Chalk is the common name for the compound calcium carbonate. In the chalk cycle shown below there are three separate chemical reactions represented:

• Reaction 1 : Chalk is heated strongly. Carbon dioxide gas is released leaving calcium oxide.
   
• Reaction 2 : Once the calcium oxide has cooled down water is added to it. Filtering the mixture will separate a solution of calcium hydroxide (the filtrate).
   
• Reaction 3: When carbon dioxide gas is passed through the calcium hydroxide solution a white precipitate is formed. This is the limewater test for carbon dioxide.

Chalk is organic in origin. It was formed over millions of years from the protective coverings of small organisms that lived in the seas and oceans. When these organisms died their protective coverings fell to the ocean bed. Here they accumulated. Earth movements caused some of these deposits to experience extremely high temperatures and pressures, forming chalk. In some cases the earth movements lifted the chalk deposits out of the water, as is the case with the White Cliffs of Dover.

Chalk is still formed by living things. The shell of a bird's egg contains calcium carbonate.

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