Hydrogen gas can be prepared in the Laboratory. Explain the following:

(i)                 Why is the gas collected by the downward displacement of water.

(ii)               Why hydrogen gas is not collected by air?

Answer:

(i)  Why is the gas collected by the downward displacement of water.

Because hydrogen is denser than water, it is gathered by displacing water downward. As a result, it rises to the surface of the water. It does not dissolve in water because it is insoluble in water.

(ii) Why hydrogen gas is not collected by air?

Although hydrogen is lighter than air, it still cannot be collected by the downward displacement of air. It is because hydrogen gets oxidized easily in presence of air. This means that hydrogen can easily mix with air and thus, cannot be collected by this method. 

Furthermore, this method is dangerous as the air and hydrogen mixture is explosive.

The hydrogen gas is highly combustible and gets oxidized when comes in contact with oxygen. It produces water and has a faint blue flame.
As a result, collecting hydrogen gas in an air medium is impossible.

And yes, we may prevent it from catching fire, but then it may also become combined with other air contaminants, making it difficult to obtain pure hydrogen gas (dihydrogen).

References:

https://www.topperlearning.com/answer/why-do-we-collect-the-hydrogen-gas-by-downward-displacement-of-water-why-not-by-air/t1wpu11

https://www.vedantu.com/question-answer/hydrogen-is-collected-by-downward-displacement-class-11-chemistry-cbse-5f17dabfe0640e6e829c30d8