Question

The potential energy of a molecule on the surface of a liquid compared to one inside the liquid is,
A. zero
B. lesser
C. equal
D. greater

Answer 1

The potential energy of molecules inside the liquid is due to the interaction of the molecules with the neighbouring atoms. Find out which molecules have more attractive interactions with the neighbouring atoms. From this we can find which molecule will have more potential energy.

Complete answer:
The molecules at the surface of a liquid have different potential energy then the molecules in the bulk. Potential energy of the molecules in a liquid can be defined in terms of the interaction of the molecules with the surrounding molecules.

The molecules in the bulk of a liquid have a higher number of surrounding molecules with whom they have attractive interaction. But the molecules at the surface of a liquid have lower numbers of neighbouring atoms because they do not have any molecules above them compared to the molecules in the bulk which have surrounding molecules in every direction.

Now, with the increasing number of surrounding or neighbouring molecules, the attractive interaction of the molecule will also increase. But at the surface we have a lesser number of surrounding atoms and so the attractive interaction for the surface molecules will also be less. Hence, the molecules need to do work to go from the bulk of the liquid to the surface of the liquid where it will have lower energy. So, the potential energy of the molecules in the surface of the liquid will increase due to this work done.

So, we can say that the potential energy of a molecule on the surface of a liquid compared to one inside the liquid is greater.

So, the correct answer is “Option d”.

Note:
Tension can be defined as a property of liquids that arises due to the difference in potential energy of the molecules in the surface of the liquid and in the bulk of the liquid. The liquid drop is spherical because of the surface tension of the liquids or we can say that the spherical shape has the minimum surface energy.