All the carbonates in this Group undergo thermal decomposition to give the metal oxide and carbon dioxide gas. Thermal decomposition is the term given to splitting up a compound by heating it. All of these carbonates are white solids, and the oxides that are produced are also white solids.

2. Thermal decomposition of calcium
carbonate
• Adapted from:
• http://www.rsc.org/learn-
chemistry/wiki/TeacherExpt:Thermal_decomp
osition_of_calcium_carbonate

3. Thermal decomposition of calcium
carbonate
• 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(S)

4. Bubbling carbon dioxide
• Thermal decomposition:
• CaCO3(s) → CaO(s) + CO2(g)
• The reaction is highly exothermic and the
small amount of water added is partly
converted to steam in the process:
CaO(s) + H2O(l) → Ca(OH)2(s)
• Lastly
• Ca(OH)2(s) + CO2(g) -> CaCO3(s) + H2O(l)

5. Thermal decomposition of calcium
carbonate

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

7. 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 ml drops of water
Blow Bubbles through solution
Add Universal indicator

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

9. What do you see?
• Add about 10 cm3 more water to the solid. What
happens now?
• 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?
• Test the remaining half of the mixture with
Universal Indicator solution.
• Write down what you observe.

10. Lets fill in chart. . . .
Method Observations
Heat for 10 min
Add 2-3 drops of water
Add 10 ml drops of water
Blow Bubbles through solution
Add Universal indicator

11. Practical applications
• This set of experiments involves a variety of
important reactions and types of
reactions, with several references to industrial
processes.
• The roasting of limestone and the hydration
ofthe quicklime formed has relevance in the
manufacture of plaster and cement, and in the
laboratory limewater is a common reagent for
the testing of carbon dioxide.

12. Answers to questions
Why does the chalk crumble slightly on strong heating?
• Carbon dioxide/a gas is evolved; this forces its way out
of the solid and breaks down its structure.
What type of reaction is taking place during the heating
process? Write an equation for the reaction.
• Thermal decomposition; CaCO3(s) → CaO(s) + CO2(g)
Why is steam evolved when drops of water are added?
Write an equation for the reactionoccurring.
• The reaction is highly exothermic and the small amount
of water added is partly converted to steam in the
process: CaO(s) + H2O(l) → Ca(OH)2(s)

13. Answers to questions
Why does the limewater turn cloudy? Write an
equation for the reaction which is occurring.
• Insoluble calcium carbonate is being precipitated:
Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
What does the colour change occurring when
limewater is added tell you about the pH of the
solution? Explain why the pH would be expected to
have this value.
• The pH is about 11 - 14; soluble metal hydroxides
are alkaline and therefore give high pH values

14. Answers to questions
• Reference
• This experiment has been reproduced from
Practical Chemistry:
• http://www.practicalchemistry.org/experimen
ts/intermediate/materials/thermal-
decompositionof-
• calcium-carbonate,282,EX.html

15. Differential thermal analysis