A soap bubble of radius r is blown up to form a bubble of radius 2r under isothermal conditions. If $\sigma$   is the surface tension of soap solution, the energy spent in doing so is 

Surface area of bubble of radius r = ${\text{r = 4πr}}^{\text{2}}$ . Surface area of bubble of radiums  $\text{2r = 4π}{\left(\text{2r}\right)}^{\text{2}}{\text{=16πr}}^{\text{2}}$ . 
Therefore, increase in surface area = 16 ${\text{πr}}^{\text{2}}{\text{- 4πr}}^{\text{2}}$  = 12 ${\text{πr}}^{\text{2}}$ . Since a bubble has two surfaces, the total
Energy spent = work done = ${\text{24πσr}}^{\text{2}}$