$A$ thin flat circular disc of radius $4.5 \,cm$ is placed gently over the surface of water. If the surface tension of water is $0.07 \,N \,m^{-1}$, then the excess force required to take it away from the surface is

Give a reason why some insects are able to walk on the water surface.

Why are clothes easily washed by soap or detergent?

Two glass plates of area $10^{-2} \,m^2$ have a $10 \,cm$ thick water film between them. The surface tension of water is $70 \times 10^{-3} \,N/m$. The force required to separate the two glass plates from each other is (in $\,N$)

Surface tension is exhibited by liquids due to the force of attraction between the molecules of the liquid. The surface tension decreases with an increase in temperature and vanishes at the boiling point. Given that the latent heat of vaporization for water $L_v = 540 \text{ kcal/kg}$,the mechanical equivalent of heat $J = 4.2 \text{ J/cal}$,density of water $\rho_w = 10^3 \text{ kg/m}^3$,Avogadro's number $N_A = 6.0 \times 10^{26} \text{ molecules/kmol}$,and the molecular weight of water $M_A = 18 \text{ kg/kmol}$.
$(a)$ Estimate the energy required for one molecule of water to evaporate.
$(b)$ Show that the intermolecular distance for water is $d = \left( \frac{M_A}{N_A \rho_w} \right)^{1/3}$ and find its value.
$(c)$ $1 \text{ g}$ of water in the vapour state at $1 \text{ atm}$ occupies $1601 \text{ cm}^3$. Estimate the intermolecular distance at the boiling point in the vapour state.
$(d)$ During vaporisation,a molecule overcomes a force $F$,assumed constant,to go from an intermolecular distance $d$ to $d'$. Estimate the value of $F$.
$(e)$ Calculate $\frac{F}{d}$,which is a measure of the surface tension.

On what does the value of surface tension depend? Explain.