Any substance that is made up of only one type of atom or molecule is said to be a pure single substance whereas substance that is made up two or more different elements or molecule is said to be a mixture.
Air: Different types of gases are present in air that is nitrogen, water vapors, oxygen etc. So, air is a mixture.
Ammonia gas: It consists of only one molecule namely ammonia throughout the substance so it is a pure single substance.
Hydrochloric acid: It is made up by dissolving hydrochloric gas in water so it is a mixture.
Salt water: It is mixture as water and salt are being mixed which are two different substances.
Hence, ammonia gas is a single substance.
