Question

Hydrogen is collected by the ------ displacement of --------?

Answer 1

Hydrogen is the most abundant element in the universe. Hydrogen has three isotopes protium, deuterium, tritium out of which protium is the most common. Frozen water is heavier than liquid water, as there is no hydrogen bonding in the frozen state of water.

Complete answer:
Most of the gases that are produced in the laboratory are generally collected by a technique called water displacement as they are insoluble in water.
Hydrogen fuses with air but does not dissolve in water, it is insoluble in water. If one of the gas is lighter than another gas, then the first gas can be collected by the downward displacement of the other gas.
Hydrogen is collected by downward displacement of water as hydrogen is less dense than water. The water displacement method is used for collecting gases that are insoluble in water.
Downward displacement of water is done by collecting an immiscible liquid/gas above the surface of the water by displacing water downward.
This can be demonstrated by an experiment:
Take a few grams of zinc granules and place them in a Woulfe’s bottle, with the help of thistle funnel dropwise pour dilute sulphuric acid on granulated zinc pieces.
The hydrogen gas evolved to be collected over water by downward displacement of water.
The reaction involved in the experiment is $Zn{\text{ + }}{{\text{H}}_2}S{O_4} \to ZnS{O_4} + {H_2}$.
So the answer to the question is Hydrogen is collected by downward displacement of water.

Note:
Hydrogen is highly combustible and is difficult to handle safely. Because of its combustible nature, it burns with the explosion. Hydrogen is lighter than air but the hydrogen can be oxidized in the presence of air, so it cannot be collected by downward displacement.