Decomposition of calcium oxide

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.

Chemicals

Calcium carbonate

Universal Indicator solution (HIGHLY FLAMMABLE)

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

Apparatus

Eye protection

Tripod

Gauze

Bunsen burner

Tongs

Boiling tubes, 2 (Note 1)

Drinking straw (Note 2)

Dropping pipette

Filter funnel, small

Filter paper

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.

Procedure

a You need to prepare a tabulated results sheet before you start your experiments.

Method Observation
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.

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:

The chalk should be seen to crumble slightly
More crumbling, steam given off, evidence that mixture has become hot
Some of the solid dissolves, white suspension
Limewater turns cloudy.
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.

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