1. The problem statement, all variables and given/known data Two soap bubbles of equal radius 'r' coalesce isothermally to form a bigger bubble of radius 'R'. If the atmospheric pressure is P0, find the surface tension of the soap solution. 2. Relevant equations Surface energy = surface tension x increase in area (W = T.ΔA) Excess pressure inside bubble = 4T/R i.e. T = p.R/4 3. The attempt at a solution Don't know where to start. From the above equation, I thought I can proceed, but I neither know the surface energy nor the surface tension. I also do not know why the term 'p' has been given or how to relate it to the problem. Probably I should take the area increase as the difference between the total areas of the smaller bubbles and the area of the larger bubble. Area of smaller bubbles = 2.4∏r2 Area of bigger bubble = 4∏R2 Difference in area = 4∏(R2 - 2r2) W = T.4∏(R2 - 2r2) = p.R.4∏(R2 - 2r2)/4 How do I proceed further?