Surface Energy - Practice Questions & MCQ

The molecules on the liquid surface experience net downward force. And because of this force, these molecules tend to move downwards. So to fill the space we need to bring a molecule from the interior of the liquid to the free surface. And to do this some work is required to be done against the intermolecular force of attraction. This work will be stored as the potential energy of the molecule on the surface. 

And this stored potential energy of surface molecules per unit area of the surface is called surface energy.

Surface energy is also defined as the amount of work done in increasing the area of the surface film through unity.

$
\begin{aligned}
& \text { I.e surface energy }=\frac{\text { work done in increasing the surface area }}{\text { increase in surface area }} \\
& \text { or surface energy }=\frac{W}{\Delta A} \ldots \text { (1) }
\end{aligned}
$

Where $W \rightarrow$ work done

$
\text { and } \Delta A \rightarrow \text { increase in area }
$

And work done in increasing the surface area is given by

$
W=T \times \Delta A \ldots . .(2)
$

where $T \rightarrow$ Surface tension

$
\text { and } \Delta A \rightarrow \text { increase in area }
$

So we rewrite equation (2) as

$
T=\frac{W}{\Delta A} \ldots(3)
$

So we can also define surface tension as the amount of work done in increasing the area of the liquid surface by unity against the force of surface tension.
Or we can say that the surface tension of a liquid is numerically equal to its surface energy.
As

$
W=T \Delta A
$

If $\Delta A=1$, then $T=W$
$T \rightarrow$ Surface tension