what are upward displacement of air, downward displacement of water and why are they not used to collect hydrogen
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.