Question

Why do molecules of liqid lying in surface flim have extra energy?

Answer 1

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A molecule on the surface of a liquid experiences a downward pull due to surface tension. ... Thus the molecules in surface film possess extra energy. This energy is called as surface energy.

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Answer 2

Answer:

Surface energy: The surface energy is defined as the extra (or increased) potential energy possessed by the molecules in a liquid surface with an isothermal increase in the surface area of the liquid. A liquid exerts a resultant cohesive force on every molecule of its surface, trying to pull it into the liquid. To increase the surface area, it is necessary to bring more molecules from inside the liquid to the liquid surface. For this, external work must be done against the net cohesive forces on the molecules. This work is stored in the liquid surface in the form of potential energy.

This extra potential energy that the molecules in the liquid surface have is called the surface energy. Thus, the molecules of a liquid in the surface film possess extra energy

Dimensions:

[surface energyl=[ML²T ^-2]

SI unit: the joule (J).