A free soap bubble in air naturally attains a nearly perfect spherical shape mainly because of which physical effect?

Introduction / Context:
Soap bubbles are beautiful everyday examples of surface tension at work. When a thin film of soap solution encloses air, the bubble almost always forms a spherical shape if it is free in air. This question asks you to identify the main physical effect responsible for that spherical shape.

Given Data / Assumptions:

• A thin soap film encloses air and forms a free bubble in still air.
• No strong external disturbances such as wind or collisions are present.
• Gravity effects on the bubble shape are small for small bubbles.
• The liquid film has surface tension and some viscosity, and the air exerts pressure.

Concept / Approach:
Surface tension is the property of a liquid surface that tends to minimise its area due to cohesive forces between molecules. For a given volume of air enclosed by a thin liquid film, the shape that minimises surface area is a sphere. Therefore, surface tension pulls the film into a spherical shape. Inertia refers to resistance to change in motion, not to static shape. Viscosity describes internal friction in the fluid and affects how quickly the shape changes, but not the final equilibrium shape. Pressure differences between inside and outside are related to curvature but alone do not explain why a spherical shape is selected; they are a consequence of surface tension.

Step-by-Step Solution:
Step 1: Recognise that the bubble encloses a fixed volume of air. Step 2: For that fixed volume, different shapes correspond to different surface areas. Step 3: A sphere is the geometrical shape that has the minimum surface area for a given volume. Step 4: Surface tension tends to pull the liquid film into a shape of minimum surface area to minimise surface energy. Step 5: Thus, the bubble naturally adopts a spherical shape to reduce surface energy. Step 6: Conclude that surface tension is the primary reason for the spherical shape of the soap bubble.

Verification / Alternative check:
If surface tension is reduced, for example by adding substances that disrupt the liquid surface, bubbles become unstable and may not form properly. Experiments with different liquids show that liquids with higher surface tension form more stable spherical drops and bubbles. Rain droplets in air, oil drops in water and small mercury droplets all tend to spherical shapes when gravity is not dominant, reinforcing the idea that surface tension prefers minimal surface area shapes, which are spheres.

Why Other Options Are Wrong:
Inertia of the liquid film: Inertia resists changes in motion but does not determine the static equilibrium shape of a bubble.

Air pressure inside the bubble only: Pressure difference across the curved surface is related to surface tension and curvature; it is not an independent cause of the spherical shape.

Viscosity of the liquid film: Viscosity affects how quickly the film flows and deforms but does not set the final equilibrium shape for a given volume.

Common Pitfalls:
Students sometimes confuse pressure and surface tension because both appear in the formula for pressure difference across a curved surface. It is important to remember that surface tension is the driving effect that tends to minimise surface area, and the pressure difference is a consequence of this curvature and tension balance. Thinking in terms of energy minimisation makes it easier to associate spherical shapes with surface tension.

Final Answer:
A soap bubble attains a spherical shape mainly because of the surface tension of the soap solution.