, which may even cause the water to boil.


Using common names instead of chemical names, this is what happens:

limestone right facing arrow with heat quicklime + carbon dioxide

quicklime + water → slaked lime

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