Illustrate the presence of water and carbon dioxide in gaseous combustion products of a solid hydrocarbon by drawing the products over a piece of cobalt chloride paper and through limewater.

Demonstration

In this demonstration a solid hydrocarbon burns and a pump is used to draw the gaseous combustion products over a piece of cobalt chloride paper and through limewater to show the presence of water and carbon dioxide respectively.

Lesson organisation

With this demonstration, the apparatus can be left running for some time and students can file past in small groups to see it more closely. Alternatively a flexicamera can be used - linked to a projector.

If students are not familiar with the cobalt chloride paper and limewater tests, either demonstrate these separately or allow students to try the tests themselves.

Assuming everything is already set up this demonstration takes only a few minutes.

Apparatus Chemicals

Eye protection

Glass funnel, about 6 cm in diameter

Boiling tubes, 2

Two-holed rubber bungs, 2, to fit the boiling tubes, and fitted with one long and one short piece of glass tubing (see diagram)

Glass or plastic tubing for connections (Note 1)

Filtering pump (Note 2)

Tea light, night light or candle

Piece of blue cobalt chloride paper (TOXIC)

Limewater (treat as IRRITANT), about 20 cm3

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. 

Cobalt chloride/cobalt chloride paper (TOXIC, DANGEROUS TO THE ENVIRONMENT) - see CLEAPSS Hazcard and CLEAPSS Recipe Book. Cobalt chloride paper can be stored in a desiccator. Minimise handling of cobalt chloride paper (Sensitiser) and wash hands after use (cobalt chloride is a category 2 carcinogen). See Standard Procedure: Preparing and using cobalt chloride indicator papers.

Calcium hydroxide solution, ‘limewater’, Ca(OH)2(aq), (treat as IRRITANT) - see CLEAPSS Hazcard and CLEAPSS Recipe Book. Ideally, the limewater should be made fresh on the day. 

1 Care should be taken with the right-angle bend connected to the funnel. If this is made of flexible tubing, it can get hot and melt.

Ideally, the glass stem of the funnel should be bent into a right-angle. Alternatively, join a standard funnel onto a right angled piece of glass tubing using epoxy resin. A more temporary arrangement is to slide one arm of a right-angled piece of glass tubing inside the stem of the funnel and seal the join on the outside with a piece of flexible tubing (see diagram). 

2Filter paper: Information about the type and use of various grades of filter papers can be found in section 9.11.4 of the CLEAPSS Laboratory Handbook. Filter pumps: The use of traditional water-operated filter pumps for vacuum filtration and for drawing air through solutions is covered in section 10.6.4 of the CLEAPSS Laboratory Handbook. It is strongly recommended that this is referred to before purchasing or using such pumps – it may not be possible or appropriate to use this type of equipment in your school or college. Alternative means of carrying out vacuum filtration and drawing air through solutions are suggested in this section of the Laboratory Handbook.

Procedure

a Before the demonstration, assemble the apparatus as shown in first diagram. Ensure that the connections to the boiling tubes are the correct way round.

b Place a piece of blue cobalt chloride paper into the first boiling tube and half-fill the second boiling-tube with limewater.

c At the start of the demonstration, turn on the pump so that a gentle stream of air is drawn through the apparatus.

d Light the tea light and leave for a few minutes until the cobalt chloride paper turns pink (from blue) and the limewater goes milky (produces a white precipitate). This indicates the presence of water and carbon dioxide respectively.

Teaching notes

Some students will know that air contains both water vapour and carbon dioxide. To show that the changes observed are not due to these alone, repeat the experiment without the tea light and note how much longer it takes for any changes to be observed.

Understanding the process of burning is important at all levels of chemistry. Emphasis that burning in air is a reaction with oxygen. The elements hydrogen and carbon are present in hydrocarbons, such as candle wax. Students will quite readily appreciate that carbon reacts with oxygen to form carbon dioxide, but often need help to grasp that hydrogen combines with oxygen to form water.

The production of carbon dioxide could lead to discussion of the role of this gas in the greenhouse effect.

The experiment could be extended to burning alcohols with a spirit burner.

Health & Safety checked, July 2016 

Credits

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

© Nuffield Foundation and the Royal Society of Chemistry

Related resource and standard techniques

Preparing and using cobalt chloride indicator papers.

Page last updated July 2016