The displacement done by collecting pure hydrogen in the laboratory is typically referred to as "water displacement." This method is often used for collecting gases that are less dense than air, such as hydrogen.
In this technique, the hydrogen gas is generated in a reaction (commonly through the reaction of an acid with a metal like zinc or through electrolysis of water) and it is channeled into an inverted container (like a gas jar or test tube) that is filled with water. As the hydrogen gas enters the container, it displaces the water, allowing for the collection of the gas while preventing any backflow of water.
This method is effective for collecting hydrogen because it ensures that the gas is not mixed with other gases in the atmosphere, and it allows for measurement of the volume of hydrogen collected.