Identifying the products of combustion

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.

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). 

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 candle and leave for a few minutes until the cobalt chloride paper turns pink (from blue) and the limewater goes milky. 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 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. Emphasise 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 and Safety checked November 2006
 

Standard Techniques

Preparing and using cobalt chloride indicator papers