Ammonia gas (NH3) is not collected over water because it has a high solubility in water. This means that a large amount of ammonia gas will dissolve in water, making it difficult to collect the gas in a pure form. Additionally, ammonia gas reacts with water to form ammonium hydroxide (NH4OH), which is a corrosive and toxic substance. Therefore, it is important to collect ammonia gas over a drying agent, such as calcium chloride (CaCl2), which will absorb the water vapor and prevent the formation of ammonium hydroxide.