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Expt:Thermal decomposition of calcium carbonate

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Calcium carbonate is strongly heated until it undergoes thermal decomposition to form calcium oxide and carbon dioxide. The calcium oxide (unslaked lime) is dissolved in water to form calcium hydroxide (limewater). Bubbling carbon dioxide through this forms a milky suspension of calcium carbonate. Students are also asked to research the large-scale applications of these processes.

Apparatus and chemicals

  • Eye protection
  • Tripod
  • Gauze
  • Bunsen burner
  • Heat resistant mat
  • Tongs
  • Boiling tubes, 2 (see note 1)
  • Drinking straw (see note 4)
  • Dropping pipette
  • Filter funnel, small
  • Filter paper
  • Calcium carbonate (Low hazard) (see notes 1, 2 & 3)
  • Universal Indicator solution

Technical notes

  • Calcium carbonate (Low hazard) Refer to CLEAPSS® Hazcard 19B
  • Large (150 x 25 mm) test-tubes.
  • The calcium carbonate used should be in the form of pea-sized lumps of chalk.
  • Blackboard chalk should not be used, as it is likely to be mostly calcium sulfate.
  • Freshly purchased drinking straws should be used.


Health & Safety

The demonstrator should wear goggles and protective gloves.

Apparatus used for decomposition of calcium carbonate


A. You need to prepare a tabulated results sheet before you start your experiments.

Method Observations
Heat for 10 min
Add 2 – 3 drops of water
Add 10 cm3 more water
Blow bubbles through solution
Add Universal Indicator

B. Set a lump of chalk (calcium carbonate) on a gauze. If your gauze has a coated central circle, use the edge where there is no coating.

C. Heat the chalk very strongly for 5 -10 minutes. Write down what you observe.

D. Let the chalk cool and use tongs to move it into a boiling tube. Add 2 – 3 drops of water with a dropping pipette. Write down your observations.

E. Add about 10 cm3 more water to the solid. What happens now?

F. Filter half the mixture into the other boiling tube and, using a straw, gently blow a stream of bubbles through the filtrate. What do you see?

G. Test the remaining half of the mixture with Universal Indicator solution. Write down what you observe.


This experiment has been adapted from Practical Chemistry: