Class practical

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.

Lesson organisation

This experiment can be carried out conveniently in groups of two or three and takes about 40 - 45 minutes.

Apparatus Chemicals

Eye protection



Bunsen burner


Boiling tubes, 2 (Note 1)

Drinking straw (Note 2)

Dropping pipette

Filter funnel, small

Filter paper

Calcium carbonate

Universal Indicator solution (HIGHLY FLAMMABLE)

Refer to Health & Safety and Technical notes section below for additional information.

Health & Safety and Technical notes

Read our standard health & safety guidance

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.


a You need to prepare a tabulated results sheet before you start your experiments. An example table is provided below:



Heat for 10min


Add 2-3 drops of water


Add 10cm3 more water


Blow bubble through the 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.

Teaching notes

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 10min

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 10cm3 more water

Some of the solid dissolves, white suspension

Blow bubble through the 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.

Some question and answers for the class after the experiment:

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 2016


This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.

© Nuffield Foundation and the Royal Society of Chemistry

Page last updated October 2015