In the 18th century, travellers to southern Italy could take part in a rather strange experiment. It happened at the Grotto del Cane, the Cave of the Dog, near an active volcano. When the tourist entered the cave, accompanied by the guide and his unsuspecting dog, the dog fell over, senseless. To revive it, the guide threw the dog into the lake.

The story provides an introduction to carbon dioxide and, after their experiments, pupils should be able to explain why the dog collapsed but the people were unaffected.

Volcanoes release large quantities of carbon dioxide and it also seeps through the ground into nearby caves. The gas is more dense than air.

Prepare some carbon dioxide by mixing a carbonate with an acid. For example, try washing soda with citric acid or chalk with strong vinegar.

If the container with the effervescent mixture is tipped, the heavy but invisible gas can be poured into another jar. Join two plastic cups with a light thread and balance them over a pulley wheel. Tip carbon dioxide into one cup and that cup will descend.

Alternatively, set up a balance with plastic cups at each end of a thin wire, pivoted in the centre. As carbon dioxide fills one cup, the balance tilts down on that side. After a few minutes, the gas diffuses into the air and the balance returns to its original position.

Children will now understand that any carbon dioxide seepage in the cave is likely to collect near the ground. It will affect the dog but not taller people.

Prepare some carbon dioxide by placing the reaction mixture at the base of a tall jar. Light a wax taper and slowly lower it into the jar. When the flame enters the carbon dioxide it is extinguished. Relate this to the use of the gas in fire extinguishers for electrical fires, where the water should be avoided.