Chlorine cannot be collected over water because it is highly soluble in water. When chlorine gas comes into contact with water, it reacts to form a mixture of hydrochloric acid (HCl) and hypochlorous acid (HClO) through the following reaction:
\[ \text{Cl}_2 (g) + \text{H}_2\text{O} (l) \rightarrow \text{HCl} (aq) + \text{HClO} (aq) \]
Since chlorine gas dissolves readily in water and reacts with it, attempting to collect chlorine over water would result in the gas quickly dissolving and reacting, leading to loss of chlorine and potentially dangerous conditions due to the release of acidic byproducts.
Moreover, the solubility of chlorine in water is significant; this means that even if you tried to capture the gas, a large portion of it would remain dissolved in the water rather than being collected as a gas. For these reasons, methods such as gas collection over mercury or using a gas jar with a less soluble gas are preferred when handling chlorine.