Hydrogen chloride (HCl) gas cannot be collected over water because it reacts with water to form hydrochloric acid (HCl(aq)). The reaction between HCl gas and water is a highly exothermic process, meaning that it releases a large amount of heat. This can cause the water to boil and splatter, which can be dangerous. Additionally, the HCl(aq) that is formed is corrosive and can damage equipment and materials.

For these reasons, it is important to collect HCl gas over a dry, non-reactive gas such as nitrogen or argon.