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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