Hydrogen chloride gas is collected by upward displacement of air.
The method for preparing and collecting a gas less dense (lighter) than air by heating solid reactants. The less-dense gas extends into, and displaces, the more dense air downwards. This method of gas preparation is called upward delivery or displacement.
Hydrogen chloride gas is collected by the downward delivery (upward displacement of air) as it is 1.28 times heavier than air. It is not collected over water, since it is highly soluble in water.