Question

which gases cant be collected by upward displacement of air and why​

Answer 1

Answer:

any gases can be collected dear...

Answer 2

Answer:

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.