Potential energy of molecules

Q: 咩為之mean separation呢?同separation有咩分別?
You need to realize that not all molecules have the same states of motion. The kinetic energy and potential energy of molecules follow a distribution, i.e. some molecules possess higher kinetic and potential energy and some molecules possess lower kinetic and potential energy. Hence, those with higher potential energy are separated farther apart from each other, whereas those with lower energy are closer to each other. The "mean separation" is a representation of the separation of the molecules at a certain temperature.

Q: 咁佢既potential energy of molecules又有冇增加到呢?點解增加/冇增加到呢?
Yes, the potential energy of molecules increases with rise of temperature. This is the reason why we observe a liquid expands in volume when heated. The heat energy absorbed by the molecules increases both the kinetic and potential energies of molecules, thus increasing the separation between molecules. The liquid is then seen to expand.

Q: 書話只有state changes(由液體變氣體)先會有增加,點解同一個state係冇呢?
State change gives a drastic increase of potential energy (with the kinetic energy keep unchanged). During boiling, molecules in a liquid acquire enough energy to free from the intermolecular force and become completely free to move around.

Please also refer to the answer in the second question. An obervation of the expansion of a liquid when heated is a simple proof that the separation between molecules, thus the potential energy, increases when its temperature rises. The statement in your book is not completely right.