Downward displacement of air is a method used to collect gases that are less dense than air, such as hydrogen (\(\rm H_{2}\)) or ammonia (\(\rm NH_{3}\)). In this technique, the gas is allowed to flow into a container from the top, pushing the lighter air downward and out of the container. Since the collected gas is lighter than air, it rises and accumulates at the top of the container.

This method is commonly employed in laboratories to efficiently collect gases without losing them to the atmosphere. It takes advantage of the differences in density between the gas being collected and the surrounding air.