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=4πr2Surface area of bubble of radius 2 r=4π(2r)2=16πr2As the bubble has two surfaces, hence the total  increase in surface area =2×16πr2−4πr2=24πr2∴  Energy needed = work done =24πr2T