Soap bubble attains spherical shape due to which of the following reasons?

Concept

A molecule inside a liquid is pulled equally in every direction by its neighbours, but a molecule at the free surface has neighbours only below it, so it feels a net inward cohesive pull. This unbalanced cohesion makes the free surface behave like a stretched elastic membrane and stores energy in proportion to the surface area. The property is called surface tension, and a free liquid surface therefore tends to shrink to the smallest area that can enclose the given volume.

Application

A soap bubble is a thin liquid film with two free surfaces enclosing a fixed amount of trapped air. Both surfaces contract until the total area is as small as possible for that enclosed volume. Of all closed shapes enclosing a given volume, the sphere has the least surface area, so the film settles into a sphere — the state of minimum surface energy. Surface tension is therefore the reason a soap bubble is spherical.

Cross-check

Comparing the shape-determining role of each quantity offered confirms the same conclusion:

• Surface tension acts along the surface and stores energy per unit area, so minimising area is exactly what it drives — this is what fixes the shape.

• Inertia depends only on mass and resists changes in motion; it supplies no restoring force that could shape a thin film.

• Pressure acts normal to a surface and pushes outward equally in all directions; the excess pressure inside a bubble, 4T/r, is itself produced by the tension T of the two surfaces, so pressure is a consequence here, not the cause.

• Viscosity is internal friction between moving layers; it only decides how quickly the bubble settles into shape, not which shape it settles into.

Result: the soap bubble is spherical because surface tension minimises the surface area, and hence the surface energy, of the enclosing film.