A soap bubble has radius r.The surface tension of the soap film is T. The energy needed to double the radius of the bubble without change of temperature is

Surface area of bubble of radius r
${\text{=4πr}}^2$
Surface area of bubble of radius 2 r
$=4\mathrm{\pi }(2\mathrm{r}{)}^2=16{\mathrm{\pi r}}^2$
As the bubble has two surfaces, hence the total  increase in surface area 
$=2\times \left[16{\mathrm{\pi r}}^2-4{\mathrm{\pi r}}^2\right]=24{\mathrm{\pi r}}^2$
$\therefore \text{ Energy needed }=\text{ work done }=24{\mathrm{\pi r}}^2\mathrm{T}$