An air filled balloon is tied to the bottom of a swimming pool which is 3 m away from the surface .If the string is cut , the balloon which has a volume of 20cm cube rises to the surface of the pool . What will the volume of the balloon be when it reaches the surface ?

Yes Volume of the balloon increases.
It can be explained basing on ideal gas equation.

As you might know PV =nRT and here in this case probably temperature is constant both in side water and on the surface.

The pressure inside the water is more since it experience it due to atmosphere as well as depth of water on the top of it.

When the balloon comes to top it experience pressure only due to atmosphere and there is no pressure due to water.

Hence volume of balloon is less under water ( p is inversely proportional to volume) and more on top.

Cheers and further doubts on this are welcome.

Thanks suresh , u r helpful .

hi
vsuresh is right, only for information. the pressure under water increases at the rate of 1 atp after every 10m of depth. so balloon will have 2atp at the depth of 10m, and so on
anwaar