Here's why:

* Dissolution: When HCl gas comes into contact with water, it readily dissolves to form hydrochloric acid (HCl(aq)). This means the HCl gas would simply disappear into the water, making it impossible to collect.

* Reaction: The dissolution of HCl in water is an exothermic reaction, meaning it releases heat. This heat can cause the water to boil, further hindering the collection process.

Instead of collecting HCl over water, we use the following methods:

* Downward displacement of air: Since HCl is denser than air, it is collected by downward displacement of air in a gas jar.

* Collection over mercury: HCl is relatively insoluble in mercury, so it can be collected over a mercury trough. However, this method is rarely used due to the toxicity of mercury.

In summary: HCl's high solubility in water makes it unsuitable for collection over water, as it dissolves and reacts, hindering the collection process.