Thermal decomposition, or thermolysis, is a chemical decomposition caused by heat. The decomposition temperature of a substance is the temperature at which the substance chemically decomposes. The reaction is usually endothermic as heat is required to break chemical bonds in the compound undergoing decomposition. If decomposition is sufficiently exothermic, a positive feedback loop is created producing thermal runaway and possibly an explosion.

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