Final answer:
Observations of a reaction can include the reduction of vanillin, the formation of a precipitate, and a change in temperature.
Step-by-step explanation:
One observation of a reaction occurring is the reduction of vanillin. Reduction is a chemical reaction that involves the gain of electrons or the decrease in oxidation state. In the case of vanillin, it can be reduced to vanillyl alcohol by the addition of reducing agents such as sodium borohydride. The reduction process can be observed by monitoring the color change from pale yellow to colorless as vanillin is converted to vanillyl alcohol.
Another observation of a reaction occurring is the formation of a precipitate. Some reactions may produce a solid product that is insoluble in the reaction mixture, resulting in the formation of a precipitate. This can be seen as a visible change in the appearance of the solution, with the formation of solid particles that settle at the bottom or float in the mixture.
Additionally, a change in temperature can be an indication of a reaction occurring. Some chemical reactions are exothermic, meaning they release heat energy, while others are endothermic and absorb heat energy. Monitoring the temperature during a reaction can help confirm if a reaction is occurring, as a significant increase or decrease in temperature may suggest a chemical change is taking place.