Q1.
Predict whether the water level will increase or decrease with respect to the mark B.
Answer
Prediction: it will decrease. After the sugar dissolves, the final level C settles below B — and, as Fig. 7.10d shows, it still stays a little above A.
A = level of the water alone
B = level after the sugar is added but before it dissolves
C = level after the sugar has dissolved
A < C < B, so the level falls from B to C
B = level after the sugar is added but before it dissolves
C = level after the sugar has dissolved
A < C < B, so the level falls from B to C
Why the prediction works: at mark B, the sugar is sitting as a heap at the bottom, taking up its own room in the vessel and pushing the water up. Once it dissolves, its particles separate and move into the interparticle spaces that already exist between the water particles — space that was not being counted before. So the volume of the solution turns out to be less than the volume of the water plus the volume of the sugar, and the level comes down.
Why C does not fall all the way back to A: there are not enough vacant spaces for every single sugar particle, so the solution still takes up slightly more room than the water did alone.