 We're going to look at an acid-base reaction between HCl gas and ammonia gas. There's still a transfer of a hydrogen ion from the acid HCl to the base ammonia. It will produce ammonium chloride, which is a solid, which we'll see as smoke coming out of the tube. I'm going to add some concentrated aqueous ammonia to one of the flask. We're going to add some concentrated hydrochloric acid to the other flask. Most of our studies with acids and bases are going to be in solution, but you do not need to have an aqueous solution to have an acid-base reaction.