The candle in the bell jar

‘The candle in the bell-jar’ was given in the article as a way of showing children how oxygen can be used up in a room that is sealed too tightly. 

This experiment may take two hours, or longer.

Equipment

Materials per group

• Candles (the students may decide that they would like to experiment with different heights and thicknesses).

Equipment per group

• Bell jar or large glass beaker
• Beehive shelf
• Glass trough
• Matches
• General laboratory ware
• Safety glasses

Health, safety and technical notes

• Read our standard health and safety guidance here. 
• Wear eye protection.
• This is an open-ended problem-solving activity, so the guidance given here is necessarily incomplete.
• Keep away from flammable/combustible materials.

Commentary

The first part of this challenge is to work out the three hypotheses given in the Student Sheet for this phenomenon: 

1. The rise in the water level is caused by the consumption of oxygen alone
2. The rise in the water level is caused by a combination of the oxygen consumed, and the carbon dioxide released
3. The rise in the water level is caused by the contraction of gases in the bell jar as they cool.

Each of these hypotheses gives rise to several lines of investigation. Students could, for instance, design a system for lighting the candle inside an enclosed volume of air to give the most accurate assessment of the rise in the water level. They might try introducing a layer of oil above the water to prevent carbon dioxide from dissolving, to see if this affects the rise in the water level.

The candle could be replaced by a spirit burner so that the amount of fuel burned can be used to calculate the volume of oxygen consumed (and carbon dioxide released). An alternative method of heating the gases inside the bell jar to the same temperature as the lighted candle can be tried, to assess the rise in water level on cooling.

A critical appraisal of the experiment was published in 1967 by Richard Kempa who showed that the combustion of a candle cannot be supported by an atmosphere containing less than about 14 per cent oxygen by volume. He suggested that the experiment may be used to introduce children to the idea that oxygen does not support combustion if it is insufficiently “concentrated”. 

Notes

This resource is part of a collection of problem-solving activities, designed to engage learners in small group work. Find out how to use these resources, and obtain a list of suggested ‘junk items’ here.