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Notes on Observations of a Chemical Reaction
Stating that an observation of a chemical reaction is the substance has changed one or more of its properties (e.g. a new substance is produced) is not an observation of a chemical reaction. It is not an observation. Observations are statements from your five senses. So, what are the four observations of a chemical reaction.
1. A permenant color change.
2. Gas is generated. Gas generated can be odorless or has an odor (so can't say gas formed and odor as two observations)
3. Energy change. Either exothermic (energy/heat released) or endothermic (energy/heat absorbed) reaction.
4. Precipitate formed. Precipitation is a solid formed in a liquid (or more correctly solution).
For a chemical reaction to occur, at least one (but can any number) of these observation must occur.