Ammonia, NH3, is a colourless gas with a sharp smell. It is the only common alkaline gas, and that is what the test uses.
To test for it, hold a piece of damp red litmus paper at the mouth of the test tube, without letting it touch the liquid. If ammonia is present, the red litmus paper turns blue.
The paper must be damp because the ammonia has to dissolve in the water on the paper. This makes an alkaline solution, and it is the alkali that changes the colour of the litmus. Dry litmus paper would not change colour.
The test is used to confirm the ammonium ion. When an ammonium compound is warmed with sodium hydroxide solution, ammonia gas is given off, and it turns damp red litmus paper blue. Never smell the gas directly, because it is irritating to the lungs.
