Final answer:
To find out why the temperature decreased when citric acid and baking soda were mixed, focus on their endothermic reaction and avoid irrelevant factors like the color of the solution.
Step-by-step explanation:
The student poured a citric acid solution into a polystyrene foam cup and then stirred in a baking soda solution, which led to a temperature change from 23.4 Celsius degrees to 17.5 Celsius.
To determine why the temperature decreased, one should consider the endothermic reaction between baking soda (sodium bicarbonate) and citric acid, which absorbs heat from the surrounding solution and surroundings, resulting in a temperature drop.
Questions that could help determine why the temperature dropped might include asking about the concentration of the reactants, the specific heat capacity of the solution, and whether the reaction is known to be endothermic or exothermic. However, a question such as "What was the initial color of the citric acid solution?" would not be useful in determining why the temperature changed, as it has no relevance to the thermodynamics of the reaction.