Question

The potential energy of a molecule on the surface of a liquid compared to the one inside the liquid is:
A. zero
B. lesser
C. Equal
D. Greater

Answer 1

Hint: The answer lies in the factors which determine the potential energy of the molecule in the liquid.
The factors are mainly the forces acting on the molecule in the liquid. In the liquid there are cohesive forces, which are the forces acting between the liquid molecules. Throughout the liquid mass, the molecules move randomly. The potential energy of a molecule, say M1 inside the liquid mass is because of the cohesive forces it overcomes on its path. The work done by the molecule to move against the cohesive forces is stored in the molecule itself in the form of potential energy.

Complete step by step solution:
Imagine a liquid mass in a container. The molecules throughout the body keep moving randomly. Suppose that the surface area of the container is increased. Surface area of the container increases, which implies that the molecules which were deep inside the liquid mass may get a chance to come to the surface.

The molecules from inside the liquid move to the surface, against all the cohesive forces acting against it during its journey from its initial position to the surface.
If a displacement is produced against a force, it means that work is done.
Here, the molecule has done some work against the cohesive force during its motion to the surface. This work done is stored in the molecule in potential form.

Thus, by the above explanation we can safely state that molecules on the surface of the liquid mass have more potential energy than that inside the liquid mass.

The right option is D, that is, the potential energy of molecules on the surface of the liquid is more than that inside the liquid.

Note: The student may compare the surface molecule with the inside molecule and on the basis of cohesive and adhesive forces acting on it, may come to the wrong conclusion.
Common mistakes- molecules on the surface have two sets of forces acting, one adhesive force (between the air molecules and water molecule) and one cohesive (between the water molecules).

The molecules inside the liquid have only cohesive forces acting on them and they may come to some wrong inference.

The point to note is that potential energy is only stored when work is done against forces.
If you compare molecules on the surface and that inside merely, then nearly no work is done against any forces so the potential comparison will bear no fruitful result.