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Question
Describe an experiment to demonstrate that air exerts pressure.
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Solution
Experiment to demonstrate that air exerts pressure:
Take a thin can fitted with an airtight stopper. The stopper is removed and a small quantity of water is boiled in the can. Gradually the steam occupies the entire space of can by expelling the air from it [Fig (a)]. Then stopper is then tightly replaced and simultaneously the flame beneath the can is removed. Cold water is then poured over the can.
It is observed that the can collapses inwards as shown in fig (b).
The reason is that the pressure due to steam inside the can is same as the air pressure outside the can [Fig (a)]. However, on pouring cold water over the can, fitted with a stopper [fig (b)], the steam inside the can condenses producing water and water vapour at very low pressure. Thus, the air pressure outside the can becomes more than the vapour pressure inside the closed can.
Consequently, the excess atmospheric pressure outside the can causes it to collapse inwards.
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