Question

difference between downward displacement of water and downward displacement of air

Answer 1

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→upward displacement of air - collection of a gas/liquid below air (i.e. by displacing air upwards)

→downward displacement of water - collection of an immiscible liquid/gas above the surface of water (i.e. by displacing water downward)

→Collection of hydrogen is not possible by upward displacement of air because hydrogen-air mixture is combustible. It is only possible by downward dispacement of water because hydrogen does not dissolve in water

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