The Limestone Cycle
What compounds are involved in the limestone cycle?
- Limestone is a raw material that is processed to form quicklime, slaked lime and limewater
Compounds involved in the limestone cycle
| Common name | Chemical name | Formula |
| limestone | calcium carbonate | CaCO3 (s) |
| quicklime | calcium oxide | CaO (s) |
| slaked lime | calcium hydroxide | Ca(OH)2 (s) |
| limewater | calcium hydroxide solution | Ca(OH)2 (aq) |
What are the four steps in the limestone cycle?
Step 1
- A lump of calcium carbonate is heated strongly to form calcium oxide and carbon dioxide in a thermal decomposition reaction
- The solid starts to glow brightly and will then turn crumbly
- The following reaction occurs:
calcium carbonate → calcium oxide + carbon dioxide
CaCO3 (s) → CaO (s) + CO2 (g)
The thermal decomposition of calcium carbonate
When calcium carbonate is heated the solid will begin to crumble and will also glow brightly
Step 2
- Drops of water are added to the calcium oxide to form calcium hydroxide
- An exothermic reaction occurs
calcium oxide + water → calcium hydroxide
CaO (s) + H2O (l) → Ca(OH)2 (s)
Formation of calcium hydroxide from calcium oxide
When water is added to calcium oxide, heat is given off
Step 3
- More water is added to form a solution of calcium hydroxide known as limewater
- Calcium hydroxide is sparingly soluble in order to remove any undissolved solid the mixture should be filtered
Formation of limewater
Limewater is calcium hydroxide solution, Ca(OH)2 (aq)
Step 4
- Carbon dioxide is bubbled through the solution of calcium hydroxide to form calcium carbonate
- The solution turns milky or cloudy as a suspension (precipitate in solution) of calcium carbonate is formed
calcium hydroxide + carbon dioxide → calcium carbonate + water
Ca(OH)2 (aq) + CO2 (g) → CaCO3 (s) + H2O (l)
Bubbling carbon dioxide through limewater
When carbon dioxide is bubbled through limewater, calcium carbonate and water are formed
Summary of the limestone cycle
Make sure your can recall the equations for the reactions that occur in the cycle
Examiner Tip
Limewater is used to test for carbon dioxide. A positive test is the limewater turning cloudy or milky. If carbon dioxide continues to be blown into the limewater the solution will turn colourless again. This is because when carbon dioxide dissolves into water the solution starts to become acidic. Therefore the acidic solution will begin to dissolve the calcium carbonate formed.
