Ammonia gas is highly soluble in water. It reacts with water to form ammonium hydroxide, which is a strong base. This reaction is highly exothermic, meaning that it releases a lot of heat. If ammonia gas were collected over water, the heat released by the reaction could cause the water to boil and the ammonia gas to escape.

In order to prevent this from happening, ammonia gas is collected by upward delivery. This means that the ammonia gas is directed upwards into a container that is filled with a dry, inert gas such as nitrogen or argon. The inert gas prevents the ammonia gas from coming into contact with water, and thus prevents the formation of ammonium hydroxide.

Additionally, collecting ammonia gas by upward delivery also helps to prevent the contamination of the gas with other substances. If ammonia gas were collected over water, it could absorb impurities from the water, such as dissolved minerals or organic matter. These impurities could then interfere with the subsequent use of the ammonia gas.

Therefore, ammonia gas is collected by upward delivery in order to prevent it from reacting with water and to prevent contamination of the gas.