Identifying the products of combustion

Read our standard health & safety guidance

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 and 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)
Pump, see Standard procedure: Filter pumps.
Glass or plastic tubing for connections (see note 3)
Candle

Piece of blue cobalt chloride paper (Toxic) (see note 1)
About 20 cm³ of limewater (treat as Irritant) (see note 2)

Apparatus to identify the products of combustion

Technical notes

Cobalt chloride/cobalt chloride paper (Toxic, Danger to the environment) (see note 1) Refer to CLEAPSS Hazcard 25 and CLEAPSS Recipe card 46

Calcium hydroxide solution, ‘limewater’ (treat as Irritant) (see note 2) Refer to CLEAPSS Hazcard 18 and CLEAPSS Recipe card 15

1 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: Make your own cobalt chloride paper.

2 Ideally, the limewater (a saturated solution of calcium hydroxide, Ca(OH)₂) should be made fresh on the day.

3 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.

Temporary right-angled bend

4 A ‘tealight’ or ‘nightlight’ is more squat and so is more stable than a table candle.

Procedure

HEALTH & SAFETY: Wear eye protection

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

Making and using cobalt chloride paper