Question 2
Why is water added up to the level of the rubber band in activity 17.3?
The right answer is the one marked above.
To make the total volume the same in all three tubes, so that concentration is the only thing that differs. Without it the volume would vary too, and we could not say what caused the result.
The water is not a reactant here — it is only the solvent. Adding it does not change the number of acid molecules; it changes only the volume they are spread through.