|MadSci Network: Chemistry|
Ammonia is a base. When ammonia (which has a chemical formula of NH3) dissolves in water (chemical formula H2O) it forms a solution of ammonium hydroxide (NH4OH). Ammonium hydroxide is a chemical relative of other bases like sodium hydroxide (lye). I don't know the details of the chemistry of the cabbage juice indicator you were using. Ammonia can undergo reactions in a different manner than other typical bases. I suspect that the cabbage juice has a slightly different reaction with ammonia to give a green color instead of the blue given by other bases.
If you want to investigate this further:
I would suggest checking on the web to see if others have observed a green color when the cabbage juice indicator is exposed to ammonia. It may also be useful to check in your library to see if this is discussed in elementary school books about science or chemistry. Books that talk about the cabbage juice indicator as a science project may discuss this observation. Finally, the chemistry faculty at a local college may be of help (Northwestern Univ. or U. of Ill. Urbana-Champaign in your case).
Dan Berger adds:
Ammonia is a weak base; this means that very little of it is present in water as "ammonium hydroxide" NH4OH. The cabbage juice indicator is responding to pH, and it may be that the pH of ammonia water is not low enough to give a nice blue color. Green is a common indicator color for "intermediate" pH, between perhaps 6 and 9.
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