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Thermal decomposition of calcium carbonate

• Adapted from:

• http://www.rsc.org/learn-chemistry/wiki/TeacherExpt:Thermal_decomposition_of_calcium_carbonate

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)

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)

Thermal decomposition of calcium carbonate

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.

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

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.

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.

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

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.

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)

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

Answers to questions

• Reference

• This experiment has been reproduced from Practical Chemistry:

• http://www.practicalchemistry.org/experiments/intermediate/materials/thermal-decompositionof-

• calcium-carbonate,282,EX.html

Differential thermal analysis