Question

Why is it said that inside a liquid, the attraction due to surrounding molecules results in a negative potential energy for a molecule ?

Answer 1

This is really a consequence of how we define the reference point of the potential energy. Typically atoms like to be close together because the electrons of one atom are attracted to the protons of the other. However, since we define the 0 of the potential energy between the two as the atoms being infinitely far apart, moving the atoms any closer reduces the potential energy between the two atoms and thus put the system at a negative potential energy.

Hope this helps :)