Answer:
Because of the solubility of ammonia gas in water, it is not collected over water.
Step-by-step explanation:
Gases prepared in the laboratory are often collected by a technique called water displacement. A bottle is filled with water and placed upside-down in a trough or container of water. The reaction flask is connected to the bottle of water by means of a rubber or glass tubing which is fed under the water bottle . As the gas is produced in the reaction flask, it enters the bottle and displaces the water in it. When the bottle is full of the gas, it is then sealed. The volume of gas in the bottle can be used to determine the amount of gas produced in the reaction. Usually, the gas collected is present in addition to water vapor in the bottle.
Gases that are collected over water must not react with or dissolve in water.
Ammonia gas is very soluble in water. The solubility is due to the readiness with which it forms hydrogen bonds with water to produce aqueous ammonia, NH₃.H₂O.
Aqueous ammonia ionizes partially to produce ammonium ions, NH₄+ and hydroxide ions, OH-. Because of the solubility of ammonia gas in water, it is not collected over water.