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. Students are also asked to research the large-scale applications of these processes.
This experiment can be carried out conveniently in groups of two or three and takes about 40 - 45 minutes.
Universal Indicator solution (HIGHLY FLAMMABLE)
Refer to Health & Safety and Technical notes section below for additional information.
Boiling tubes, 2 (Note 1)
Drinking straw (Note 2)
Filter funnel, small
Health & Safety and Technical notes
Wear eye protection.
Calcium carbonate, CaCO3(s) - see CLEAPSS Hazcard. 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.
Universal indicator solution (HIGHLY FLAMMABLE) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.
1 Use large (150 x 25 mm) test-tubes (boiling tubes).
2 Freshly purchased drinking straws should be used and each student issued with their own straw.
|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.
Keep an eye on less mature students who might be tempted to suck rather than blow through the filtrate.
The results expected are as follows:
|Heat for 10 min||The chalk should be seen to crumble slightly|
|Add 2 – 3 drops of water||More crumbling, steam given off, evidence that mixture has become hot|
|Add 10 cm3 more water||Some of the solid dissolves, white suspension|
|Blow bubbles through solution||Limewater turns cloudy.|
|Add Universal Indicator||Indicator goes from green to blue/purple|
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 of the 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. Students could be asked to carry out web research on these applications.
Student questions and answers
Here are some questions for your students, with answers
1 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.
2 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)
3 Why is steam evolved when drops of water are added? Write an equation for the reaction occurring.
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)
4 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)
5 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
Health & Safety checked August 2008
Science is Fun - a detailed description of some of the reactions and commercial uses of calcium carbonate
Page last updated on 31 July 2012