Identifying the products of combustion

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

Lesson organisationWith this demonstration, the apparatus can be left running for some time and students can file past in small groups to see it moreclosely. 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 allowstudents to try the tests themselves.

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

Apparatus Chemicals

Eye protectionGlass funnel, about 6 cm in diameterBoiling tubes, 2Two-holed rubber bungs, 2, to fit theboiling tubes, and fitted with one longand one short piece of glass tubing(see diagram)Glass or plastic tubing forconnections (Note 1)Filtering pump (Note 2)

CandlePiece of blue cobaltchloride paper (TOXIC)Limewater (treat asIRRITANT), about 20 cm3

Refer to Health & Safetyand Technical notessection below foradditional 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 andCLEAPSS 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 andusing cobalt chloride indicator papers .

Calcium hydroxide solution, ‘limewater’, Ca(OH)2(aq), (treat as IRRITANT) - see CLEAPSS Hazcard and CLEAPSS RecipeBook. 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 andmelt.

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

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

Procedurea Before the demonstration, assemble the apparatus as shown in first diagram. Ensure that the connections to the boiling tubesare 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 candle 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 notesSome students will know that air contains both water vapour and carbon dioxide. To show that the changes observed are notdue to these alone, repeat the experiment without the candle 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 withoxygen. The elements hydrogen and carbon are present in hydrocarbons, such as candle wax. Students will quite readilyappreciate that carbon reacts with oxygen to form carbon dioxide, but often need help to grasp that hydrogen combines withoxygen 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, September 2014

CreditsThis 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 techniquesPreparing and using cobalt chloride indicator papers .

Page last updated October 2015