Objective: To show that burning needs
oxygen Equipment: A plate, three candles, two drinking glasses
of same size and one with a bigger size. Introduction: At lower classes we teach that air contains
oxygen and burning needs oxygen. This statement can be supported
by this demo experiment. Procedure: Put a candle vertically in a plate. Light the
candle. The candle keeps on burning. Cover the burning candle by
an inverted glass. The candle goes off. Now explain that the
glass originally had air in it and when covered only that much
of air was made available to the candle. This air had some
amount of oxygen and when that was consumed, the candle went
off.
Now use two plates and put one candle each in them. Light them.
Take two glasses of quite different sizes (one may be a glass
and the other may be a glass jug), in the two hands and cover
the candles with these simultaneously. The candle in the smaller
glass goes off earlier than the one in the bigger glass. Explain
that the bigger glass contains more air and hence more oxygen.
Now put one candle in one plate and two candles in the other
plate. Light all of them. Take two glasses of the same size in
the two hands and simultaneously cover the burning candles in
the two plates. The single candle lasts longer than the double
candle. Explain that same amount of oxygen was available in the
two glasses but two candles together was consuming oxygen faster
than a single candle. Discussion: This is a slight modification of the famous
experiment given in most of the science textbooks for lower
classes to demonstrate that there is 21% oxygen in air. We
haven’t put water in the plate in this experiment.
When to do: Class 6 or so. Instrument cost and availability: Zero cost. Acknowledgement: Self designed
