Question

though hydrogen is lighter than air it cannot be collected by downward displacement of air.why?

Answer 1

Even though hydrogen is lighter than air it is not collected by downward displacement of air because of these reasons:

Hydrogen  gets oxidised when it is exposed to oxygen which makes it burn with a pale blue flame and the residue is water. This means that it mixes with oxygen.

Downward displacement of air requires a gas that does not mix with air and is lighter than air. Hydrogen is lighter than air but mixes with air.

Answer 2

Answer:

Although the hydrogen is lighter than air but then also cannot be collected by downward displacement of air and this is due to the fact that hydrogen gets oxidized easily in presence of air, this means that hydrogen can easily mix with their and thus cannot be collected by this method.